beginning C programming

Tutorial 8.4: Passing Arrays to Functions

2009-06-01T10:28:00.016+08:00

An array can be passed as an argument to a function. Remember the mechanism of call-by-value. When we pass variables to a function, copy of their values are made for the function. Therefore any changes to the copy do not affect the original variables' value. But in the case of passing an array to a function, we are actually passing the memory location of the array which is its address. It is because the name of arrays evaluates to the address of the first element of the array. Thus, the called function can modifies the actual values in the original array since it knows exactly where the array is stored in the memory. The following program demonstrates that an array name is actually the address of its first element.

Sample output:

&a[0] = 0022FF20
   &a = 0022FF20
    a = 0022FF20

Printing the address of the array's first element and its name yields the same output. The %p is a conversion specifier for printing address. As usual we will look at an example on how to pass an array to a function. It also demonstrates that a function can modify the elements in an array.

Sample output:

1    2    3     4    5
1    4    9    16    25

Explanation:

First, an integer array a which contains five elements is declared.

Then we use for repetition statement to display the elements of the array. \t is an escape sequence that is used to insert horizontal tab. Once all the elements are displayed, a newline escape sequence is outputted to position the cursor at the beginning of the next line.

Next function SquareArray is invoked. The array a and its size are passed as the arguments to the called function.

Finally we re-display the elements of the array to see changes made by function SquareArray.

The following is the function definition of SquareArray.

The function expects two arguments, an integer array and integer variable. It does not returns any value to the caller.The definition consists of integer variable i declaration and for repetition statement.

The for statement repeats for the number of elements (size of the array) and performs square operation on each element of the array.

Tutorial 8.3: Two-dimensional Arrays

2009-05-30T14:50:00.011+08:00

Introduction
Up until now, we have been dealing with arrays with one index value as reference to its elements. These arrays are called one-dimensional arrays. In C we can declare arrays of two dimensions. Two-dimension arrays is illustrated in the following figure.

The array contains three rows and four columns (3-by-4 array). Two-dimensional arrays are commonly used to represent table of values that are arranged in rows and columns. For example, we could use a two-dimensional array to store information of student's course works for C programming course. What if we have several courses? We could use a three-dimensional array to store the information. One dimension would be for student's ids, one dimension for the scores and the third for the courses.

Defining and Initializing Two-dimensional Arrays
Now lets look at how we can declare a two-dimensional array. A two-dimensional array is declared as

int coursework[20][5];

The statement declares an array coursework containing twenty rows and five columns. As one-dimensional array, the index value of two-dimensional arrays with 0. Therefore, the reference for the first row and columns is coursework[0][0].

The basic structure of initializing a two-dimensional array much like a one-dimensional array. For example

int a[3][4] = { {2, 4, 6, 8}, {1, 3, 5, 7}, {2, 5, 3, 6} };

The values for each row are grouped in braces. The first group of values in the braces initializes the row 0. The next group initializes row 1 and the last group is for row 2. If we specify not enough initializing values for a given elements in a row, that elements will be initialized to zero.

Next: Passing Arrays to Functions

Tutorial 8.2: Using Character Arrays

2009-05-26T00:19:00.004+08:00

Defining and Initializing Character Arrays
We have discussed integer arrays in the previous tutorials. Now we will be looking at how character arrays can be used to store strings. For example, we initialize a char array string which has 20 elements as follows

char string1[20];

Although array string1 is defined to have 20 elements, actually it can hold up to 19 characters only. In C, a special string-termination character '\0' called null character is added at the end of a string. The null-character is there to indicate where a string end. Therefore when declaring a char array, we should always remember to define it large enough to hold the number of characters and the null character. The following figure illustrates a char array s holding a string "hello". The null character is added as the last character in the string.

Now lets look at how to initialize a char array. For example

char string2[] = "Good Morning";

Each element of the array is initialized to contain one character of the string. Note that we haven't specified the size of the array. We let the compiler assigns the size of the array based on the length of the string. Character arrays also can be initialize as follows.

char string3[20] = "The end";

The first seven elements will contain the specified string and the null character will be added to the seventh element. The rest still can be used because the memory locations are already allocated for string3.

I/O Character Arrays
A string can be inputted into a character array using scanf with conversion specifier %s. For example,

char string1[10]; scanf("%s", string1);

Note that the normally used & is omitted in the statement. This is because the name of the array is already an address which is corresponding to the address of the first element of the array. Function scanf will read characters until a space, tab or newline is encountered. Another important matter that we have to consider is the length of the string inputted. It should be no longer than 9 characters because the last element is reserved for the null character. Therefore it is better to use the conversion specifier 9%s to make sure only 9 characters will be read during the process of input.

A character array is printed using printf with conversion specifier %s. For example,

printf("The string is %s.\n", string1);

Function printf will print the characters in the array until null character is encountered.

An example of using character array.

Sample output:

Enter your name: Haris
Good Morning! Haris
Haris

Explanation:

Here we have declared integer variable i and two character arrays namely string1 and string2. string1's size is set to 20 while string2 is initialized to hold a string "Good Morning!".

A prompt message is printed to ask the user to enter his/her name. The entered name is stored in array string1. Then both arrays' contents are printed using function printf.

The for repetition statement prints the content of string1 element-by-element until the null character is encountered. This is achieved by specifying the loop condition to string1[i] != '\0'. Note that conversion specifier %c is used since we are printing a character in each iteration.

Next: Two-dimensional Arrays

Tutorial 8.1: Arrays

2009-05-22T09:18:00.003+08:00

Introduction
Lets revise what is a variable? A variable is a memory location that can store a value. An array is sequence of memory locations that have the same name and type. Thus we can say that an array is a collection of the same type of variables.

The following illustrates the graphical representation of an array.

The array name is a and it has 10 elements. The name of an array can be declared like other variable names. It can contain letters, digit, underscore and cannot begins with digit. All the array's elements have the same data type, int, char or any legal type in C. Each element has its index number and we refer to an element using their index number. Thus if we want to access the fifth element for example, we refer it as a[4]. Notice that the first element of arrays start with zero.

So what is the usefulness of arrays? Arrays are used when we want to store information that have relation such as student's scores, item's prices etc. Of course we can use variables to store the student's scores such as follows.

int s1,s2,s3,s4,s5,s6,s7,s8,s9,s10;
printf("Enter 7 scores: \n");
scanf("%d%d%d%d%d%d%d%d%d%d",&s1,&s2,&s3,&s4,&s5,&s6,&s7,&s8,&s9,&s10);

This would work but what if we need to store 100 scores or 1000 scores. Its almost impossible to program such task. Well, this is where an array can be useful.

Defining and Initializing Arrays
Lets look at how to declare an array. Its like how we declare a variable except that we need to specify its size in square bracket.

int a[10];

Here we declare an integer array a that has 10 elements. We may define arrays to contain other data types such as char, float and double.

char a[10]; float a[10]; double a[10];

Now we know how to declare an array, lets looks at how to initialize it. The simplest way to initialize an array is as follows.

int a[10] = {32, 78, 64, 90, 27, 82, 18, 60, 94, 58};

Here we assign 32 to the the 1st element, 78 to the second element and so on. If there are fewer specified values than elements, the remaining elements are initialized to zero. For example,

int a[10] = {32, 78, 64, 90};

All elements of an array could have been initialized to zero as follows.

int a[10] = {0};

Now we know how to declare and initialize an array, lets look at an example. The program utilizes for statement structure to store 10 student's scores into an array.

Sample output:

Enter student's scores:
15
48
68
92
37
82
64
79
57
62

Your student's scores are:
15
48
68
92
37
82
64
79
57
62

Next: Character Arrays

Tutorial 7.4: Computer-aided Learning Multiplication Problem

2009-05-22T08:51:00.025+08:00

PROBLEM
We are writing a program that will aid an elementary school student learns multiplication. Use random function to generate positive one digit integers and it should display questions of multiplication such as:

How much is 5 x 8 ?

The student then input the answer. The program should check the answer and response with correct or wrong message. Then another multiplication question is displayed. The process should be repeated until -1 is inputted by the student meaning to exit the program. Finally the program displays the number of correct answer and questions asked.

The program also should have three level of difficulties of question. The first level is multiplication of two integers. As the level goes up, the multiplication question consists of three integers and four integers for the highest level. The program should be able to adjust the level difficulty automatically according to the performance of the student. For example it should increase the level if the student able to answer correctly for five consecutive questions and vice-versa.

SOLUTION

PROGRAM ALGORITHM (FLOW OF PROGRAM)
1. Display user instructions
2. Display a multiplication question and compute the answer
3. Get data: user answer
4. Check the user answer with the computed answer
5. Determine the level of difficulty
6. Display the performance of the user

Steps 2-5 are repeated until the user exit the program.

PROGRAM DESIGN
We use functions to perform all required task in the program. There are six (6) functions have been defined accordingly, display the user instructions, generate integers, print the question, read user answer and check it with the correct answer, adjust the level of difficulty and display user performance.

Design of Function PrintUserInstructions

Input Parameter
None

Program Variable
None

Return Value
None

The definition consists of three statements to display user instructions.

Design of Function GenerateNumber

Input Parameter
None

Program Variable
None

Return Value
rand() % 9 + 1

The expression rand() % 9 + 1 always returns a value in the range 1 ≤ integer ≤ 9. Function rand() returns a pseudo-random number each time it is invoked. Pseudo-random number is a sequence of numbers that appears to be random that is generated using an initial value (seed). For a given seed, the sequence is always the same. If we use function rand() with the default seed, we will always get the same sequence. Therefore, in order to obtain a random number, we have to randomize the seed.

In C, there is a function called srand() that can be used to initialize the pseudo-random generator by supplying a seed value as an argument to it. Now what we have to do is to supply a different value each time the program is run. This can be achieved by using function time() which returns the number of seconds that have passed since 00:00:00 GMT January 1, 1970.

Design of Function PrintQuestion

Input Parameter
int level /* level of difficulty */

Program Variables
int i /* loop counter */
int num /*store generated integer */
int ans /* store the computed answer */

Return Value
int ans /* store the computed answer */

Function PrintQuestion uses for loop statement to print the multiplication question. In each iteration, an integer is generated by calling function GenerateNumber and multiplied with variable ans. The loop condition uses level (function's argument) in the loop condition. Initially, level has the value of 1 (level 1), therefore the loop is repeated twice which means the multiplication question will be consisted of two numbers.

Design of Function InputAnswer

Input Parameter
int correct_ans /* store the correct answer */

Program Variable
int user_ans /* store the user answer */

Global Variables
int cnt_question /* count the total questions have been asked */
int cnt_true /* count total true answer */
int cnt_consecutive_true /* count consecutive true answer */
int cnt_consecutive_false /* count consecutive false answer */

Return Value
QUIT 0
CONT 1

User answer is read by using scanf and stored in variable user_ans. Then user_ans is tested. If the value is -1, symbolic constant QUIT is returned. Else variable cnt_question is incremented and user_ans is tested. If user_ans is equal to correct_ans, variables cnt_true and cnt_consecutive_true are incremented and zero is assigned to cnt_consecutive_false.

Design of Function AdjustDifficulty

Input Parameter
int level

Program Variable
None

Return Value
int level

Difficulty level is adjusted based on the value of variables cnt_consecutive_true and cnt_consecutive_false.

Design of Function PrintResult

Input Parameter
None

Program Variable
None

Return Value
None

The definition consists of two statements to display the total number of correct answers and total number of questions asked.

IMPLEMENTATION

Begin by writing the #define directives for the constant data and global variables. In the main(), declare the variables for correct answer, level of difficulty and loop control variable. Then start coding each program algorithm steps. Call function PrintUserInstructions to display the user instruction/prompt. Then call the functions PrintQuestion, InputAnswer and AdjustDifficulty. These functions should be placed in the body of the program loop since they will be repeated until the user wants to exit. Finally call the function PrintResult to display the result. Don't forget to use comments in your program so as you won't get lost. Good luck and Happy Coding!

Tutorial 7.3: Defining a Function

2009-05-21T17:47:00.004+08:00

Defining a Function
Function definition consists of two parts, the function header and the function body (block). The function header specifies the function's return value type, function name and function parameters. The function body defines the operation that the function performs when it is called. The general form of a function definition is as follows.

return-type function-name (parameters)
{
declarations and statements;
}

The function-name can be any valid identifier that is not a keyword such as int, break, char etc. Of course you can't use a name that already been assigned to another function. The parameters is the list of variable names and their types that carry the values received by the function from the caller. The values supplied to the function is called arguments. The return-type specifies the type of the value returned by the function. The type of value can be specified as any of the available type in C. If the return-type is omitted, int is assumed. The void return-type indicates that the function does not returns a value.

The following program uses a function GetAverage to compute an average of three values.

Sample output:

The average is 20.433334

Explanation:

In the pre-processor section, we have the following line.

This is a function prototype. It is there to tell the compiler the specification of a function. The function prototype includes specification for the return type, the parameters and the function name. Basically function prototype is the same as the function header plus the semicolon at the end.

Lets look at the function definition of GetAverage. The function has three parameters namely num1, num2 and num3 which are of type float. It is defined to computes the average of three values and returns a float type value. The return statement is used when the function is required to return a value. If the function is defined not to return a value, we are not required to put the return statement because the control is returned simply when the close brace is reached. Note that the return value should be of the same type as the declared return type in the function header.

As usual, in the function main(), we have variables declaration and assignment. In this case we have three float variables. Then we have the function call statement.

We simply use the function's name followed by the arguments to be supplied to the function to call a function. It is important to note that the supplied arguments must be of the same type as the expected type. When the arguments are supplied to a function, copies of the values in n1, n2 and n3 are made for the function. The copies are then stored in num1, num2 and num3 which have been declared in the function header. This means the function can't modifies the values in n1, n2 and n3. This mechanism is called call-by-value. There is another mechanism called call-by-reference. These two mechanism will be discussed in the next section. The result (returned value) of the function is assigned to variable res. Finally the value of res is displayed using printf.

Next: Arrays

Tutorial 7.2: Variable Scope and Sequence of Execution

2009-05-21T11:55:00.005+08:00

Variable Scope
All examples that have been presented in the previous tutorials declared variables in the block of function main(). These variables are known only within the block of function main() which means, the variables are not accessible outside of the block. This is also applied to variables which are defined within a block that is inside other block. But variables which are defined in the outer block are accessible in the inner block. This is illustrated in the following example.

In the example there are two variables declared,. Variable a is declared within function main() block and variable b is declared within while loop block. a is the outer block variable therefore it is accessible within inner block. But b is not accessible in the outer block because it is while loop variable. Thus if you try to print the value of b outside the while loop block, you will get an error message when you compile the program.

Variables declared outside any function is called global variables. A global variable is a variable that can be accessed in any function. Global variables are declared in the normal way except that (normally) they are declared before function main().

Sequence of Execution
Before we discuss the details on how to define a function, lets look the sequence of execution when functions are invoked. When a function is invoked, the code within the function's body is executed. Then execution continues from the point at which the function was called. The operation is illustrated in the following figure.

For example, the function main() calls function1. The statements within the block of function1 are executed. When it reaches at the end of the block, it returns to the main function and continues at the point at which the function1 was called. Similarly, a function can be invoked by another function. The sequence of execution is the same as when the main function calls a function. This is illustrated in the figure where function3 calls function4.

Next: Defining a Function

Tutorial 7.1: Functions

2009-05-21T09:00:00.003+08:00

Introduction
Programs that solve the real-world problem are usually consist of thousands (or more) of lines of code. In that case, it is not easy to develop, maintain and manage a large source code in a single function - main(). So when we are writing a big program, the best way to deal with it is to break the large program into smaller sets of statements (functions) according to their tasks. For example we could define functions that performs the printing of user instructions, reading inputs from user, calculating equation and etc . Whenever we need to do something, we just call the relevant function in the main() that do the specific task. In that case, our program will be neat and tidy and errors can be easily detected and solved.

A) Program with no functions

B) Program with functions

Sample output:

Enter scores for class A (-1 to end):
75
85
64
-1
Class A average is 74.67

Enter scores for class B (-1 to end):
37
97
71
-1
Class B average is 68.33

Both program A and B are written to perform the same task which is to calculate average score for two classes. But program A does not use functions and program B uses function. As you can see in program B, the codes of user instructions, read inputs, calculate and display the average are segmented into functions. In function main(), we invoke/call the relevant functions to perform the required tasks.

Next: Variable Scope and Sequence of Execution

Tutorial 6.5: Infinite Loop

2009-05-19T13:43:00.006+08:00

Infinite Loop
Infinite loop is a repetition structure that loops forever, either due to it contains no condition that can cause the loop to be terminated or having condition that can never be satisfied. Let us see how while, do while and for loop statement can be set up to get an infinite loop.

Having an expression within the parenthesis of while and do while (second expression of for loop) that always returns true (non-zero value) can result an infinite loop. The following programs are example of infinite loop of while, do while and for repetition structure.

The loop condition (expression) is set to 1, resulting an infinite loop.

Sometimes infinite loop can be useful, for example when we want to write a program that computes the average of an arbitrary number of integers. A sample program is given below. The program should be able to allow the user to input any number of integers until he is satisfied. In this case, we have to define an infinite loop so as the program can perform the specific task. But what can we do to have the means of exiting the infinite loop? The answer is the statements in the loop's body must contains the means of exiting from the loop. Remember keyword break from Tutorial 5.4. We have seen how break can be used to jump out of switch selection bypassing the following statements. Similarly break can be used to exit a loop from any point of its body.

Sample output:

This program computes the average of any number of integers.
Enter integers (-1 to stop input):
10
5
7
-1
The average is 7.33

Explanation:

Here we declare four (4) variables namely i, num, sum and average. Variable i is used to count the number of integers entered by the user. The entered integer is stored in variable num. The summation of all integers is stored in variable sum and lastly variable average is used to store the average.

Then we display the user instruction using printf. We inform the user to input -1 if he wants to end input process.

We set up an the loop using while statement. Since loop condition has the value of 1, the block of statements will be repeated forever. In the body of the loop, we have read input from user statement. The input integer is stored in variable num. Then we compare the value of num with -1. If the comparison is true indicating the user wants to end the input process, break statement will be executed. Else the i is incremented and the value entered is added to variable sum.

Finally we calculate the average and display the result.

Next: Functions

Tutorial 6.4: Nested Loop

2009-05-18T23:46:00.005+08:00

Nested Loop
Remember nested if, the same can be applied to repetition structure. A loop within a loop is called nested loop. Any type of loop can be placed in any type of loop. Its means, you can put a while loop in a for loop, do while loop in a while loop etc. The following is an example of nested loop.

Sample output:

1 1
2 3
3 6
4 10
5 15
6 21
7 28
8 36
9 45
10 55

Explanation:

The program computes the sum of all numbers from 1 up to the value of j which is 10. Lets look at the loop structure.

The for loop is called the outer loop and the do while loop is called inner loop. Each time outer loop is repeated, inner loop will be executed and all its iteration are performed. This means, inner loop is executed or all its iteration are completed 10 in this example since the outer loop is repeated 10 times.

The for loop begins by assigning value of 1 to i. The loop condition is i < 11 thus the for loop will repeat for 10 times. For each iteration of for loop, variable sum is initialise to 0 and the do while is executed.

The do while loop computes the sum of all values of j each time j is incremented until the current value of i. You may wonder what is the statement

is doing. Actually that statement is equivalent to

Next: Infinite Loop

Tutorial 6.3: for Repetition Structure

2009-05-18T12:03:00.004+08:00

for Loop Statement
Unlike while and do while statement, for statement handles all the details such as variable initialisation, variable update and loop condition within single parantheses. If you still confuse with what i mean, lets revise the while loop example.

As you can see, the statements to control the loop operation are separated. In the for statement, these statements are combined together. Lets look at the for statement structure to have better understanding.

for ( variable initialisation; loop condition; variable update ) {
...statements...
}

for statement structure has three (3) fields which are variable initialisation, loop condition and variable update. These fields in the for statement are optional. we may omit variable initialisation and place it elsewhere in the program for example before for statement. If loop condition is omitted, C will assume that the condition is true thus resulting an indefinite loop. Variable update may be ommited if we want to place it in the body of for statement.

An example of for statement is given. The purpose of this program is to display zero (0) to nine (9).

Sample output:

0
1
2
3
4
5
6
7
8
9

Explanation:

As usual, we declare an integer variable count which acts as the loop counter. We do not initialise the variable unlike while and do while statement because it will be taken care of in the for statement.

Here we have the for repetition structure. The operation of the loop is controlled by the three expression between the for statement parantheses. In the variable initialisation field, we initialise variable to 0. The specified loop condition is count less than 10

Next: Nested Loop

Tutorial 6.2: do while Repetition Structure

2009-05-18T11:06:00.003+08:00

do while Loop
The do while loop is similar to the while loop. In the while loop, the loop condition is tested at the beginning of the loop. The do while loop tests the loop condition at the end of the loop structure or after the body of the loop is executed. Thus in do while loop, the compounded statement will be executed at least once.

Lets look at an example of do while loop.

Sample output:

0
1
2
3
4
5
6
7
8
9

Explanation:

This is also a very simple program of do while loop. The function of the program is exactly the same as the example of while loop. We skip the counter declaration and initialisation since it is already covered in the previous example. We go straight to the do while loop structure.

Similarly, it contains two (2) statements action printing the value of count and ++count. Lets go through it line-by-line. When the do while is executed, it begins by executing the printf() function. Thus count is displayed on the monitor screen. Then count is incremented and it reaches the brace at the end of the loop. Here loop condition is tested and if it is true, it will go back up to the beginning and the body of the loop will be executed again. This action continues until the condition is false.

Next: for loop

Tutorial 6.1: while Repetition Structure

2009-05-17T22:27:00.004+08:00

INTRODUCTION
In the previous tutorial, we have learned about sequence and selection structures. We know that selection structure allows us to specify how the computer will react to certain inputs. But there is still a limitation in what we can do in programming. For example what if we want to display numbers from 1 to 100? Of course we can do hundred lines of printf(), but what if we want to repeat a thousand times of 10 statements. We would have to write ten thousand of statements to achieve it and it is possible to do it. But it is not a convenience and it would take a long time to complete. Thus the ability to specify repetition of compound statements is as important in programming. Repetition of statements execution is called loop. Looping in a program is the process of executing series of instruction repeatedly in a given number of times.

There are two types of loop namely definite loop and indefinite loop. Definite loop also called counter-controlled loop is a repetition in which it will continue looping until certain condition is met. On the contrary indefinite loop has no certain condition in which it will continue looping forever. Thus statements enclosed by indefinite loop should contain the means of exiting from the loop. Typically, there are three requirements have to be specified in defining a definite loop. They are counter initialization, increment of counter and loop condition. The counter will determine how many times the loop will be executed. There are three types of repetition structure available in C namely while loop, do while loop and for loop.

But before going further, you should understand the concept of True and False in C. We have covered this concept in Tutorial 5. You might want to look at it if you still have not grasped the basic concept. We begin by discussing the while loop followed by do while loop. Then we will discuss for loop because I believe for loop is a bit complicated compared to the other loops.

while LOOP
while loop allows the repetition of a set of statements execution to continue for as long as a specified expression (condition) is true. When the condition becomes false, the repetition terminates and the first statement after the repetition structure is executed. Basically while loop is similar to if selection in the sense that their set of statements are executed if the condition is true. The different is while loop repeats the statement execution as long as the condition is true and if selection does not have the repetition feature.

Lets look at an example to have an idea what is while loop all about.

Sample output:

0
1
2
3
4
5
6
7
8
9

Explanation:

This is a very simple while loop. It has an expression as the mechanism of repetition and a set of statements. Now lets look at the first statement.

Here, we declare an integer variable count and initialize it to 0. Variable count acts as the loop counter for the while loop.

Now we look at our while loop. It contains two statements action printing the value of count and ++count. If you are not sure what ++count is doing, you might want to look at Tutorial 4 Increment and Decrement Operators.

Lets go through it line-by-line how while loop works in this example. It begins by testing the while loop condition. Variable count has value of 0, thus the condition of count < 10 is true. printf() prints the value of count and count is incremented. When it reaches the close brace, it jump back up to the beginning of the loop and the condition is tested again. This process is repeated until the condition becomes false and the loop is terminated.

Next: do while Loop

Tutorial 5.5: Electricity Bill Problem

2009-05-14T19:23:00.005+08:00

PROBLEM
We are going to write a program that computes a customer's electricity bill. The rate of charge for the first 200 unit of electricity used is RM0.218. For the following unit of electricity used is RM0.334. A penalty of 1.5% of the unpaid balance if the unpaid balance is greater than zero. Electricity unit, unpaid balance are inputted by user.

SOLUTION
PROGRAM DATA REQUIREMENT
List of data required in this problem. There are three (3) categories of data.

Constants
RATELESS 0.218 /* rate of charge first 200 unit */
RATEMORE 0.334 /* rate of charge following unit */
PEN 0.015 /* penalty for unpaid balance */

Inputs
int unit /* unit of electricity used */
float UnpaidBal /* unpaid balance */

Outputs
float Penalty /* charge of penalty */
float UseCharge /* charge for the current electricity use */
float TotalBill /* total charge */

PROGRAM ALGORITHM (FLOW OF PROGRAM)
1. Display user instructions
2. Get data: unpaid balance and electricity unit used
3. Determine electricity unit used and compute use charge
4. Determine unpaid balance and compute penalty
5. Compute total charge
6. Display the total bill amount

DESIGN OF COMPUTATION OF USE CHARGE
The data required to compute the use charge are listed. We separate the involved data categories into Input, Process and Output.

Input Data
int unit /* unit of electricity used */

Process Data
RATELESS 0.218 /* rate of charge first 200 unit */
RATEMORE 0.334 /* rate of charge following unit */

Output Data
float UseCharge /* charge for the current electricity use */

Algorithm for Computation of Use Charge
We know that different rate will be used if the electricity unit used is more than 200. Thus we use if else selection to design the algorithm.
if unit > 200
compute use charge for more than 200 unit
else
compute use charge for less than 200 unit

Formula for Use Charge
UseCharge=(unit-200)*RATEMORE+200*RATELESS /* more than 200 */
UseCharge=unit*RATELESS /* less than 200 */

DESIGN OF COMPUTATION OF PENALTY
The data required to compute the penalty are listed. We separate the involved data categories into Input, Process and Output.

Input Data
float UnpaidBal /* unpaid balance */

Process Data
PEN 0.015 /* penalty for unpaid balance */

Output Data
float Penalty /* charge of penalty */

Algorithm for Computation of Penalty
Penalty is dealt when there is an unpaid balance. We use if selection to design the algorithm.
if unpaid balance > 0
compute penalty

Formula for Penalty
Penalty=PEN*UnpaidBal

DESIGN OF ELECTRICITY BILL LAYOUT
Displaying bill involves only input data (variables).

Input Data
float Penalty
float UnpaidBal
float UseCharge.

Algorithm for Displaying Bill
if unpaid balance > 0
compute penalty

IMPLEMENTATION
Begin by writing the #define directives for the constant data. In the main(), declare the input and output variables. Then start coding each program algorithm steps. Use printf() to display the user instruction/prompt and scanf() to read the user inputs (UnpaidBal and unit). Implement the computation of use charge and penalty using if else selection. Then, use relevant arithmetic operators to construct the formulas under their corresponding if else selection. Finally use comments in your program so as you won't get lost. Good luck and Happy Coding!

Tutorial 5.4: switch Selection Structure

2009-05-14T14:39:00.005+08:00

switch Selection Statement
The switch statement allow us to select one from multiple options. It is especially useful when we want to write a code that makes a selection based on an expression or a variable. switch selection structure consists of a series of case label sand optional default case as shown below.

switch statement works by comparing the value of variable or expression within parentheses with values specified in each case. If there is a match, the statements following the case will be executed. If there is no match occurs, the default statement is executed. At the end of each case, keyword break must be placed to exit from the switch immediately after statements execution.

Lets look at an example.

Sample output:

1. Create a new database
2. Edit a database
3. Delete a database
4. Merge databases
5. Exit system
Choose an option: 2
Editing...

Explanation:

We declare an integer variable choice. Then we display the options that user can choose and ask the user to input his/her choice. scanf() reads the input value and store it in variable choice.

Next we have the switch statement that will select from the specified cases. A case is selected depending on the entered value of choice. Statement belong to the selected case will be executed and the break at the end of the case causes the program to jump to the statement following the switch block. In the case of user enters a value other than one (1) to five (5), default case will be executed displaying a message or invalid input.

Next: while Loop

Tutorial 5.3: Nested if Selection Structure

2009-05-14T13:56:00.004+08:00

Nested if Statement
It is possible to have if within if statement. The if within an if is called nested if. The flowchart illustrates the concept of nested if. Do take note that if can also be place within else statement.

Lets look at an example of nested if.

Sample output:

Enter a number between -10 and 10: 6
6 is a positive number
6 is an even number

Explanation:

Here we have if else selection structure within the if statement that tests whether the variable num is an even or odd number. Notice that the nested if will only be checked if the condition of the if statement is true. The braces enclosing the nested if are necessary to make the nested if a part of if (first) statement.

Next: switch Statement

Tutorial 5.2: if else Selection Structure

2009-05-14T13:20:00.005+08:00

Extending the if Statement
Lets revise the last session, if statement allows a statement to be executed if the condition is true. If it is false the statement will be skipped. What if we want the program to execute certain statement if the condition is false. Remember the previous program's if statement.

The message will be displayed only when the condition is true. Lets make a few modification to the if selection structure.

As you can see we add else statment after the if statement. In the if else selection structure, the else statement will be executed when the condition is false. For example, if variable num is less than 0, the first printf() will be skipped and the second printf() will be executed. The function of if else selection is illustrated in the flowchart.

The full program and the sample output are given below. The sample output shows two different outputs for positive number and negative number.

Sample output:

Enter a number between -10 and 10: 6
6 is a positive number

or

Enter a number between -10 and 10: -2
-2 is a negative number

Next: Nested if Statement

Tutorial 5.1: if Selection Structure

2009-05-14T12:01:00.004+08:00

Introduction
Up until now, we have been writing a program that executes the statements one by one until the defined end. This type of program is called sequential structure. The functionality of this type of program is limited since it flows in single direction. Now we will be looking at one of the most powerful programming feature - the ability to change the flow of program execution. It is achieve by establishing the truth or falsity of an expression (condition). This type of program is called selection structure.

Selection structure is extensively used in programming because it allows the program to decide an action based upon user's input or other processes. The application of selection structure can be seen at systems that have password checking such as ATM Machine, handphone and etc.. The system allows user access only if the user input password is the same as the corresponding password stored in the system. In C, there are three types of selection structure can be implemented, namely if selection statement, if else selection statement and switch selection statement.

Before we go further into the selection structure, you need to know the meaning of True and False in programming. An expression that evaluates to non-zero (1) is considered true expression while false expression evaluates to zero (0). Refer to example of Tutorial 4

if Selection Statement
if is the primary tool of selection structure. if is used when we want a single or a group of statements to be executed if the condition is true. If the condition is false, the statement is skipped and program continues by executing the first statement after the if selection structure. The flowchart illustrates the function of if selection.

Lets look at an example that implement if selection structure. The program asks the user to input a number between -10 and 10. Then it message if the number is a positive number.

Sample output:

Enter a number between -10 and 10: 6
6 is a positive number

Explanation:

We declare a variable called num and then we prompt the user to enter a number between -10 and 10. The scanf statement reads the user input and store it in variable num.

The if statement compare the value in num with the value of 0. If num is greater than 0, the printf() will be executed which displays a message and continue with the next part of the program. If it is not greater than 0, the printf() will be skipped.

Next: Extending the if Statement

Tutorial 4: Operators

2009-05-13T21:59:00.003+08:00

Arithmetic Operators
Arithmetic operations can be performed in C programming language. There are five operators available in C which are + (addition), - (subtraction), * (multiplication), / (division) and % (modulus). +, -, * and / are self explained. % operator is to obtain the remainder of when we are dividing a number with a number. For example,

x % y

produce the remainder when x is divided by y.

The multiplication, division and modulus have higher precedence over the addition and subtraction. Its means /, * and % will be evaluated before + and - operators.

Relational and Logical Operators
There are six (6) relational operators which are < (Less than), > (Greater than), <= (Less or Equal to), >= (Greater or Equal to), == (Equal to) and != (Not equal to). Note that relational operators have lower precedence than arithmetic operators.

There are two (2) logical operators which are || (OR) and && (AND). Expression consists of logical operators is evaluated left to right. The precedence of && is higher than || and logical operators have lower precedence than relational operators. There is a unary negation operator ! that converts a zero (0) to one (1) and vice-verse.

For example lets look at the following expression. Assume x = 2, y = 9 and z = 3
x > 5 || y < 7 && z != 2

Since relational operators have higher precedence, they will be evaluated first.
x > 5 returns False (0)
y < 7 returns True (1)
z != 2 returns True (1)

Thus, the expression can be rewritten as
0 || 1 && 1

Now, && is evaluated then ||. Expression 1 && 1 returns 1, then the expression becomes
0 || 1

which returns 1.

Increment and Decrement Operators
C provides two operators for incrementing and decrementing variables. If we have
++x

the expression is adding x by 1. Similarly if we have
--x

the expression is subtracting x by 1.

Postfix and Prefix
Writing the increment or decrement operator in front of a variable is called prefix form. We may write the operator after a variable and this is called postfix form. Post-incrementing or pre-incrementing variable has the same effect if it is not used in an expression. Ultimately the value of the variable is added by 1. Post-incrementing and pre-incrementing must be considered when it is used in an expression. This is because for a variable that is post-incremented, the incrementing of the variable is performed after the variable is used while for pre-incrementing, the incrementing is performed before the variable is used. This is a bit confusing but lets look at an example to have better understanding. Assume i = 20 and j = 4 in the statement

sum = i - j++;

sum will have the value of 16 since j is incremented after the expression is evaluated. After evaluation j will have the value of 5.

Now consider the following statement.

sum = i - ++j;

sum will have the value of 15 since j is incremented before the expression is evaluated. After evaluation j will have the value of 5. Now you can see the different. Post-incrementing and pre-increment do can effect the result of an expression. Therefore we have to be careful when using with it in an expression.

Summary of Operators Precedence and Associativity¹

¹(Kernighan & Ritchie) The Ansi C Programming Language

Tutorial 3: Output and Input

2009-05-13T21:50:00.002+08:00

Outputting Information Using printf

printf is a standard library function which can be used to output information to the display screen. It function is to output whatever information contained between the parentheses. For example,

printf("C program example\n");

The printf is used to print everything between the quotation marks on monitor screen. Notice that there is a ‘\n’ character at the end of the printf statement. This character is called newline escape sequence. The function of this escape sequence is to print a newline. There are more escape sequences available in C. We will be looking at the escape sequences later.

The function printf() can also be used to print the value stored in a variable. For example,

printf("%d\n", num);

This printf has two arguments. "%d\n" and num. The first argument consists of conversion specifier "%d" indicates that an integer is to be displayed on the monitor screen. The second argument contains the variable that stores the value to be printed which in this case variable num.

Inputting Information Using scanf

scanf is a standard library function which is used to input information from keyboard. It takes whatever entered from the keyboard and interprets it as specified by the conversion specifier and stores the result in a variable. The following is an example of scanf function statement.

scanf("%d", &variable);

The first argument consists of conversion specifier "%d" indicates that an integer is to be read from the standard input which is usually the keyboard. The second argument is where we specify the variable to be used to store the input integer. Notice that the variable name starts with the & (ampersand) character. The & is the address-of operator in C programming. It tells the scanf function the location (address) of the memory is stored in memory.

Lets look at an example of I/O operations

Sample output:

Enter two numbers:
5
7
v1 is 5
v2 is 7

Explanation:

We begin with integer variables declaration namely v1 and v2.

Then we display a prompt message asking user to input two numbers.

These statements read the user inputs from the keyboard and store the values in variable v1 and v2. In the sample output, the entered number are 5 and 7. So 5 is stored in v1 and 7 is stored in v2.

The first argument of printf statement consists of string to be displayed and a conversion specifier. The second argument specifies the variable that stores the value to be printed.

Tutorial 2: Variables and Data Types

2009-05-13T21:44:00.005+08:00

Variable is a memory location that can store a value. The stored value can be retrieved or modified when necessary. Initialization of a variable means a value is given to the variable during its declaration. When a variable is declared, its name and data type are specified.

A variable name can contain letters, digit and underscore (_). It cannot begins with a digit. C is case sensitive, therefore v and V are considered two different variable names. Its data type is given to specify what type of data can be stored in the variable.

There are four basic data types in C programming language: int, float, double and char. Each data type has its own size. The size of a variable is the memory space allocated in the memory required to store its value. The following example demonstrates on how to declare a variable for each data type.

The statements declare four variables of types int, float, double and char with the name of num1, num2, num3 and letter. Now that we have declared the variables, lets look at how to initialize them.

We use '=' (assignment operator) to initialize the variables such as the value of 5 into variable num1. We could also perform the declaration and initialization on a single statement. This is demonstrated in the following program.

Tutorial 1: Introduction to C Programming

2009-05-13T21:28:00.003+08:00

Introduction
C was developed in 1972 by Dennis Ritchie based on a programming language called B (which is derived from BCPL language). It is one the most used programming language in software development area. At the beginning, C was designed and developed for the development of UNIX operating system. Today C has been widely used to develop many type of application software such as basic calculator, word processor, games, operating system etc.

Getting Started - New to C Language?
Before we discuss C syntax in details, we look at the fundamental stages of C program development. Typically there are four development stages namely editing, pre-processing and compiling, linking and executing.

Editing
Editing is the process of writing the C source code. Programmers write C source code using editor program. There are many editor program available for C. But two editors widely used on Linux platform are vi and emacs. On Windows platform, we can use Notepad as the editor program but most peoples use software package for C such as C++ Builder, Code::Blocks, Dev-C++, Microsoft Visual Studio etc. that have integrated editor. These software provides complete programming environment for writing, managing, compiling, debugging and testing C source code. This is also known as Integrated Development Environment (IDE).

Pre-processing and Compiling
A C source has to be translated into machine language code. Machine language is a language that computer can understand in the form of binary number. The process of code translation is called compiling. During compilation process, the compiler can detect any syntax error due to unrecognised program code as well as structural errors. The output of compilation process is an object code which is saved in a file called object file. In C, before a source code is compiled, there is a stage called pre-processing. During this stage, C pre-processor performs directives which are usually including header or other C files to be compiled and text substitution.

Linking
Linking is a process of combining generated object files, add required codes from C standard library and produce an executable file. C standard library is collection of header files. Header files contain C source code that performs common operation such as input/output, mathematical operation and string handling.

Execution
The execution stage is where you run the program to check whether it produce the desired output. This stage can be a testing stage where you check the output of your program. If it does not produces the suppose output, then you have made logic (semantic) error in your source code. Unlike syntax error, logic error won't be detected during compilation process. Therefore you have to find out the error's location by yourself.

C Program Layout
Basically, a C program consists of two sections:
1. pre-processor directives
2. main function

The pre-processor section is the place where we specify what compiler should do before compiling the source code. Usually we would specify the included header file which will be used in our program. A header file keeps all the information about functions available in standard C library. All pre-processor directives begins with the # symbol. The second section begins with the function main() definition. The function main() is where the program starts its execution. Thus function main() must present in any C program.

An Example of C Program
We begin by programming a very simple C program. Write the following program using your editor program. When you finish writing it, compile it and execute the program. You will get a text message displayed on your monitor screen. The program's output is given at the sample output section. Don't worry if what you type does not make sense. I'll explain everything later.

Sample output:

C program example

Explanation:

The first line of the program is

This line is not actually part of the program. It is simply a comment and it is included to provide information on what the program does. A comment begins with /* and end with */. It is a good habit to include comments in every part of your program especially when you are writing a big program/project. It will keep you inform what are the code actually doing since you may not always remember what they do.

Then we have the next line which is

This is also not part of the program, but it is a very important line in C program. In fact, if this line is omitted, the program won't actually works. The symbol # indicates that this is a pre-processor directive. Remember about header file that I mentioned during compilation process. This line is a directive to the compiler to include stdio.h header file in our program. stdio.h contains information on input/output routines which will be needed during compilation process. In this case we are using the printf() function from stdio.h.

The next line is

This is also another comment line.

Then we have the main function section.

The main function has only two (2) statements. The first is printf() statement and the second is return statement. I'll explain both statements in details later.

Now we look at the first line of function main() definition.

This defines the start of the function main(). Function main() has the keyword int signifies what type of value will be returned by the function. In this case the function main() will return an integer value. There parentheses following the main. This is the place where we specify information to be transferred to function main(). However, in this example we don't transfer any information.

Then we have

The open brace specifies the start of the function whilehe close brace specifies the end of the function. The portion within the braces is called block or body. Within the block is where we write the statements that defines what our program does.

Now lets look at the first statement.

printf() function instructs the computer to display characters or a string enclosed by the qoutation marks on the monitor screen. A string is a combination of numerous characters. Items enclosed by the parentheses are called arguments.

The last statement in this program is

At the end of function main() we have to include return 0 statement because function main is defined to return a value of integer type. This statement also indicates that the program has terminated successfully.

Symbolic Constant - #define Directive
I would like to discuss another pre-processor directive that is usually used in C programming. #define directive is used to define a symbol (name) that represents a constant value. Basically #define directive acts as a constant converter. It causes the compiler to go through all the program's codes and whenever it encounters the defined symbol, it will substitute the symbol with the specified value. For example,

#include<stdio.h>
#define PI 3.14159

int main() {
...
...
}

Symbol PI is defined and it represents the value of 3.14159.

Computer-aided Learning Multiplication Problem's Source Code

2009-05-12T19:08:00.000+08:00

Electricity Bill Problem's Source Code

2009-05-12T18:53:00.000+08:00