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C Programming - Decision Making - Looping

Author: Exforsys Inc. Published on: 4th Apr 2006 | Last Updated on: 21st Sep 2011

Loops are group of instructions executed repeatedly while certain condition remains true. There are two types of loops, counter controlled and sentinel controlled loops (repetition).

Counter controlled repetitions are the loops which the number of repetitions needed for the loop is known before the loop begins; these loops have control variables to count repetitions. Counter controlled repetitions need initialized control variable (loop counter), an increment (or decrement) statement and a condition used to terminate the loop (continuation condition).

Sentinel controlled repetitions are the loops with an indefinite repetitions; this type of loops use sentinel value to indicate end of iteration.

Loops are mostly used to output the data stored in arrays, however they are also used for sorting and searching of data.

For Loop

For loops is a counter controlled repetition; therefore the number iterations must be known before the loop starts.

Sample Code

for(control-variable; continuation-condition;increment/decrement-control) {
code to iterate
}

Hint: if the code to iterate is only a single line then the braces ({ }) can be ignored.

Diagram 1 illustrates for statement operation.

Diagram SEQ Diagram * ARABIC 1 for statement

Example: consider the case of repeating the same line of code for 10 times. We write the code below.

Sample Code

int counter;
for(counter=1; counter<=10; counter++)
printf("n Number: %d",counter);

Figure 1 simple program counting from 1 to 10

Description: statement 1 declares the control variable of this for loop. Statement 2 is the most important part of this code. It initializes the control variable; it sets the continuation condition and it increments the control variable after every successful iteration. Diagram 2 clearly shows how this code works.

Diagram 2 code 1's operation

The While Statement

while statement is a sentinel controlled repetition which can be iterated indefinite number of times. Number of iterations is controlled using a sentinel variable (test expression).

Sample Code

while(test-expression)
{
code to execute
}

Hint: test-expression must be initialized otherwise errors will be generated when trying to compile the code.

Diagram 3 illustrates how while statement operates

Diagram 3 while statement

Hint: sentinel variable (test expression) must be controlled within the while statement, otherwise the loop will run forever.

Example: here we will consider the same example as we did for for loop; this is to see how two loops differ from each other.

Sample Code

int counter=1;
while(counter <=10)
{
printf("n Number: %d",counter);
counter++;
}

Figure 2 counting from 1 to 10 using while loop

Description: this code does the same job as the code 1 but using while statement instead of for statement.

The do..while statement

do..while statement is a sentinel controlled repetition which is quite different from the other two statements we covered earlier. This statement runs the code first and then checks the test-expression, so there is always a guarantee that the code runs at least once. This type of loop is generally used for password checks and menus.

Sample Code

do{
code to iterate
}while(test-expression)

Hint: you must initialize the test-expression to avoid possible errors.

Diagram 4 illustrates the operation of do..while statement.

Diagram 4 do..while statement

Example #4: here we will consider a simple menu using do..while this code will repeat the menu until 0 is inputted.

Sample Code

int input;
do
{
printf("n press 1 to print "hello" or 0 to exit : ");
scanf("%d",&input);
switch(input)
{
case 1: printf("hello"); break;
case 0: printf("exitting");break;
}
}
while(input !=0);

Figure 3 simple menu

Description: switch statement is used to create a simple menu which allows actions for two different cases. If and input is 0 then the while loop will be terminated once the test-expression is reached causing the program to exit.

The Break Statement

break statement is used to exit the iteration. This statement is usually used when early escape from the loop is acceptable by the programmer. For example if we are searching for a data, we can use break as soon as we find the data to exit the loop. You can simply use break by writing break; at the point where you want to escape the loop. break can be used with all the statements we have covered in this tutorial.

Diagram 5 shows how the program flow changes by break statement.

Diagram 5 use of "break"

Example: in this example, we will write a program which will escape the loop using break as soon as the number is equal to 2.

Sample Code

int counter;
for(counter=1;counter<=10;counter++)
{
printf("nnumber: %d before break",counter);
if(counter==2)
break;
printf("nnumber: %d after break",counter);
}
printf("nloop was escaped at %d",counter);

Figure 4 use of "break"

Description: counter value is initialized to 1. Therefore statements 1 and 2 will all be executed in the first iteration, but when the counter is incremented; statement 1,2 and 4 will be executed because statement 2 will escape out of the loop and go to the first line after the loop.

The Continue Statement

continue statement is used to skip the remaining code of the loop and start the new one. This statement can be implemented by continue;.

Diagram 6 shows the flow of the loop.

Diagram 6 "continue" statement

Example: we will consider the case in which the loop will skip all the even numbers between 1 and 10.

Sample Code

int counter;
for(counter=1;counter<=10;counter++)
{
if(counter%2==0)
continue;
printf("nnumber: %d was not skipped",counter);
}

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Figure 5 odd numbers between 1 and 10

Description: for statement will iterate 10 times counting from 1 to 10. Every time the counter reaches a number which is a multiple of 2 (counter%2==0), continue statement will be used to skip to the next number.

Read Next: C Programming - Arrays

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