Essentials of Repetition, The while Loop

Lecture 5

Essentials of Repetition

Most C programs involve in repetition or looping. A loop is a group of instructions the computer executes while some loop continuation condition remains true. There are two types of repetition.

1. Counter-controlled repetition

2. Sentinel-controlled repetition

Counter-controlled repetition

Also called definite repetition. We know in advance exactly how many times the loop will be executed. A control variable is used to count the number of repetitions. The control variable is incremented/decremented each time the group of instructions is performed. When the value of the control variable indicates that correct number of repetitions has been performed, the loop terminates and the computer continues executing the statement after the repetition structure.

Sentinel-controlled repetition

Also called indefinite repetition. It is not known in advance how many times the loop will be executed. It is particularly useful when-

a. The precise number of repetition is not known in advance and

b. The loop includes statements that obtain data each time the loop is performed.

The sentinel value indicates “End of Data”. The sentinel is entered after all regular data items have been supplied to the program. Sentinels must be distinct from regular items.

The while Loop

The general form of while loop is as follows-

initialize loop counter;

while (test loop counter using a condition){

      do this;

      and do that;

      and this;

      increment or decrement loop counter;

}

The following program prints number 1 to 10 using a while loop.

#include <stdio.h>

#include <conio.h>

void main(){

      int count=1; //initializing loop counter

      while (count<=10){

      printf (“I am in loop number %d”, count);

      count++;//incrementing loop counter

      }

}

           

Case Study 1

The lecturer of CSE department has been assigned a very tough (!) task. He will have to count total number of passes and fails for his class after taking the final exam. The program will tell how many students passed and how many failed. Moreover, if his class has number of passes over 80%, then we will call his class BEST. Otherwise, we will ask him to try better next time.

Now, see, what you will have to take care of:

1. Input: The input of the program will be pass/fail only for each student. You will have to collect the number of students from the user. This number of students will be your loop control variable.

2. In Loop: Inside the loop, you will have to keep track of number of fail and number of pass based on the input.

3. Output: You will have to show number of passed students and number of failed students. Also, you will have to see if the number of passed students is 80% or higher.

So, let’s get started. See, what you will have to create:

1. Variables, for holding number of students (will be used in loop condition), number of pass, number of fail, loop counter, and the user input (pass/fail)

2. A loop. Inside of the loop, you will have to have an if/else structure (if passed then increase pass, else increase fail).

3. At the end of the loop, you will have to have another if/else structure (if pass is above or equal to 80% it is the BEST class, else the teacher will be asked for better luck next time).

Simple! Now, we will develop the pseudocode.

Header files

main(){

      initialize pass and fail with 0, result, number of students, loop counter with 1;

      ask the user for putting number of students, take it into number of students variable;

      while (loop counter <=number of students){

            ask the user to put result (pass/fail), take it into result variable;

            if result is pass

                  increment pass by one;

            else

                  increment fail by one;

            increment loop counter by one;

      }

print number of fail and pass;

if ((pass/number of students) >= 0.8)

      print BEST;

else

      print BETTER LUCK NEXT TIME;

}

Case Study 2

It will be a similar program. The problem statement is as follows.

The user is teacher once again. The user will give the program- number of students. Then he will provide their marks. In the end, the program will respond with the average marks. The program can really help a teacher to understand the merit of his class.

Can you write the pseudocode by yourself?