Does the Do While loop test its condition before or after it performs an iteration?

Question

The while loop is used to repeat a section of code an unknown number of times until a specific condition is met. For example, say we want to know how many times a given number can be divided by 2 before it is less than or equal to 1. If we know a specific number, such as 32, we can say 5 times, but for a given symbolic variable "NUMBER" which represents any number in the world, how many times is not known a priori (before hand). In this case, we could use a while loop to determine that answer:

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C, C++, or Java
Actionscript
Why While Loops?
Example 1: How to assure proper input
Design Pattern:
C, Java, or Actionscript
Introduction to Test Before Loops
Counting Loops
Does the loop test its condition before or after it performs an iteration?
Which loop can be test the condition first?
What executes first in a do

The "pseudocode" for such an algorithm is: while the number is bigger than one keep dividing it by two. additionally, keep a count of how many times we do the division.

Pseudocode

            
                get our number
                set our initial count to 0
                while our number is greater than 1
                  divide the number by 2
                  increase our count by 1
                end

Matlab

            
                count = 0;
                while (number > 1)
                  number = number / 2; %  must "move" toward end of loop
                  count  = count + 1; 
                end

C, C++, or Java

            
                int count = 0;
                while (number > 1)
                {
                  number = number / 2;
                  count++; // the same as count = count + 1;
                }

Actionscript

            
                var count:int = 0;
                while (number > 1)
                {
                  number = number / 2;
                  count++; // the same as count = count + 1;
                }

Why While Loops?

Like all loops, "while loops" execute blocks of code over and over again.
The advantage to a while loop is that it will go (repeat) as often as necessary to accomplish its goal.

Generic Syntax:

            
  while ( condition is true )
    do something
    % Note: the "something" should eventually result
    % in the condition being false
  end

Infinite loops:

If the action inside the loop does not modify the variables being tested in the loops condition, the loop will "run" forever. For example:

            
  while ( y < 10 )
    x = x + 1;
  end

            
  while (  true )
    printf('hello');
  end

Example 1: How to assure proper input

Ask the user to input a value.
while the input is incorrect.
ask the user to input another value.
go back to line 2 (the while)

Matlab

            
% MATLAB
%
% Using a while loop to ask the user to input a number
% between 1 and 10 (inclusive).
%
%   Variables:
%        value  : variable to store the input 
%

value = input ('Please Enter a Number between 1 and 10 (1-10)');

while ( value < 1 || value > 10)

  fprintf('Incorrect input, please try again.\n');
  value = input ('Enter a Number between 1 and 10 (1-10)');

end % while

C

            
/*
 * C
 * Using a while loop to ask the user to input a number
 * between 1 and 10 (inclusive).
 *
 *   Variables:
 *        value  : variable to store the input 
 */
printf("Please Enter a Number between 1 and 10 (1-10): ");
scanf("%d", &value);

while ( value < 1 || value > 10)
  {

    printf("Incorrect input, please try again.\n");
    printf("Enter a Number between 1 and 10 (1-10): ");
    scanf("%d", &value);

  }

Design Pattern:

A design pattern is the syntax that you have to memorize in order to do well in programming and on tests.

The design pattern for a while loop is:

Matlab

            
            while ( some condition is true )
              % Do this code 
              % Something here should modify the condition above 
            end

C, Java, or Actionscript

            
          while ( some condition is true )
            {
              // Do this code 
              // Something here should modify the condition above 
            }

Back to Topics List

Kenneth Leroy Busbee

Overview

A while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The while loop can be thought of as a repeating if statement.[1]

Discussion

Introduction to Test Before Loops

There are two commonly used test before loops in the iteration (or repetition) category of control structures. They are: while and for. This module covers the: while.

Understanding Iteration in General – while

The concept of iteration is connected to possibly wanting to repeat an action. Like all control structures we ask a question to control the execution of the loop. The term loop comes from the circular looping motion that occurs when using flowcharting. The basic form of the while loop is as follows:

initialization of the flag 
while the answer to the question is true then do
    some statements or action
    some statements or action
    some statements or action
    update the flag

In most programming languages the question (called a test expression) is a Boolean expression. The Boolean data type has two values – true and false. Let’s rewrite the structure to consider this:

initialization of the flag 
while the expression is true then do
    some statements or action
    some statements or action
    some statements or action
    update the flag

Within the while control structure there are four attributes to a properly working loop. They are:

Initializing the flag
Test expression
Action or actions
Update of the flag

The initialization of the flag is not technically part of the control structure, but a necessary item to occur before the loop is started. The English phrasing is, “While the expression is true, do the following actions”. This is looping on the true. When the test expression is false, you stop the loop and go on with the next item in the program. Notice, because this is a test before loop the action might not happen. It is called a test before loop because the test comes before the action. It is also sometimes called a pre-test loop, meaning the test is pre (or Latin for before) the action and update.

Human Example of the while Loop

Consider the following one-way conversation from a mother to her child.

Child: The child says nothing, but mother knows the child had Cheerios for breakfast and history tells us that the child most likely spilled some Cheerios on the floor.

Mother says: “While it is true that you see (As long as you can see) a Cheerio on the floor, pick it up and put it in the garbage.”

Note: All of the elements are present to determine the action (or flow) that the child will be doing (in this case repeating). Because the question (can you see a Cheerios) has only two possible answers (true or false) the action will continue while there are Cheerios on the floor. Either the child 1) never picks up a Cheerio because they never spilled any or 2) picks up a Cheerio and keeps picking up Cheerios one at a time while he can see a Cheerio on the floor (that is until they are all picked up).

Infinite Loops

At this point, it is worth mentioning that good programming always provides for a method to ensure that the loop question will eventually be false so that the loop will stop executing and the program continues with the next line of code. However, if this does not happen, then the program is in an infinite loop. Infinite loops are a bad thing. Consider the following code:

Pseudocode infinite loop

loop_response = 'y'
While loop_response == 'y'
    Output "What is your age? "
    Input user_age
    Output "What is your friend's age? "
    Input friend_age
    Output "Together your ages add up to: "
    Output user_age + friend_age

The programmer assigned a value to the flag before the loop which is correct. However, they forgot to update the flag. Every time the test expression is asked it will always be true. Thus, an infinite loop because the programmer did not provide a way to exit the loop (he forgot to update the flag). Consider the following code:

loop_response = 'y';
While loop_response = 'y'
    Output "What is your age? "
    Input user_age
    Output "What is your friend's age? "
    Input friend_age
    Output "Together your ages add up to: "
    Output user_age + friend_age
    Output "Do you want to try again? y or n "
    Input loop_response

No matter what the user replies during the flag update, the test expression does not do a relational comparison but does an assignment. It assigns ‘y’ to the variable and asks if ‘y’ is true? Since all non-zero values are treated as representing true, the answer to the test expression is true. Viola, you have an infinite loop.

Counting Loops

The examples above are for an event controlled loop. The flag updating is an event where someone decides if they want the loop to execute again. Often the initialization sets the flag so that the loop will execute at least once.

Another common usage of the while loop is as a counting loop. Consider:

counter = 0
While counter < 5
    Output "I love ice cream!"
    counter += 1

The variable counter is said to be controlling the loop. It is set to zero (called initialization) before entering the while loop structure and as long as it is less than 5 (five); the loop action will be executed. But part of the loop action uses the increment operator to increase counter’s value by one. After executing the loop five times (once for counter’s values of: 0, 1, 2, 3 and 4) the expression will be false and the next line of code in the program will execute. A counting loop is designed to execute the action (which could be more than one statement) a set of given number of times. In our example, the message is displayed five times on the monitor. It is accomplished by making sure all four attributes of the while control structure are present and working properly. The attributes are:

Initializing the flag
Test expression
Action or actions
Update of the flag

Missing an attribute might cause an infinite loop or give undesired results (does not work properly).

Infinite Loops

Consider:

counter = 0;
while counter < 5
    Output "I love ice cream!"

Missing the flag update usually causes an infinite loop.

Variations on Counting

In the following example, the integer variable age is said to be controlling the loop (that is the flag). We can assume that age has a value provided earlier in the program. Because the while structure is a test before loop; it is possible that the person’s age is 0 (zero) and the first time we test the expression it will be false and the action part of the loop would never be executed.

While 0 < age
    Output "I love candy!"
    age -= 1

Consider the following variation assuming that age and counter are both integer data type and that age has a value:

counter = 0;
While counter < age
    Output "I love corn chips!"
    counter += 1

This loop is a counting loop similar to our first counting loop example. The only difference is instead of using a literal constant (in other words 5) in our expression, we used the variable age (and thus the value stored in age) to determine how many times to execute the loop. However, unlike our first counting loop example which will always execute exactly 5 times; it is possible that the person’s age is 0 (zero) and the first time we test the expression it will be false and the action part of the loop would never be executed.

Key Terms

counting controlledUsing a variable to count up or down to control a loop.event controlledUsing user input to control a loop.infinite loopA sequence of instructions which loops endlessly, either due to the loop having no terminating condition, having one that can never be met, or one that causes the loop to start over.[2]initialize itemAn attribute of iteration control structures.loop attributesItems associated with iteration or looping control structures.might not happenIndicating that test before loops might not execute the action.whileA test before iteration control structure.

References

cnx.org: Programming Fundamentals – A Modular Structured Approach using C++

Does the loop test its condition before or after it performs an iteration?

The condition is tested at the beginning of each iteration of the loop. If the condition is true ( non-zero ), then the body of the loop is executed next. If the condition is false ( zero ), then the body is not executed, and execution continues with the code following the loop.

Which loop can be test the condition first?

Exit Controlled Loop The loop body is executed first and then the given condition is checked in case If the test condition is false, the loop body will be executed at least once.

Is a do

The do-while Loop is a Posttest Loop This means it does not test its expression until it has completed an iteration. As a result, the do-while loop always performs at least one iteration, even if the expression is false to begin with.

What executes first in a do

The do-while loop is an exit-condition loop. This means that the body of the loop is always executed first. Then, the test condition is evaluated. If the test condition is TRUE, the program executes the body of the loop again.

Is a do Do-while vs while Condition while loop Do-while in Java While in C While loop