Programming Fundamentals/While Loop

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.

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" loop.

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 between a parent and their child.

It's just after breakfast and the parent knows their child had Cheerios. Past experience has taught them that their child usually spills Cheerios on the floor.

The parent tells their child: “While it is true that there is a Cheerio on the floor, pick it up and put it in the garbage.”

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 (Is there a Cheerio on the floor?) has only two possible answers (true or false), the action will continue while there are Cheerios on the floor. Therefore, 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 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 and will not stop until the program is terminated externally or the computer is restarted. 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, it's 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 controlled

Using a variable to count up or down to control a loop.

infinite loop

A 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.

initialize item

An attribute of iteration control structures.

loop attributes

Items associated with iteration or looping control structures.

might not happen

Indicating that test before loops might not execute the action.

while

A test before iteration control structure.

References

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

Programming Fundamentals