6.3: while Loops

A while loop is called a conditional loop because it repeats as long as a condition is true. The loop does not necessarily know ahead of time how many times it will repeat. Instead, MATLAB checks the condition before each repetition.

General Format of a while Loop 

while condition
    statements to repeat
end

The condition is usually a logical expression. MATLAB evaluates the condition. If it is true, MATLAB executes the statements inside the loop. Then MATLAB goes back to the top of the loop and checks the condition again. The loop stops when the condition becomes false. A while loop must include some action that can eventually make the condition false. Otherwise, the loop may run forever. This is called an infinite loop.

 

Example: Creating a Vector of Positive Numbers

Suppose we want to ask the user to enter positive numbers one at a time. As long as the user enters a positive number, we add that number to a vector. When the user enters a non-positive number, such as 0 or a negative number, the loop stops.

 

% positiveNumbers.m
% This script asks the user to enter positive numbers.
% The loop stops when the user enters 0 or a negative number.

clc; clear;

inputNum = input('Enter a positive number: ');
posNums = [];

while inputNum > 0
    posNums = [posNums inputNum];
    inputNum = input('Enter a positive number: ');
end

fprintf('Here is your array: [');
fprintf('%4.1f', posNums);
fprintf(' ]');

Let us walk through this code carefully. First, we ask the user to enter a number. Then we create an empty vector called posNums. The while loop checks whether inputNum is greater than zero. If it is, that number is added to the end of the vector.

Inside the loop, we ask the user for another number. This step is very important. Without it, inputNum would never change, and if the first number were positive, the loop would run forever.

Caution

Common Beginner Mistake

Forgetting to update the condition variable inside a while loop is one of the most common causes of infinite loops.

 

How MATLAB Evaluates a while Loop

The basic logic of a while loop is:

1.       Check the condition.

2.       If the condition is true, run the loop body.

3.       Return to the top of the loop.

4.       Check the condition again.

5.       Stop when the condition is false.

This means the loop body may run many times, one time, or not at all. If the condition is false the first time MATLAB checks it, the loop body is skipped completely.

Exercise \(\PageIndex{1}\)

What happens if the first number entered by the user in the previous example is -5? Does the loop run at all?

Answer

The loop does not run at all because the condition inputNum > 0 is false immediately. The vector posNums remains empty.