PHP Question: While And Do While Looping

16th May 2011 - 2 minutes read time

Question

What does the $count variable equal after each of these loops?

// Loop 1 - while
$count = 0;

while ($count < 0) {
    ++$count;
}
// Loop 2 - do while
$count = 0;

do {
    ++$count;
} while  ($count < 0);









 

Answer

The $count variable will equal 0 after loop 1 and 1 after loop 2. This is because when a while loop is run the condition is looked at before the first iteration, exiting if it equals false. Conversely, a do while loop will run the condition after the first iteration. This means that a do while loop will always run at least once, even if the condition equals false.

In the above code the first loop will not run because the result of the comparison is false. The second loop will run once, incrementing the $count value by 1, and then exit after testing the comparison

This is is an important distinction between how while and do while loops work and can be useful in certain circumstances. For example, a while loop is a handy way of replacing an if statement and a for loop with a single while statement.

If the condition initially equals true then both loops will work in the same way. For example, if we made both loop conditions exit when the count is greater than 10, and run the code again, the output of both loops would equal 10.

PHP

Comments

The first loop will return 0 and the second one as 1. reason being, while, first checks the condition and fails and do while, first executes it and then checks the condition. 

Submitted by Sirish on Tue, 05/17/2011 - 21:57

Permalink

I've got a question about the do while order, what will happen if you rotate it to while do? I am sorry if it sounds like a stupid question I admit I am a beginner and really would like to learn, thank you.

Submitted by Daniel on Tue, 04/13/2021 - 09:06

Permalink

Hi Daniel,

It's fine to ask questions, especially if you're learning!

So I'm assuming you mean converting the structure to this:

while ($count < 0) {
  ++$count;
} do

This results in a syntax error as PHP doesn't know what to do with the "do" at the end of the statement. In PHP there is a while loop and a do-while loop, but not a while-do loop.

The question in this post is meant to show the differences between while and do-while, showing that the code in the do-while loop is run at least once, even if the outcome of the while check is false.

I hope that helps a little?

Philip Norton

Submitted by philipnorton42 on Tue, 04/13/2021 - 11:05

Permalink

Add new comment

The content of this field is kept private and will not be shown publicly.
CAPTCHA
This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.
4 + 0 =
Solve this simple math problem and enter the result. E.g. for 1+3, enter 4.

Related Content

PHP:CSI - To Switch, Or Not To Switch?

11th September 2022

I was writing unit tests for a API mapping function recently and came across this interesting issue. The code I was writing tests for was in a legacy codebase that I was making changes to, and it made sense to have some unit tests in there before I started work to ensure everything worked before and after.

Read more

Using PSR-4 With Composer

28th August 2022

The PHP Standards Recommendations (called PSR) are a set of standards that aim to make certain aspects of working with PHP easier.

Read more

PHP:CSI - Date Is Less Than One Month Ago

27th February 2022

Working with logic surrounding dates can sometimes be difficult and it's fairly common to come across really subtle date and time based bugs.

I was recently shown a bug in a PHP application that looks like it should be working at face value, but doesn't actually produce the correct result.

Read more