Differences between a while loop and a for loop in PHP?

Question

I'm reading an ebook on PHP right now, and the author noted that the difference between a while loop and a for loop is that the for loop will count how many times it runs.

So take this:

<?php
    for ($i = 1; $i < 10; $i = $i + 1) {
        print "Number $i\n";
    }
?> 

But wouldn't this be the same as

<?php
    $i = 1;
        while ($i < 10) {
            $i = $i + 1;
            print "Number $i\n";
        }
?>

Or is there some other differences that he didn't point out? (Aside from using while loop for when you're unsure of how long the condition will remain true, such as selecting rows from a database)

I mean, if that's the only difference, can't I just not use the for loop and use the while loop instead?

Answer 1

Can you? Yes, certainly. But whether or not you should is an entirely different question.

The for loop is more readable in this scenario, and is definitely the convention you'll find used within virtually every language that has looping directives. If you use the while loop, people are going to wonder why you didn't use a for loop.

Answer 2

It's a matter of taste, personal preference and readability. Sometimes a while loop works better logically. Sometimes, a for.

For my personal rule, if I don't need a variable initializer, then I use a while.

But a foreach loop is useful in its own way.

Plus, in the case of PHP's scoping, where all variables not inside of functions are global, it the variable will continue living after the loop no matter which loop control you use.

Answer 3

Functionally, a for loop is equivalent to a while loop; that is, each can be rewritten as the other with no change to the outcome or side effects. However, each has different connotations. A while loop runs while a condition holds; the condition is static, though circumstances change. A for loop runs over a sequence. The difference is important to programmers but not programs, just as choice of variables names are important to programmers even though they can be changed to produce functionally equivalent code. One loop construct will make more sense than the other, depending on the situation.

Answer 4

A for-loop

for (INIT; CONDITIONS; UPDATE) {
    BODY
}

is basically the same as a while-loop structured like this:

INIT
while (CONDITIONS) {
    BODY
    UPDATE
}

While you could technically use one or the other, there are situations where while works better than for and vice-versa.

Answer 5

Functionally, your two examples are the same. The for for loop is preferable because:

1. It's more concise and puts all the information about the loop in one place
2. It makes $i a local variable for the loop
3. It's the more common way of iterating a set number of times, and therefore makes more intuitive sense to other programmers

Semantics

The third point is really what most people are reacting to, I think.

Conceptually, while means 'I don't know how long this condition will last, but as long as it does, do this thing.'

On the other hand, for means 'I have a specific number of repetitions for you to execute.'

So it's mostly a matter of semantics. Just like in HTML, you can use a <span> like a <p> if you style it like one, but it makes less sense.

Don't forget foreach

Personally, the loop I use most often in PHP is foreach. If you find yourself doing things like this:

for ($i=0; $i < count($some_array); $i++){
  echo $some_array[$i];
}

...then try this:

foreach ($some_array as $item){
   echo $item;
}

Faster to type, easier to read.