10

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?

Rob
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6 Answers6

13

"For" expresses your intentions more clearly

Functionally, your two examples are the same. But they express different intentions.

  • while means 'I don't know how long this condition will last, but as long as it does, do this thing.'
  • for means 'I have a specific number of repetitions for you to execute.'

You can use one when you mean the other, but it's harder to read the code.

Some other reasons why for is preferable here

  • It's more concise and puts all the information about the loop in one place
  • It makes $i a local variable for the loop

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.

Community
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Nathan Long
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11

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.

Adam Robinson
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  • So mostly it's a matter of preference? And standardization? – Rob Apr 23 '10 at 13:39
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    Which loop is preferred is a matter of what suits the code, not your personal preference. If you understand how both loops work, you'll come to understand where/when to use each type with experience. – Dolph Apr 23 '10 at 13:42
  • While loops are generally more prone to errors, so in general, in my code, I only do them when its a multiline initialization or something tricky that doesn't fit in a for loops declaration – DevelopingChris Apr 23 '10 at 13:43
  • @Rob: The `++` operator is the increment operator. `$i++` is semantically equivalent to `$i = $i + 1`. – Adam Robinson Apr 23 '10 at 15:00
  • Oh thanks Adam. That's nifty. Will $i++ ALWAYS be $i = $i + 1? – Rob Apr 23 '10 at 15:01
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    @Rob: Yes, though you can write it both as `$i++` and `++$i`. If it's a statement by itself, there's no behavioral difference. The only difference is if it's part of an expression, like `$j = $i++` and `$j = ++$i`. Putting the `++` at the end makes the increment happen *after* referencing the variable, so `$j` would have the value of `$i` *before* the increment. In the latter example (with `++` before the `$i`), `$j` will be given the value *after* the increment. In other words, after either statement is over, the first statement makes `$j` equal `$i-1` and the second makes `$j` equal to `$i`. – Adam Robinson Apr 23 '10 at 15:11
  • Now you lost me. If $j = $i++ ($i + 1) wouldn't the first statement make $j = $i, and the second statement make it equal $i + 1? – Rob Apr 23 '10 at 15:14
  • @Rob: I mean comparing the values *after the satements are done executing*. Remember that `$i` is being incremented by 1 in each statement. – Adam Robinson Apr 23 '10 at 15:17
  • Oh okay, I see what you're saying now. Thanks, this was really helpful. – Rob Apr 23 '10 at 15:24
  • @Rob: It's also worth noting that it appears that `++$i` is (slightly) faster than `$i++` (this is a PHP-only scenario) – Adam Robinson Apr 23 '10 at 15:26
  • By slightly faster, do you mean like the difference in speed between echo and print, or an actual real world necessity? – Rob Apr 23 '10 at 15:27
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    @Rob: Not a necessity, but given that it's just as easy to type `++$i` as it is to type `$i++`, why not? :) – Adam Robinson Apr 23 '10 at 15:54
6

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.

outis
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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.

gix
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1

The Main Difference Between for() and While() is that we have to define the limit or count but in while() loop we don't define limit or count it works until reached the last item

FOR LOOP Initialization may be either in loop statement or outside the loop. It is normally used when the number of iterations is known. Condition is a relational expression. It is used when initialization and increment is simple. for ( init ; condition ; iteration ) { statement(s); }

WHILE LOOP Initialization is always outside the loop. It is normally used when the number of iterations is unknown. Condition may be expression or non-zero value. It is used for complex initialization. while ( condition ) { statement(s); }

Engr Mazhar Mukhtar
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0

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.

amphetamachine
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  • So with a for loop, the variable will reset after the loop is complete? Or did I misunderstand that? – Rob Apr 23 '10 at 13:41
  • In many other languages *i* would not exist outside a for-loop, and trying to access it outside the loop would cause an error. In PHP however, *i* will be available in the function scope after it has been declared, no matter where. – Internet Friend Apr 23 '10 at 14:27
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    Pretend your loop variable doesn't exist outside of your for-loop. If you want to use $i again, be sure to assign it a new value before using it again (re-initialize it). Otherwise, you're teaching yourself a bad habbit that won't work at all in stricter languages (e.g. C or Java) as Internet Friend suggests above. This is the concept of "variable scope." – Dolph Apr 24 '10 at 10:07
  • @Rob - Not in PHP, but since how a particular hard-scoped language (like C or Java) interprets the variable initializer is pretty much up to the language, `for (int x = 5;;)` may or may not initialize `x` to within the `for` block. If you need the variable outside of the loop, it's safer to say `int x; for (x=5;;)` to make sure the variable is declared in a parent scope. – amphetamachine Apr 28 '10 at 17:22