Consider the following PHP Code:
//Method 1
$array = array(1,2,3,4,5);
foreach($array as $i=>$number){
$number++;
$array[$i] = $number;
}
print_r($array);
//Method 2
$array = array(1,2,3,4,5);
foreach($array as &$number){
$number++;
}
print_r($array);
Both methods accomplish the same task, one by assigning a reference and another by re-assigning based on key. I want to use good programming techniques in my work and I wonder which method is the better programming practice? Or is this one of those it doesn't really matter things?
Since the highest scoring answer states that the second method is better in every way, I feel compelled to post an answer here. True, looping by reference is more performant, but it isn't without risks/pitfalls.
Bottom line, as always: "Which is better X or Y", the only real answers you can get are:
It depends on what you're after/what you're doing
Oh, both are OK, if you know what you're doing
X is good for Such, Y is better for So
Don't forget about Z, and even then ...("which is better X, Y or Z" is the same question, so the same answers apply: it depends, both are ok if...)
Be that as it may, as Orangepill showed, the reference-approach offers better performance. In this case, the tradeoff one of performance vs code that is less error-prone, easier to read/maintan. In general, it's considered better to go for safer, more reliable, and more maintainable code:
'Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.' — Brian Kernighan
I guess that means the first method has to be considered best practice. But that doesn't mean the second approach should be avoided at all time, so what follows here are the downsides, pitfalls and quirks that you'll have to take into account when using a reference in a foreach loop:
Scope:
For a start, PHP isn't truly block-scoped like C(++), C#, Java, Perl or (with a bit of luck) ECMAScript6... That means that the $value variable will not be unset once the loop has finished. When looping by reference, this means a reference to the last value of whatever object/array you were iterating is floating around. The phrase "an accident waiting to happen" should spring to mind.
Consider what happens to $value, and subsequently $array, in the following code:
$array = range(1,10);
foreach($array as &$value)
{
$value++;
}
echo json_encode($array);
$value++;
echo json_encode($array);
$value = 'Some random value';
echo json_encode($array);
The output of this snippet will be:
[2,3,4,5,6,7,8,9,10,11]
[2,3,4,5,6,7,8,9,10,12]
[2,3,4,5,6,7,8,9,10,"Some random value"]
In other words, by reusing the $value variable (which references the last element in the array), you're actually manipulating the array itself. This makes for error-prone code, and difficult debugging. As opposed to:
$array = range(1,10);
$array[] = 'foobar';
foreach($array as $k => $v)
{
$array[$k]++;//increments foobar, to foobas!
if ($array[$k] === ($v +1))//$v + 1 yields 1 if $v === 'foobar'
{//so 'foobas' === 1 => false
$array[$k] = $v;//restore initial value: foobar
}
}
Maintainability/idiot-proofness:
Of course, you might say that the dangling reference is an easy fix, and you'd be right:
foreach($array as &$value)
{
$value++;
}
unset($value);
But after you've written your first 100 loops with references, do you honestly believe you won't have forgotten to unset a single reference? Of course not! It's so uncommon to unset variables that have been used in a loop (we assume the GC will take care of it for us), so most of the time, you don't bother. When references are involved, this is a source of frustration, mysterious bug-reports, or traveling values, where you're using complex nested loops, possibly with multiple references... The horror, the horror.
Besides, as time passes, who's to say that the next person working on your code won't foget about unset? Who knows, he might not even know about references, or see your numerous unset calls and deem them redundant, a sign of your being paranoid, and delete them all together. Comments alone won't help you: they need to be read, and everyone working with your code should be thoroughly briefed, perhaps have them read a full article on the subject. The examples listed in the linked article are bad, but I've seen worse, still:
foreach($nestedArr as &$array)
{
if (count($array)%2 === 0)
{
foreach($array as &$value)
{//pointless, but you get the idea...
$value = array($value, 'Part of even-length array');
}
//$value now references the last index of $array
}
else
{
$value = array_pop($array);//assigns new value to var that might be a reference!
$value = is_numeric($value) ? $value/2 : null;
array_push($array, $value);//congrats, X-references ==> traveling value!
}
}
This is a simple example of a traveling value problem. I did not make this up, BTW, I've come across code that boils down to this... honestly. Quite apart from spotting the bug, and understanding the code (which has been made more difficult by the references), it's still quite obvious in this example, mainly because it's a mere 15 lines long, even using the spacious Allman coding style... Now imagine this basic construct being used in code that actually does something even slightly more complex, and meaningful. Good luck debugging that.
side-effects:
It's often said that functions shouldn't have side-effects, because side-effects are (rightfully) considered to be code-smell. Though foreach is a language construct, and not a function, in your example, the same mindset should apply. When using too many references, you're being too clever for your own good, and might find yourself having to step through a loop, just to know what is being referenced by what variable, and when.
The first method hasn't got this problem: you have the key, so you know where you are in the array. What's more, with the first method, you can perform any number of operations on the value, without changing the original value in the array (no side-effects):
function recursiveFunc($n, $max = 10)
{
if (--$max)
{
return $n === 1 ? 10-$max : recursiveFunc($n%2 ? ($n*3)+1 : $n/2, $max);
}
return null;
}
$array = range(10,20);
foreach($array as $k => $v)
{
$v = recursiveFunc($v);//reassigning $v here
if ($v !== null)
{
$array[$k] = $v;//only now, will the actual array change
}
}
echo json_encode($array);
This generates the output:
[7,11,12,13,14,15,5,17,18,19,8]
As you can see, the first, seventh and tenth elements have been altered, the others haven't. If we were to rewrite this code using a loop by reference, the loop looks a lot smaller, but the output will be different (we have a side-effect):
$array = range(10,20);
foreach($array as &$v)
{
$v = recursiveFunc($v);//Changes the original array...
//granted, if your version permits it, you'd probably do:
$v = recursiveFunc($v) ?: $v;
}
echo json_encode($array);
//[7,null,null,null,null,null,5,null,null,null,8]
To counter this, we'll either have to create a temporary variable, or call the function tiwce, or add a key, and recalculate the initial value of $v, but that's just plain stupid (that's adding complexity to fix what shouldn't be broken):
foreach($array as &$v)
{
$temp = recursiveFunc($v);//creating copy here, anyway
$v = $temp ? $temp : $v;//assignment doesn't require the lookup, though
}
//or:
foreach($array as &$v)
{
$v = recursiveFunc($v) ? recursiveFunc($v) : $v;//2 calls === twice the overhead!
}
//or
$base = reset($array);//get the base value
foreach($array as $k => &$v)
{//silly combine both methods to fix what needn't be a problem to begin with
$v = recursiveFunc($v);
if ($v === 0)
{
$v = $base + $k;
}
}
Anyway, adding branches, temp variables and what have you, rather defeats the point. For one, it introduces extra overhead which will eat away at the performance benefits references gave you in the first place.
If you have to add logic to a loop, to fix something that shouldn't need fixing, you should step back, and think about what tools you're using. 9/10 times, you chose the wrong tool for the job.
The last thing that, to me at least, is a compelling argument for the first method is simple: readability. The reference-operator (&) is easily overlooked if you're doing some quick fixes, or try to add functionality. You could be creating bugs in the code that was working just fine. What's more: because it was working fine, you might not test the existing functionality as thoroughly because there were no known issues.
Discovering a bug that went into production, because of your overlooking an operator might sound silly, but you wouldn't be the first to have encountered this.
Note:
Passing by reference at call-time has been removed since 5.4. Be weary of features/functionality that is subject to changes. a standard iteration of an array hasn't changed in years. I guess it's what you could call "proven technology". It does what it says on the tin, and is the safer way of doing things. So what if it's slower? If speed is an issue, you can optimize your code, and introduce references to your loops then.
When writing new code, go for the easy-to-read, most failsafe option. Optimization can (and indeed should) wait until everything's tried and tested.
And as always: premature optimization is the root of all evil. And Choose the right tool for the job, not because it's new and shiny.
As far as performance is concerned Method 2 is better, especially if you either have a large array and/or are using string keys.
While both methods use the same amount of memory the first method requires the array to be searched, even though this search is done by a index the lookup has some overhead.
Given this test script:
$array = range(1, 1000000);
$start = microtime(true);
foreach($array as $k => $v){
$array[$k] = $v+1;
}
echo "Method 1: ".((microtime(true)-$start));
echo "\n";
$start = microtime(true);
foreach($array as $k => &$v){
$v+=1;
}
echo "Method 2: ".((microtime(true)-$start));
The average output is
Method 1: 0.72429609298706
Method 2: 0.22671484947205
If I scale back the test to only run ten times instead of 1 million I get results like
Method 1: 3.504753112793E-5
Method 2: 1.2874603271484E-5
With string keys the performance difference is more pronounced.
So running.
$array = array();
for($x = 0; $x<1000000; $x++){
$array["num".$x] = $x+1;
}
$start = microtime(true);
foreach($array as $k => $v){
$array[$k] = $v+1;
}
echo "Method 1: ".((microtime(true)-$start));
echo "\n";
$start = microtime(true);
foreach($array as $k => &$v){
$v+=1;
}
echo "Method 2: ".((microtime(true)-$start));
Yields performance like
Method 1: 0.90371179580688
Method 2: 0.2799870967865
This is because searching by string key has more overhead then the array index.
It is also worth noting that as suggested in Elias Van Ootegem's Answer to properly clean up after yourself you should unset the reference after the loop has completed. I.e. unset($v); And the performance gains should be measured against the loss in readability.
There are some minor performance differences, but they aren't going to have any significant effect.
I would choose the first option for two reasons:
It's more readable. This is a bit of a personal preference, but at first glance, it's not immediately obvious to me that $number++ is updating the array. By explicitly using something like $array[$i]++, it's much clearer, and less likely to cause confusion when you come back to this code in a year.
It doesn't leave you with a dangling reference to the last item in the array. Consider this code:
$array = array(1,2,3,4,5);
foreach($array as &$number){
$number++;
}
// ... some time later in an unrelated section of code
$number = intval("100");
// now unexpectedly, $array[4] == 100 instead of 6
I guess that depends. Do you care more about code readability/maintainability or minimizing memory usage. The second method would use slightly less memory, but I would honestly prefere the first usage, as assigned by reference in foreach definition does not seem to be commonplace practice in PHP.
Personally if I wanted to modify an array in place like this I would go with a third option:
array_walk($array, function(&$value) {
$value++;
});
The first method will be insignificantly slower, because each time it will go through the loop, it will assign a new value to the $number variable. The second method uses the variable directly so it doesn't need to assign a new value for each loop.
But, as I said, the difference is not significant, the main thing to consider is readability.
In my opinion, the first method makes more sense when you don't need to modify the value in the loop, the $number variable would only be read.
The second method makes more sense when you need to modify the $number variable often, as you don't need to repeat the key each time you want to modify it, and it is more readable.
Have you considered array_map? It is designed to change values inside arrays.
$array = array(1,2,3,4,5);
$new = array_map(function($number){
return $number++ ;
}, $array) ;
var_dump($new) ;
I'd choose #2, but it's a personal preference.
I disagree with the other answers, using references to array items in foreach loops is quite common, but it depends on the framework you're using. As always, try to follow existing coding conventions in your project or framework.
I also disagree with the other answers that suggest array_map or array_walk. These introduce the overhead of a function call for each array element. For small arrays, this won't be significant, but for large arrays, this will add a significant overhead for such a simple function. However, they are appropriate if you're performing more significant calculations or actions - you'll need to decide which to use depending on the scenario, perhaps by benchmarking.
Most of the answers interpreted your question to be about performance.
This is not what you asked. What you asked is:
I wonder which method is the better programming practice?
As you said, both do the same thing. Both work. In the end, better is often a matter of opinion.
Or is this one of those it doesn't really matter things?
I wouldn't go so far as to say it doesn't matter. As you can see there can be performance considerations for Method 1 and reference gotchas for Method 2.
I can say what matters more is readability and consistency. While there are dozens of ways to increment array elements in PHP, some look like line noise or code golf.
Ensuring your code is readable to future developers and you consistently apply your method of solving problems is a far better macro programming practice than whatever micro differences exist in this foreach code.
I'm trying to loop through an associative array with the help of the functions current(), next() and reset(). The first two functions work great for me but when I want to loop through it again and use the reset() function it won't work.
Here's the code:
while ($availability_per_date = mysql_fetch_assoc($availability)) {
//it won't go in to the loop below a second time
while (current($room_types_available)) {
$key= key($room_types_available);
if ($availability_per_date["{$key}"] == 0) {
$room_types_available["{$key}"] = 0;
}
echo $key;
next($room_types_available);
}
reset($room_types_available);
}
First off, try to use built-in functions that can easily work better with your code, here's an example:
while ($availability_per_date = mysql_fetch_assoc($availability)) {
//it won't go in to the loop below a second time
foreach($room_types_available as $key=>$value){
if ($availability_per_date["{$key}"] == 0) {
$room_types_available["{$key}"] = 0;
}
echo $key;
}
}
If it gives any bugs with your app, post it and we'll fix :)
Is it possible that the return of current($room_types_available) the second time through returns a value that casts to false?
Using the each() function is a good way to solve the problem, it avoids ambiguity on false.
Not to copy on someone else's answer, but preinheimer is correct.
In the first iteration of the loop, you are setting a number of values to false (the string of "0" is false in PHP). While is then detecting these in subsequent calls and then terminating prematurely (because current, in this case, is returning false). Your two options are using each (as suggested by preinheimer) or foreach instead of while (as suggested by Khez).
Personally, (as I stated in the comments above), I view foreach as far more intuitive and therefore better practice, but neither of the two are functionally incorrect.
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?
"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.
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.
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.
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.
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); }
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.