CODEWITHSUNDEEP
php
user890748
user890748

Reputation: 13

Why does PHP post increment not behave as expected when in an array [ ] operation?

My Question is why does the post increment not function on the first element but then works as expected when it is contained within an array [ ] element ?

Here is a snippet of what I was doing that behaves unexpectedly:

When I use this code ...

$lessonIndex = 0;
$EventArray[$lessonIndex]['Title'] = 'Lesson 1';
$EventArray[$lessonIndex]['Start'] = $TimeArray[$EventArray[$lessonIndex++]['Title']]['Start'];

$EventArray[$lessonIndex]['Title'] = 'Lesson 2';
$EventArray[$lessonIndex]['Start'] = $TimeArray[$EventArray[$lessonIndex++]['Title']]['Start'];

$EventArray[$lessonIndex]['Title'] = 'Lesson 3';
$EventArray[$lessonIndex]['Start'] = $TimeArray[$EventArray[$lessonIndex++]['Title']]['Start'];

The $EventArray[$lessonIndex] contents will be skip index 0 then index 1 will get what was destined for index 0 and the increment proceeds as expected through the rest of the code, but with the results being off by one.

If I do this instead:

$lessonIndex = 0;
$EventArray[$lessonIndex]['Title'] = 'Lesson 1';
$EventArray[$lessonIndex]['Start'] = $TimeArray[$EventArray[$lessonIndex]['Title']]['Start'];
$lessonIndex++;

$EventArray[$lessonIndex]['Title'] = 'Lesson 2';
$EventArray[$lessonIndex]['Start'] = $TimeArray[$EventArray[$lessonIndex]['Title']]['Start'];
$lessonIndex++;

$EventArray[$lessonIndex]['Title'] = 'Lesson 3';
$EventArray[$lessonIndex]['Start'] = $TimeArray[$EventArray[$lessonIndex]['Title']]['Start'];
$lessonIndex++;

Everything works as expected.

Upvotes: 1

Views: 312

Answers (3)

Konerak
Konerak

Reputation: 39773

You should not use a variable that gets incremented more than once in a line, else you risk this kind of unexpected behaviour.

$EventArray[$lessonIndex]['Start'] = $TimeArray[$EventArray[$lessonIndex++]['Title']]['Start'];

What happens is that

  1. First the rvalue (The value right of the equals sign =) gets evaluated.
  2. After that evaluation, $lessonIndex will get incremented.
  3. Only then will the lvalue be fetched, but by that time $lessonIndex has already been incremented.

So this would be equal to writing

$lessonIndex = 0;
$EventArray[$lessonIndex]['Title'] = 'Lesson 1';
$somevalue = $TimeArray[$EventArray[$lessonIndex]['Title']]['Start'];
$lessonIndex++;
$EventArray[$lessonIndex]['Start'] = $somevalue;

Upvotes: 1

Marc B
Marc B

Reputation: 360922

PHP has order of operations. The ++ is evaluated BEFORE PHP calculates what array index you're referring to. So you increment the value, THEN that new value is returned and used as the array index.

$a = 0;
$x = $arr[$a++];

is functionally identical to

$a = 0;
$a++;
$x = $arr[$a]; // aka $arr[1];

Upvotes: 0

George Cummins
George Cummins

Reputation: 28936

In your first example, the ++ operator incremented the value, then sent the result as the array index.

In your second example, the value was sent as the array index, then incremented.

Upvotes: 1

Related Questions