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Asked: 2026-05-25T02:15:19+00:00

My Question is why does the post increment not function on the first element

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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.

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1 Answer

  1. Editorial Team

    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;
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