Quiz query, $d.$d--=20?

Question

$d = 5;
echo $d*$d--;

The answer I believed was 25 but apparently it is 20. To try debugging, I did:

echo $d.$d--;

And I get 45.

5*4 alone is confusing for post decrement. Why is it 4*5? Can someone explain? Thanks!

Answer 1

https://www.php.net/manual/en/language.operators.precedence.php#example-96

Answer 2

Ipang you did it right, it took me a while too, to figure that out in C

Answer 3

Ipang maybe test em all the time before use, in a different language, with time you won't need that again

Answer 4

This is really odd, we commonly understood postfix operator doesn't change the operand's value at least until the next statement line was executed. It's funny how PHP treats postfix & prefix operator the same. I'm looking forward to hearing something about this odd behaviour.

$d = 5;
echo '$d * $d++ = ' . $d * $d++ . '<br>'; // output: 30
$d = 5;
echo '$d * ++$d = ' . $d * ++$d . '<br>'; // output: 36
?>

Answer 5

Ipang I think it's working from right to left just like in C when you put such in a printf function
⚡Prometheus ⚡ this is probably the case

Answer 6

So I did a little test on different languages, and found that this operator associativity is shared amongst C, C++ and PHP. Java, C# and JavaScript seems to have different way in determining associativity and thus different output. I guess we just need to be more careful with this difference.

Answer 7

*AsterisK*
Out of curiosity I tested printing the results of the following expressions in several languages (in Code Playground) and I got different results:

* Note before every evaluation <d> = 5

1. (d * d++)
C, C++, PHP => 30
Java, C#, JS => 25

2. (d * ++d)
C, C++, PHP => 36
Java, C#, JS => 30

3. (d * d--)
C, C++, PHP => 20
Java, C#, JS => 25

4. (d * --d)
C, C++, PHP => 16
Java, C#, JS => 20

I seem to get the impression that C, C++ and PHP does associativity evaluation differently with Java, C# and JavaScript. Or maybe I wasn't doing it right that the output differs.

Answer 8

*AsterisK*
I'm having difficulty when it comes to evaluating expressions, especially now that I'm aware of this various operator associativity evaluation from different languages, wish there was a way to easily remember which language does it how 😁

Answer 9

*AsterisK*
Good idea, I'll keep a note on this for future. Thanks mate 👍