Question

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Could anyone explain these undefined behaviors (i = i++ + ++i , i = i++, etc…)

I have encountered with a strange problem regards increment operator.

I get different output of same expression in PHP and C.

In C language

main()
{
    int i = 5;
    printf("%d", i++*i++); // output 25;
}

In PHP

$i = 5;
echo $i++*$i++; // output 30

Can anyone explain this strange behavior? Thanks.

Answer 1

In C the result is undefined because either of the two operands could be evaluated first, thus reading it a second time is erroneous.

And, well, in PHP I wouldn’t be surprised if the result was 42 pending some changes to php.ini.