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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.
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.
It a matter of precedence. Have a look at http://php.net/manual/en/language.operators.precedence.php
The behaviour of ++ is undefined when used in this style, as you don't know exactly when the ++ operation will occur and when the values will be "returned" from x++.
This is undefined behavior since i++ or ++i or --i or i-- do not increment/decrement in any particular order when passed as function parameter.
Not only that but if I'm not mistaken I believe that printf("%d", i++*i++); is just outputting 5*5 and then increments i twice.
remember ++i increments before operation, and i++ increments after operation. Consider this code:
int i, x = 5;
int i = x++; // i is now equal to 5 and x is equal to 6 because the increment happened after the = operation.
x = 5; //set x back to 5
i = ++x; //i is now equal to 6 and x is equal to 6 because the increment happened before the = operation.
This is the case for C I however cannot vouch for PHP.