From perlop:

Note that just as in C, Perl doesn't define when the variable is incremented or decremented. You just know it will be done sometime before or after the value is returned. This also means that modifying a variable twice in the same statement will lead to undefined behaviour. Avoid statements like:

$i = $i ++;
print ++ $i + $i ++;
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Perl will not guarantee what the result of the above statements is.

"Undefined behaviour" means "anything could happen".

This explanation is very close. 'C' is very well defined about what is does in terms of statements with lvalues. 'C' is also explicitly ambiguous about what it does with subroutine parameters - in subroutine call, there is NO defined order of evaluation of expressions in the sub call: sub a(expr1, expr2)! C can do expr1 first or expr2 first. Often in C expr2 will be done first because that is the first value that will be pushed onto the calling stack. But there is nothing that mandates that or prohibits a different order. I seem to remember from a discussion with a higher Monk (Ikegami) that Perl always goes left to right, even in a sub arg evaluation, but I also seem to remember that this is not guaranteed by the language spec and therefore I wouldn't count on it.

I have to mention that $i=$i++ or in C i=i++ is nonsensical. "i" gets assigned back to "i" and then "i" gets incremented and that value is "thrown away".

In my opinion, a lexically scoped $var is a very cheap thing and should be used if there is any doubt about some subsequent statement. I'm not advocating creating extra unnecessary vars, just ones that "add clarity".

In my opinion, a lexically scoped $var is a very cheap thing and should be used if there is any doubt about some subsequent statement. I'm not advocating creating extra unnecessary vars, just ones that "add clarity".

I agree with you. I had posted this query because I thought about the expression behaving in a certain manner but it didn't. So it was pure curiosity. Thanks for that addition. I too wouldn't use such ambiguity in my work.

Officially, the behaviour is undefined. The following documents what actually happens. It boils down to the fact that pre-increments and pre-decrements leave the input variable on the stack while post-increments and post-decrements leave a copy of it.

$a++ + ++$a
                                  $a  Stack
1. Push copy of $a on stack       10  10
2. Increment $a                   11  10
3. Increment $a                   12  10
4. Push $a on stack               12  10,$a
5. Add                            12  22
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++$a + $a++
                                  $a  Stack
1. Increment $a                   11
2. Push $a on stack               11  $a
3. Push copy of $a on stack       11  $a,11
4. Increment $a                   12  $a,11
5. Add                            12  23
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Note: These steps don't correspond one-for-one to the ops actually involved for the sake of simplicity.

                                  $a  Stack
1. Push copy of $a on stack       10  10
2. Increment $a                   11  10
3. Increment $a                   12  10
4. Push $a on stack               12  10,$a
5. Add                            12  22
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++$a + $a++
                                  $a  Stack
1. Increment $a                   11
2. Push $a on stack               11  $a
3. Push copy of $a on stack       11  $a,11
4. Increment $a                   12  $a,11
5. Add                            12  23
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Thanks for a very clear explanation. I appreciate that.

Looks like your answer was very well covered by the other responders. Mine is just a nit since I think I know what you meant; but you said in your post:

"It should be 21 instead of 20 because..."

Did you mean "...11 instead of 10..." since you initialize your variable to 10 rather than to 20?

Did you mean "...11 instead of 10..." since you initialize your variable to 10 rather than to 20?

I was talking about the answer of the operation $a++ + $a--. But yes, as per your observation.the value of $a should be 11 instead of 10.

What should be the output of

perl -le '$a = 10; $a++ + $a--'
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Now, if you mean, "what is the value of $b after $a = 10; $b = $a++ + $a--?", then I suggest you RTFM instead of starting the same heated debate which runs through Perlmonks a few times a year.

Just RTFM. Then, if any question remains, come back.

@JavaFan - I meant -

perl -le '$a = 10; print $a++ + $a--'
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Sorry for missing out on the print statement.

Thanks davorg, wfsp for pointing to perlop and the Autoincrement magic link. I guess as it is undefined behavior I will leave it at that.