2009/9/11 John W. Krahn <jwkr...@shaw.ca>:
> Philip Potter wrote:
>> Does ?: guarantee that only one arm of its conditions will be executed?
>> eg:
>>
>> # @_ has 3 elements
>>
>> $x = $flag ? shift : push;
>
> $ perl -e'$x = $flag ? shift : push;'
> Not enough arguments for push at -e line 1, near "push;"
> Execution of -e aborted due to compilation errors.
>
> Perhaps you meant:
>
> $x = $flag ? shift : pop;
I did indeed, thanks.
>> I ask because this is the behaviour I expect from C and C++, it's very
>> important (Steve's original ? shift : {} is broken without it), but
>> it's not explicitly stated in perldoc perlop. perlop *does* explain
>> the short-circuit behaviour of qw(|| && // or and) and their use for
>> control flow, but is silent on ?:'s control flow behaviour. Is this
>> documented in perldoc, and if so where?
>
> Operators borrowed from C keep the same precedence
> relationship with each other, even where C’s precedence is
> slightly screwy. (This makes learning Perl easier for C folks.)
>
>
> Conditional Operator
>
> Ternary "?:" is the conditional operator, just as in C. It works
> much like an if-then-else. If the argument before the ? is true,
> the argument before the : is returned, otherwise the argument
> after the : is returned.
That language doesn't guarantee that the other side won't be
evaluated, just that it won't be returned. Compare the language in the
C draft standard, n1256 6.5.15p4:
"The first operand is evaluated; there is a sequence point after its
evaluation. The second
operand is evaluated only if the first compares unequal to 0; the
third operand is evaluated
only if the first compares equal to 0; the result is the value of the
second or third operand
(whichever is evaluated), converted to the type described below."
Here, it's very explicit wrt evaluation as well as return value.
I'm going to drop this issue now, because I've made my point and we've
strayed beyond perl-beginners topicality.
Philip
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