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In article <>,
Henry Law <> wrote:
>I'm checking parameters to a little utility I'm writing. It uses
>Getopt::Std, which returns the parameters in a hash %opts.
>
>The logic of the utility requires both flags -h and -p to be specified
>or neither. I'm coding the part that checks whether that is true.
>
>I'm sure there's a neater way than this (which does work, admittedly):
>
>die "Flags -h and -p must be specified together\n"
> if (exists $opts{p} && !exists $opts{h}) || (exists $opts{h} &&
>!exists $opts{p});

if exists $opts{p} == exists $opts{h}

--
Tim McDaniel,

In article <k1g6rk$ct1$>,
Tim McDaniel <> wrote:
>In article <>,
>Henry Law <> wrote:
>>I'm checking parameters to a little utility I'm writing. It uses
>>Getopt::Std, which returns the parameters in a hash %opts.
>>
>>The logic of the utility requires both flags -h and -p to be specified
>>or neither. I'm coding the part that checks whether that is true.
>>
>>I'm sure there's a neater way than this (which does work, admittedly):
>>
>>die "Flags -h and -p must be specified together\n"
>> if (exists $opts{p} && !exists $opts{h}) || (exists $opts{h} &&
>>!exists $opts{p});
>
> if exists $opts{p} == exists $opts{h}

To expand on that,

IF you have Perl boolean values, by which I mean there are only two
possible values, where one evaluates to true and one evaluates to
false (and I believe that exists fulfils that),

THEN != is the exclusive-or function: it is true if an only if exactly
one of its operands is true.

And therefore == is the inverse: true if and only if both operands are
true or both operands are false.

You may not have such a boolean. For example, you might have a
function returning a number, and you want to just check that both are
zero or both are non-zero. You can't do
somefunc($x) == somefunc($y)
because maybe that's (for example)
12 == 27
which is incorrect.

So !! is Perl's "convert to boolean" operator:
!!(something that evaluates to false) -> ''
!!(something that evaluates to true) -> 1

So the more general way to do exclusive or for values that are simply
either Perl true or Perl false, but might be any true or false value,
!!val1 != !!val2
and so the general "both or neither" is
!!val1 == !!val2

--
Tim McDaniel,

In article <bvrsg9->,
Ben Morrow <> wrote:
>
>Quoth :
>> >
>> > if exists $opts{p} == exists $opts{h}
>>
>> To expand on that,
>>
>> IF you have Perl boolean values, by which I mean there are only two
>> possible values, where one evaluates to true and one evaluates to
>> false

which was silly. Sorry. For != as exclusive or and similar ==, you
just need two distinct values that can be compared numerically. A
true number and 0 fulfil the condition, but 17 and 42 would work too.
I was just trying to express that you can't have more than two
values.

>> So !! is Perl's "convert to boolean" operator:
>> !!(something that evaluates to false) -> ''
>> !!(something that evaluates to true) -> 1
>
>It's worth being clear that !! is not an operator in its own right, but
>simply two instances of !. I'm sure you know this, but I have seen
>people confused about it in the past.

Quite so. Sorry for being confusing.

>Or, you know, use exclusive or? For some reason I have never been able
>to work out there is no ^^ operator, so you have to use 'xor':

I tend not to remember "and", "or", and "xor" because they came in
well after I started Perling, and there is no ^^. Of course, you need
to remember that "and", "or", and "xor" have very low precedence, so
while you don't need parens in
$both = $a && $b;
$either = !!$a == !!$b
you do need parens in
$both = ($a and $b);
$either = ($a xor $b);

--
Tim McDaniel,