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Hello all,

please help me the check the following code

use strict;

my $something = 'phal';
my $otherthing = 'paul';

my $note_whom = 'szmuzu\'mi';

defined($something) ? $note_whom = $something : $otherthing =
'singapore';

print "$note_whom \n $otherthing \n";

The result is: singapore
singapore

I don't know why the scalar $note_whom get the value that is assigned
to $otherthing.

Please give me the explaination, and thank you.

paul wrote:
> Hello all,
>
> please help me the check the following code
>
> use strict;
>
> my $something = 'phal';
> my $otherthing = 'paul';
>
> my $note_whom = 'szmuzu\'mi';
>
> defined($something) ? $note_whom = $something : $otherthing =
> 'singapore';
>
> print "$note_whom \n $otherthing \n";
>
> The result is: singapore
> singapore
>
> I don't know why the scalar $note_whom get the value that is assigned
> to $otherthing.
>
> Please give me the explaination, and thank you.

note precendence...

C:\tmp>perl -MO=Deparse,-p tmp.pl
use warnings;
use strict 'refs';
(my $something = 'phal');
(my $otherthing = 'paul');
(my $note_whom = q[szmuzu'mi]);
((defined($something) ? ($note_whom = $something) : $otherthing) =
'singapore');
print("$note_whom \n $otherthing \n");

HTH,
-jp

paul wrote:
>
> please help me the check the following code
>
> use strict;
>
> my $something = 'phal';
> my $otherthing = 'paul';
>
> my $note_whom = 'szmuzu\'mi';
>
> defined($something) ? $note_whom = $something : $otherthing =
> 'singapore';
>
> print "$note_whom \n $otherthing \n";
>
> The result is: singapore
> singapore
>
> I don't know why the scalar $note_whom get the value that is assigned
> to $otherthing.
>
> Please give me the explaination, and thank you.

Because of precedence you have:

(defined($something) ? $note_whom = $something : $otherthing) = 'singapore';

You need to add parentheses:

defined($something) ? ($note_whom = $something) : ($otherthing = 'singapore');



John
--
Perl isn't a toolbox, but a small machine shop where you can special-order
certain sorts of tools at low cost and in short order. -- Larry Wall

"John W. Krahn" <> writes:
> Because of precedence you have:
>
> (defined($something) ? $note_whom = $something : $otherthing) = 'singapore';
>
> You need to add parentheses:
>
> defined($something) ? ($note_whom = $something) : ($otherthing = 'singapore');

Or better yet, write it out:

if (defined($something)) {
$note_whome = $something;
} else {
$otherthing = 'singapore';
}

I always recommend against the ?: operator unless it's for something
small and self-contained. I avoid using it except for rvalues, unless
not using it will make things unacceptably ugly. Yes, this is a vague
standard. Here' an exampe of what is acceptable:

my $foo = $bar ? 0 : 1;

Here's something that isn't:

($var == 'foo' and $bar < 6 or $x > $y) ? ($this = something($else, 'maybe') or die "um, I dunno") : call_cows($potatoes);

-=Eric

Thank you everyone, I see the mistake already.

John W. Krahn wrote:
> paul wrote:
> >
> > please help me the check the following code
> >
> > use strict;
> >
> > my $something = 'phal';
> > my $otherthing = 'paul';
> >
> > my $note_whom = 'szmuzu\'mi';
> >
> > defined($something) ? $note_whom = $something : $otherthing =
> > 'singapore';
> >
> > print "$note_whom \n $otherthing \n";
> >
> > The result is: singapore
> > singapore
> >
> > I don't know why the scalar $note_whom get the value that is assigned
> > to $otherthing.
> >
> > Please give me the explaination, and thank you.
>
> Because of precedence you have:
>
> (defined($something) ? $note_whom = $something : $otherthing) = 'singapore';
>
> You need to add parentheses:
>
> defined($something) ? ($note_whom = $something) : ($otherthing = 'singapore');
>
>
>
> John
> --
> Perl isn't a toolbox, but a small machine shop where you can special-order
> certain sorts of tools at low cost and in short order. -- Larry Wall

Eric Schwartz <> wrote in comp.lang.perl.misc:
> "John W. Krahn" <> writes:

> > defined($something) ? ($note_whom = $something) : ($otherthing = 'singapore');
>
> Or better yet, write it out:
>
> if (defined($something)) {
> $note_whome = $something;
> } else {
> $otherthing = 'singapore';
> }
>
> I always recommend against the ?: operator unless it's for something
> small and self-contained. I avoid using it except for rvalues, unless
> not using it will make things unacceptably ugly. Yes, this is a vague
> standard. Here' an exampe of what is acceptable:
>
> my $foo = $bar ? 0 : 1;
>
> Here's something that isn't:
>
> ($var == 'foo' and $bar < 6 or $x > $y) ? ($this = something($else,
> 'maybe') or die "um, I dunno") : call_cows($potatoes);

One point about ternary ?: is that it is an expression that has a
value, not primarily a means of flow control. The value should be
the main point if the construct is used, not the side-effects of
the code in the branches. In my view, small and self-contained
enters the image only in so far as it can be hard to find a readable
format for arbitrarily nested ?: with big complex branches. If
such a format can be found, there is no limit to the complexity.

I am of a split mind about using ?: in lvalue context. It *is*
kind of obscure, but there are situations that simply call for
it. One is the loop of a binary search, for example

while ( $low < $high - 1 ) {
my $mid = ($low + $high)/2;
$list->[ $mid] le $targ ? $low : $high = $mid;
}

The if-else equivalent to ?: is four lines, even cuddling the else.
I can never bring myself to use it.

Anno

Eric Schwartz wrote:
> "John W. Krahn" <> writes:
>>Because of precedence you have:
>>
>>(defined($something) ? $note_whom = $something : $otherthing) = 'singapore';
>>
>>You need to add parentheses:
>>
>>defined($something) ? ($note_whom = $something) : ($otherthing = 'singapore');
>
> Or better yet, write it out:
>
> if (defined($something)) {
> $note_whome = $something;
> } else {
> $otherthing = 'singapore';
> }

Or better yet, don't use "cuddled else".

perldoc perlstyle



John
--
Perl isn't a toolbox, but a small machine shop where you can special-order
certain sorts of tools at low cost and in short order. -- Larry Wall

<> wrote in comp.lang.perl.misc:
> Eric Schwartz <> wrote in comp.lang.perl.misc:
> > "John W. Krahn" <> writes:

[...]

> while ( $low < $high - 1 ) {
> my $mid = ($low + $high)/2;
> $list->[ $mid] le $targ ? $low : $high = $mid;
> }
>
> The if-else equivalent to ?: is four lines, even cuddling the else.

Five!

> I can never bring myself to use it.

wrote:
> Eric Schwartz <> wrote in comp.lang.perl.misc:
>>"John W. Krahn" <> writes:
>
>>>defined($something) ? ($note_whom = $something) : ($otherthing = 'singapore');
>>Or better yet, write it out:
>>
>>if (defined($something)) {
>> $note_whome = $something;
>>} else {
>> $otherthing = 'singapore';
>>}
>>
>>I always recommend against the ?: operator unless it's for something
>>small and self-contained. I avoid using it except for rvalues, unless
>>not using it will make things unacceptably ugly. Yes, this is a vague
>>standard. Here' an exampe of what is acceptable:
>>
>>my $foo = $bar ? 0 : 1;
>>
>>Here's something that isn't:
>>
>>($var == 'foo' and $bar < 6 or $x > $y) ? ($this = something($else,
>>'maybe') or die "um, I dunno") : call_cows($potatoes);
>
> One point about ternary ?: is that it is an expression that has a
> value, not primarily a means of flow control. The value should be
> the main point if the construct is used, not the side-effects of
> the code in the branches. In my view, small and self-contained
> enters the image only in so far as it can be hard to find a readable
> format for arbitrarily nested ?: with big complex branches. If
> such a format can be found, there is no limit to the complexity.
>
> I am of a split mind about using ?: in lvalue context. It *is*
> kind of obscure, but there are situations that simply call for
> it. One is the loop of a binary search, for example
>
> while ( $low < $high - 1 ) {
> my $mid = ($low + $high)/2;
> $list->[ $mid] le $targ ? $low : $high = $mid;
> }
>
> The if-else equivalent to ?: is four lines, even cuddling the else.
> I can never bring myself to use it.

I assume that you meant to write:

( $list->[ $mid] le $targ ? $low : $high ) = $mid;

because your example never assigns a value to $low.



John
--
Perl isn't a toolbox, but a small machine shop where you can special-order
certain sorts of tools at low cost and in short order. -- Larry Wall

John W. Krahn <> wrote in comp.lang.perl.misc:
> wrote:


> > while ( $low < $high - 1 ) {
> > my $mid = ($low + $high)/2;
> > $list->[ $mid] le $targ ? $low : $high = $mid;
> > }
> >
> > The if-else equivalent to ?: is four lines, even cuddling the else.
> > I can never bring myself to use it.
>
> I assume that you meant to write:
>
> ( $list->[ $mid] le $targ ? $low : $high ) = $mid;
>
> because your example never assigns a value to $low.

Hmmm?

perl -le '$_ ? $low : $high = 123 for 0, 1; print "$low $high"'

prints

123 123

showing that both are set.

Anno