Magic $a $b

[Achim Peters]

Hi,

I'm a noob to perl and a frequent reader of this group. Whenever anyone
in here uses $a and/or $b in a non-sort context s/he gets a "Don't!
They're magic." as a response. Now, I believe in magic and the wisdom of
regulars and that's why I mentioned this general rule in
de.comp.lang.perl.misc.

But there happened to be a non-believer and he dared me with "Oh,
really? Perl this:"

use strict;
use warnings;

$a = 4711;

my @arr = qw /x a u q r/;
@arr = sort { $a cmp $b } @arr;

print "$a\n";

which in fact does print the 4711 (a German "magic" number) with Perl
5.8.2, thus no side effects of the sorting (except of course that the
missing "my" in front of $a does not cause an compile error.

Is there any example to prove his impertinence of questioning the gurus'
wisdom?

TIA

Bye
Achim

 

[Peter Makholm]

But there happened to be a non-believer and he dared me with "Oh,
really? Perl this:"

use strict;
use warnings;

$a = 4711;

See mom, no warning! - That's magic.

Using $a and $b defies parts of of using strict.

//Makholm

 

[Joost Diepenmaat]

which in fact does print the 4711 (a German "magic" number) with Perl
5.8.2, thus no side effects of the sorting (except of course that the
missing "my" in front of $a does not cause an compile error.

Is there any example to prove his impertinence of questioning the gurus'
wisdom?

I would think:

sub srt {
my @arr = qw /x a u q r/;
@arr = sort { $a cmp $b } @arr;
}

srt();
my $a = 4711;
print "$a\n";

versus:

srt();
my $a = 4711;
print "$a\n";

sub srt {
my @arr = qw /x a u q r/;
@arr = sort { $a cmp $b } @arr;
}

would be enough reason not to use $a and $b.

 

[Achim Peters]

I would think:

my $a = 4711; [...]

would be enough reason not to use $a and $b.

Tnx! He converted and now is a true fellow believer. ;-)

Bye
Achim

 

[szr]

I would think:

sub srt {
my @arr = qw /x a u q r/;
@arr = sort { $a cmp $b } @arr;
}

srt();
my $a = 4711;
print "$a\n";

versus:

srt();
my $a = 4711;
print "$a\n";

sub srt {
my @arr = qw /x a u q r/;
@arr = sort { $a cmp $b } @arr;
}

would be enough reason not to use $a and $b.

When I try the latter, I get the following, using Perl 5.8.8:

Can't use "my $a" in sort comparison at line 9.

0001: #!/usr/local/bin/perl -w
0002: my @arr = srt();
0003: my $a = 4711;
0004: print "$a\n";
0005: print join (', ', @arr);
0006:
0007: sub srt {
0008: my @arr = qw /x a u q r/;
0009: @arr = sort { $a cmp $b } @arr;
0010: }


The error message itself seems to be wrong, or more percisely, the wrong
error for the given situation. There is no "my" in the "sort { ... }"
clause. Is this be a bug?

Furthur, why does it generate an error in the first place?

 

[J. Gleixner]

When I try the latter, I get the following, using Perl 5.8.8:

Can't use "my $a" in sort comparison at line 9.

0001: #!/usr/local/bin/perl -w
0002: my @arr = srt();
0003: my $a = 4711;
0004: print "$a\n";
0005: print join (', ', @arr);
0006:
0007: sub srt {
0008: my @arr = qw /x a u q r/;
0009: @arr = sort { $a cmp $b } @arr;
0010: }


The error message itself seems to be wrong, or more percisely, the wrong
error for the given situation. There is no "my" in the "sort { ... }"
clause. Is this be a bug?

Furthur, why does it generate an error in the first place?

Are you kidding??????????

You're supposed to get an error. That's the reason Joost
posted the very concise example - to show the OP why
it might be bad to use $a.

 

[Ben Morrow]

When I try the latter, I get the following, using Perl 5.8.8:

Can't use "my $a" in sort comparison at line 9.

The error message itself seems to be wrong, or more percisely, the wrong
error for the given situation. There is no "my" in the "sort { ... }"
clause. Is this be a bug?

No, it's not. The point is that the $a in scope at line 9 is the 'my $a'
from five lines earlier, rather that the global $main::a in scope at the
start of the program, and sort (for various nasty implementation-specfic
reasons) can't use a lexical $a. This is exactly why you shouldn't use
$a other than for sort.

Ben

 

[szr]

No, it's not. The point is that the $a in scope at line 9 is the 'my
$a' from five lines earlier, rather that the global $main::a in scope
at the start of the program, and sort (for various nasty
implementation-specfic reasons) can't use a lexical $a. This is
exactly why you shouldn't use $a other than for sort.

Ah. Its very clear now. Thank you (and J. Gleixner) I'm nto sure why I
didn't catch that.