moderated, Using a my expression with an if clause

comp.lang.perl.moderated - Perl language. (Moderated)


moderated >> Using a my expression with an if clause

by matt » Sat, 09 Jun 2007 05:32:27 GMT

A co-worker asked me about some odd behavior in a perl script. It
looked like a my variable in a loop was not getting reset to undef
during iterations. The my variable was declared with an if clause
(which is what causes the issue).

Here is a small script that duplicates the effect. The output of the
script follows.

############### SCRIPT START ############
#!/usr/bin/perl
use strict;

foreach my $i (0..10) {

print "start a=$i\n";

my $var = 'TWO' if $i == 2;
print "A: var = $var\n";

$var = 'FIVE' if $i == 5;
print "B: var = $var\n";

$var = 'EIGHT' if $i == 8;
print "C: var = $var\n";
}
############## SCRIPT END ################

Output of the script:
start a=0
A: var =
B: var =
C: var =
start a=1
A: var =
B: var =
C: var =
start a=2
A: var = TWO
B: var = TWO
C: var = TWO
start a=3
A: var =
B: var =
C: var =
start a=4
A: var =
B: var =
C: var =
start a=5
A: var =
B: var = FIVE
C: var = FIVE
start a=6
A: var = FIVE
B: var = FIVE
C: var = FIVE
start a=7
A: var = FIVE
B: var = FIVE
C: var = FIVE
start a=8
A: var = FIVE
B: var = FIVE
C: var = EIGHT
start a=9
A: var = EIGHT
B: var = EIGHT
C: var = EIGHT
start a=10
A: var = EIGHT
B: var = EIGHT
C: var = EIGHT

At the end of iteration a=2, the value of $var was set back to undef
(which is what I expected).
However, at the end of iteration a=5, $var seems to retain its value
during the next iteration of the loop (which I did not expect).

We fixed the issue by separating out the my $var declaration from the
if clause:
my $var;
$var = 'TWO' if $i == 2;

However, I don't understand why it behaved the way it did.

I tried searching the web, but the only thing close I found was
"variable suicide" which doesn't seem to be the issue.

My perl version (fromCentOS release 4.4) :
$ perl -v
This is perl, v5.8.5 built for i386-linux-thread-multi
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moderated >> Using a my expression with an if clause

by Gunnar Hjalmarsson » Sat, 09 Jun 2007 08:10:27 GMT



When used like:

my $var = 'TWO' if $i == 2;

the "if $i == 2" part is called a "statement modifier". Quoted from the
"Statement Modifiers" section in "perldoc perlsyn":

"NOTE: The behaviour of a my statement modified with a statement
modifier conditional or loop construct (e.g. my $x if ...) is undefined.
The value of the my variable may be undef, any previously assigned
value, or possibly anything else. Don't rely on it. Future versions of
perl might do something different from the version of perl you try it
out on. Here be dragons."


You should have tried the Perl documentation...

--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl
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moderated >> Using a my expression with an if clause

by Jim Gibson » Sat, 09 Jun 2007 08:48:37 GMT

In article < XXXX@XXXXX.COM >,



[rest of test program and output snipped]


The effect of 'my' in a conditional statement is undefined. See the
section "Statement Modifiers" in 'perldoc perlsyn', especially the
'NOTE' at the end. You have already arrived at the solution, which is
"don't do that".
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moderated >> Using a my expression with an if clause

by Brian McCauley » Sat, 09 Jun 2007 15:28:10 GMT


Why after so long is it still undefined?

Why is it not simply an error?
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moderated >> Using a my expression with an if clause

by Gaal Yahas » Sat, 09 Jun 2007 20:52:22 GMT


Possibly because "my $x if 0" is used In the Wild as a trick to get
static variables.

Sure, there exists a clearly better way to do it (surround the function with a
new scope and declare my $x there), but making this an error will
break old code.

ObPerl6: "state $x"

--
Gaal Yahas < XXXX@XXXXX.COM >
http://gaal.livejournal.com/
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moderated >> Using a my expression with an if clause

by Uri Guttman » Sun, 10 Jun 2007 01:53:03 GMT

>>>>> "BM" == Brian McCauley < XXXX@XXXXX.COM > writes:


>> "NOTE: The behaviour of a my statement modified with a statement
>> modifier conditional or loop construct (e.g. my $x if ...) is
>> undefined.

BM> Why after so long is it still undefined?

BM> Why is it not simply an error?

it was discovered early on that this is a bastard technique to get a
static lexical inside a block/sub. so it is used a bit even if it was
never defined to work this way. too much code uses it to allow it to be
just made into an error now. what is more annoying is that the proper
way to do this (an outer block with the lexical in it) is easy enough
and wasn't used by those who took this error route.

uri

--
Uri Guttman ------ XXXX@XXXXX.COM -------- http://www.stemsystems.com
--Perl Consulting, Stem Development, Systems Architecture, Design and Coding-
Search or Offer Perl Jobs ---------------------------- http://jobs.perl.org
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moderated >> Using a my expression with an if clause

by Ilya Zakharevich » Wed, 13 Jun 2007 00:27:26 GMT

[A complimentary Cc of this posting was sent to
Jim Gibson
< XXXX@XXXXX.COM >], who wrote in article <080620071748378798% XXXX@XXXXX.COM >:

This is too strong a summary. 'my' is fine in the "condition" part of
the statement (as far as you agree to not use this variable in the
same statement; sigh...); not, if it GOVERNED by a conditional.

In a pedantic mode,
Ilya
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