glob behavior

vagabonding electron has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks,
I cannot explain the following behavior of glob in the script.
Given is a directory U:/TEMP/TEMP with three subdirectories /A, /B and /C.
Each directory contains a file:

U:/TEMP/TEMP/A/Test01.txt
U:/TEMP/TEMP/C/Test03.txt
U:/TEMP/TEMP/B/Test02.txt
[download]

If I write the script the following way:

#!/usr/bin/perl
use strict;
use warnings;

my %address =
(
    A     => qq{U:/TEMP/TEMP/A},
    B    => qq{U:/TEMP/TEMP/B},
    C    => qq{U:/TEMP/TEMP/C},
);

for my $t (keys %address)
{
    print "$t : $address{$t} : ", glob ("$address{$t}/Test*"), "\n";
}
[download]

then it prints as shown:

A : U:/TEMP/TEMP/A : U:/TEMP/TEMP/A/Test01.txt
C : U:/TEMP/TEMP/C : U:/TEMP/TEMP/C/Test03.txt
B : U:/TEMP/TEMP/B : U:/TEMP/TEMP/B/Test02.txt
[download]

If I write it in the form:

#!/usr/bin/perl
use strict;
use warnings;

my %address =
(
    A     => qq{U:/TEMP/TEMP/A},
    B    => qq{U:/TEMP/TEMP/B},
    C    => qq{U:/TEMP/TEMP/C},
);

for my $t (keys %address)
{
    my $file = glob ("$address{$t}/Test*");
    print "$t : $address{$t} : $file, \n";
}
[download]

then the output is as follows:

A : U:/TEMP/TEMP/A : U:/TEMP/TEMP/A/Test01.txt,
Use of uninitialized value $file in concatenation (.) or string at ...
+ line 15.
C : U:/TEMP/TEMP/C : ,
B : U:/TEMP/TEMP/B : U:/TEMP/TEMP/B/Test02.txt,
[download]

Why it behaves itself different?
Thanks in advance.
VE

Comment on glob behavior Select or Download Code

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Re: glob behavior by davido (Cardinal) on Mar 20, 2012 at 08:14 UTC
In scalar context (your second example), glob will iterate over the items that match -- one item per call, and return undef if no more items remain. In list context (your first example), glob will return the entire list of items that matched, and that list could possibly be empty. So your two examples do differ in the context in which glob is being called, and that is documented to provide different behavior. Essentially, in scalar context `glob` acts like an iterator. In list context it doesn't iterate; it just returns a list. What I can't explain is why the list would be empty if nothing else changed. Dave	[reply] [d/l]
Re^2: glob behavior by rovf (Priest) on Mar 20, 2012 at 08:34 UTC
What I can't explain is why the list would be empty if nothing else changed. From perlop: A (file)glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. I think this explains the effect we can see: The first time in the loop, the arguments for `glob` are evaluated and return one file name. The second time in the loop, the arguments are not evaluated again (since the list is not "exhausted" yet). Instead, glob tries to find the next file, but there is none, and returns undef to signal this fact. On the third iteration, the arguments are evaluated again, yielding one filename. -- Ronald Fischer <ynnor@mm.st>	[reply] [d/l] [select]
Re^3: glob behavior by vagabonding electron (Curate) on Mar 20, 2012 at 08:56 UTC
It seems to be the case since after I added `U:/TEMP/TEMP/D/Test04.txt U:/TEMP/TEMP/E/Test05.txt` [download] it prints in the first version: `A : U:/TEMP/TEMP/A : U:/TEMP/TEMP/A/Test01.txt D : U:/TEMP/TEMP/D : U:/TEMP/TEMP/D/Test04.txt C : U:/TEMP/TEMP/C : U:/TEMP/TEMP/C/Test03.txt E : U:/TEMP/TEMP/E : U:/TEMP/TEMP/E/Test05.txt B : U:/TEMP/TEMP/B : U:/TEMP/TEMP/B/Test02.txt` [download] and in the second version: `A : U:/TEMP/TEMP/A : U:/TEMP/TEMP/A/Test01.txt, Use of uninitialized value $file in concatenation (.) or string at ... + line 17. D : U:/TEMP/TEMP/D : , C : U:/TEMP/TEMP/C : U:/TEMP/TEMP/C/Test03.txt, Use of uninitialized value $file in concatenation (.) or string at ... + line 17. E : U:/TEMP/TEMP/E : , B : U:/TEMP/TEMP/B : U:/TEMP/TEMP/B/Test02.txt,` [download] Thank you very much! What I do not understand though - to be honest - is why glob is not evaluated again on the second loop pass. VE Update: After reading perlop and your post twice I did catch it :-) Sometimes it takes longer to savvy. Thanks again!	[reply] [d/l] [select]
Re^2: glob behavior by vagabonding electron (Curate) on Mar 20, 2012 at 09:34 UTC
Thank you very much davido.	[reply]
Re: glob behavior by Anonymous Monk on Mar 20, 2012 at 08:45 UTC
Why it behaves itself different? It probably doesn't :) put both loops in the same file and see what happens Also, report your perl version and File::Glob version	[reply]
Re^2: glob behavior by vagabonding electron (Curate) on Mar 20, 2012 at 09:01 UTC
I use ActivePerl v.5.14.1 on WinXP. The version of File::Glob is 1.12 Update: I changed the loop to: `for my $t (keys %address) { my $file = glob ("$address{$t}/Test"); print "$t : $address{$t} : $file, \n###\n"; print "$t : $address{$t} : ", glob ("$address{$t}/Test"), "\n---\ +n"; }` [download] It prints: A : U:/TEMP/TEMP/A : U:/TEMP/TEMP/A/Test01.txt, ### A : U:/TEMP/TEMP/A : U:/TEMP/TEMP/A/Test01.txt --- Use of uninitialized value $file in concatenation (.) or string at ... + line 17. D : U:/TEMP/TEMP/D : , ### D : U:/TEMP/TEMP/D : U:/TEMP/TEMP/D/Test04.txt --- C : U:/TEMP/TEMP/C : U:/TEMP/TEMP/C/Test03.txt, ### C : U:/TEMP/TEMP/C : U:/TEMP/TEMP/C/Test03.txt --- Use of uninitialized value $file in concatenation (.) or string at ... + line 17. E : U:/TEMP/TEMP/E : , ### E : U:/TEMP/TEMP/E : U:/TEMP/TEMP/E/Test05.txt --- B : U:/TEMP/TEMP/B : U:/TEMP/TEMP/B/Test02.txt, ### B : U:/TEMP/TEMP/B : U:/TEMP/TEMP/B/Test02.txt --- [download]	[reply] [d/l] [select]
Re^2: glob behavior by vagabonding electron (Curate) on Mar 20, 2012 at 15:45 UTC
... and now after changing the line from `my $file = glob ("$address{$t}/Test");` [download] to `my ($file) = glob ("$address{$t}/Test");` [download] the output is as follows: A : U:/TEMP/TEMP/A : U:/TEMP/TEMP/A/Test01.txt, ### A : U:/TEMP/TEMP/A : U:/TEMP/TEMP/A/Test01.txt --- D : U:/TEMP/TEMP/D : U:/TEMP/TEMP/D/Test04.txt, ### D : U:/TEMP/TEMP/D : U:/TEMP/TEMP/D/Test04.txt --- C : U:/TEMP/TEMP/C : U:/TEMP/TEMP/C/Test03.txt, ### C : U:/TEMP/TEMP/C : U:/TEMP/TEMP/C/Test03.txt --- E : U:/TEMP/TEMP/E : U:/TEMP/TEMP/E/Test05.txt, ### E : U:/TEMP/TEMP/E : U:/TEMP/TEMP/E/Test05.txt --- B : U:/TEMP/TEMP/B : U:/TEMP/TEMP/B/Test02.txt, ### B : U:/TEMP/TEMP/B : U:/TEMP/TEMP/B/Test02.txt --- [download]	[reply] [d/l] [select]
Re^3: glob behavior by davido (Cardinal) on Mar 20, 2012 at 19:23 UTC
Once again you're dealing with different contexts. `my $file = glob ...` evaluates glob in scalar context, rendering it an iterator function, and `my ($file) = glob ...` evaluates glob in list context, rendering it a list function that returns a list, which can be empty. Dave	[reply] [d/l] [select]
Re^4: glob behavior by vagabonding electron (Curate) on Mar 23, 2012 at 15:38 UTC