Warning with file test(-M)

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

I got a warning message which I can't understand.

$ perl ./a
Useless use of numeric gt (>) in void context at /work/tmp/a line 4.
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where script './a' goes:

#!/usr/bin/perl
use warnings;
my $flag=  ((-M '/tmp/a') > (-M '/tmp/b'));          #no warnings
my $flag2= $flag and ((-M '/tmp/c') > (-M '/tmp/d')); #warning at here
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My guess is that the Perl optimizer considers "-M '/tmp/c'" as just a boolean expression by itself, not a part of scalar comparison.
In real case, I want to write a condition which tests 1) existence of some file $file1. 2) when $file1 exists, test if $file1 is older than the other file($file2).

if(-f $file1 and  ((-M _) > (-M $file2))){ ...
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raise a waring message. How should I write in this case?
My perl version is 'v5.8.4 built for i386-linux-thread-multi'.

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Re: Warning with file test(-M) by borisz (Canon) on May 01, 2005 at 08:04 UTC
The problem is that `and` has a lower priority as =. This means that you wrote `( my $flag2= $flag ) and ((-M '/tmp/c') > (-M '/tmp/d'));` [download] instead of `my $flag2= $flag && ((-M '/tmp/c') > (-M '/tmp/d'));` [download] Boris	[reply] [d/l] [select]
Re: Warning with file test(-M) by eibwen (Friar) on May 01, 2005 at 08:28 UTC
You can find operator precendence in perlop. Additionally, don't forget to `use strict;` You may also be interested in `use diagnostics;` which yielded a considerable amount of diagnostic information for your code: Useless use of numeric gt (>) in void context at ./tmp line 8 (#1) (W void) You did something without a side effect in a context that does nothing with the return value, such as a statement that doesn't return a value from a block, or the left side of a scalar comma operator. Very often this points not to stupidity on your part, but a failure of Perl to parse your program the way you thought it would. For example, you'd get this if you mixed up your C precedence with Python precedence and said Read more... (943 Bytes) Use of uninitialized value in numeric gt (>) at ./tmp line 7 (#2) (W uninitialized) An undefined value was used as if it were already defined. It was interpreted as a "" or a 0, but maybe it was a mistake. To suppress this warning assign a defined value to your variables. Read more... (462 Bytes) Line numbers pertain to the following code (OP's code with addition of `use strict;` and `use diagnostics;`): `#!/usr/bin/perl use strict; use warnings; use diagnostics; my $flag= ((-M '/tmp/a') > (-M '/tmp/b')); #no warnings my $flag2= $flag and ((-M '/tmp/c') > (-M '/tmp/d')); #warning at here` [download]	[reply] [d/l] [select]