Interesting Mistake

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

Hi Monks, I make a dumb typo while looping through a match. I did end up with an interesting result. I was wondering if someone could tell me what the mistake is doing? What I meant to do was loop through a folder and grab certain .htm files... as such:

my @files = <*.HTM>;
foreach $file (@files) {
if ($file =~ m/IDX/gi){print $file."\n";}
}
[download]

Which results in:

ACIDX.HTM
APIDX.HTM
...
[download]

What I did instead was (note the ~ placement) this:

my @files = <*.HTM>;
foreach $file (@files) {
if ($file = ~m/IDX/gi){print $file."\n";}
}
[download]

Which results in:

4294967295
4294967295
...
[download]

What is my mistake doing?

Comment on Interesting Mistake Select or Download Code

Replies are listed 'Best First'.
Re: Interesting Mistake by cdarke (Prior) on Feb 22, 2010 at 15:37 UTC
The tilde ~ is a unary bitwise operator to negate the bits (ones complement), see perlop. The actual number returned is all bits set: `printf "0%08x\n",4294967295;` [download] Gives: `0ffffffff` m returns 0 on a failure to match: `printf "0%08x\n",~0;` [download] also gives `0ffffffff`. Updated to explain more.	[reply] [d/l] [select]
Re: Interesting Mistake by FunkyMonk (Bishop) on Feb 22, 2010 at 15:46 UTC
m/IDX/ returns true (1) if $_ matches the pattern and false otherwise. ~ is the bitwise not operator (aka one's complement - see Symbolic Unary Operators). It inverts all the bits in a number. Your match doesn't match $_ and so returns false, which is complemented to 0xffffffff, which, in decimal, prints as 4294967295. Try `perl -e 'print ~0'` [download] or, for 64-bit perl: `perl -e 'print ~0 & 0xffffffff'` [download] Update: Added code examples & link to ~ in perlop	[reply] [d/l] [select]
Re: Interesting Mistake by toolic (Bishop) on Feb 22, 2010 at 15:48 UTC
What is my mistake doing? In addition to what cdarke said, you should use strict and warnings, which would warn you that something is wrong. `=~` is one of the Binding Operators, but `= ~` are two separate operators (`=` and `~`). You are assigning the value of the `~m/IDX/gi` to the `$file` variable. Since the assignment succeeds, you print the variable's value.	[reply] [d/l] [select]
Re: Interesting Mistake by biohisham (Priest) on Feb 22, 2010 at 15:58 UTC
Check Symbolic Unary Operators. Bitwise String Operators. and Integer Arithmetic. for references. UPDATE: The number you got in essence is the complement of 0 which btw is also the unsigned integer capacity of your 32-bit machine. `print ~0,"\n"; print 2**32 - 1,"\n"; printf "%e",~0,"\n";` [download] Excellence is an Endeavor of Persistence. Chance Favors a Prepared Mind.	[reply] [d/l] [select]