Infinity loop

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

Hello fellow monks. I am a n00b and I have stumbled upon a very perplexing question. I was hoping that with your ultimate powers of programming, you could help me. Here it goes:

I have this loop:

while ( $_[0]->seq() =~ m/CG/g ) {++$CG;}

It goes on to infinity.

$_[0]->seq() : returns a big string -- a dna sequence

And the rest you probably know what it does. So why can't it just work. A being that is higher than me( my so-called teacher) has suggested it's an "order of operations" problem.

Hope it helps.

How can I solve it without writing other lines of code???

Yours truly,
=d(o_o)b=

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Re: Infinity loop by Tanktalus (Canon) on Nov 20, 2005 at 16:31 UTC
Well, that's because `$_[0]->seq()` continues to return strings that have "CG" in them. And it probably returns a new string each time, or a copy of a string (which would be new), and then the match tries again. What you probably want is: `my $seq = $_[0]->seq(); while ($seq =~ /CG/) { ++$CG }` [download] But I'm not sure about that. Are you trying to count the number of CG sequences? Then you want: `$CG += () = $_[0]->seq() =~ /CG/g;` [download] I think that "+=()=" may be called the "pimply-goatse-operator". Not sure. Ignore that. What is it that this code is really trying to accomplish?	[reply] [d/l] [select]
Re^2: Infinity loop by doob (Pilgrim) on Nov 20, 2005 at 16:36 UTC
my $seq = $_[0]->seq(); while ($seq =~ /CG/) { ++$CG } is exactly what I had before my ...mentor...told me to use this other method...so I am still scratching my head in perplexion...he told me to seek the answer to how i can make the new line work without adding new lines of code. Thanks, though =d(o_o)b=	[reply]
Re^3: Infinity loop by nobull (Friar) on Nov 20, 2005 at 17:59 UTC
Well you could avoid the temporary variable by using //g in a LIST context instead. I wouldn't recommend it if the temporary list was likely to be of non-trivial size. `for ( $_[0]->seq() =~ /CG/g ) { ++$CG }` [download] If course if the body of your loop is really just `++$CG` then all you are doing is counting the number of elements in the list. In that case the "pimply-goatse-operator" mentioned earlier in ths thread is what you want.	[reply] [d/l] [select]
Re^3: Infinity loop by thospel (Hermit) on Nov 20, 2005 at 22:03 UTC
If your string result is so huge that the copy really is a problem, you can temporarily make an alias like this: `for my $seq ($_[0]->seq()) { # in case you need to reset the first match position # (avoid pos() on huge strings) # $seq =~ /\za/g; $CG++ while $seq =~ /CG/g; }` [download] update Interesting enough, due to the way the perl internals work, this doesn't save you memory compared to `my $seq = $_[0]->seq();` [download] during the processing, but the gain is less wasted memory when $seq goes out of scope again (this isn't a memory leak, perl will reuse that memory if the scope is entered again)	[reply] [d/l] [select]