in reply to Re: Re: Efficienty truncating a long string
in thread Efficienty truncating a long string

I don't know whether substr is optimized not to bother returning anything in void context (probably is), but this is how you do it 
my $long_string = 'string' x 5;
print $long_string,$/;

substr( $long_string, 6 ) = ''; # truncate to 6 characters

#
# same thing, only using 4 arg substr
# substr( $long_string, 6, length($long_string) - 6, '' );
#

print $long_string,$/;
__END__
stringstringstringstringstring
string

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Re: Re: Re: Re: Efficienty truncating a long string
by ysth (Canon) on Dec 18, 2003 at 10:03 UTC
    4-arg substr isn't optimized for void context. It will set up the replaced string to be returned regardless.

    The lvalue substr doesn't actually return the string but a special lvalue indicating the offset and length. Creating this and setting it to '' involves a fair amount of overhead.

    Which is actually more efficient will probably depend on how much you are truncating.

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      It was originally part of a log reader. I found that reading in blocks and working pattern matching on those blocks was much more efficient than on line by line. Obviously you can get partial lines, so I rindex the last /n, save the fragment at the end for the next pass and return the main block truncated. It works like a charm. I was reinventing it and seing how I could make it more efficient.
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        Then you probably want to use 4-arg substr to truncate the buffer and save the fragment all at once: 
        $end = rindex($buffer, "\n");
if ($end < 0) { whoops, incomplete line }
$fragment = substr($buffer, $end+1, length($buffer), '');

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        Note that length($buffer) is bigger than the actual length(buffer)-$end-1 you want, but specifying a length larger than is available is guaranteed to not cause problems; you'll just get what is there. If the buffer ends with "\n", you'll get an empty $fragment.
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Re: Re: Re: Re: Efficienty truncating a long string
by dino (Sexton) on Dec 18, 2003 at 09:50 UTC
    Ty, How embarrasing, that simple eh? Thanks again, sigh. :)
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