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

Well I've looked at substr and can understand its normal non lvalue usage, but as far as I can see its a copying operator, returning a substr of the original. I want to truncate a string without the copying overhead. The lvalue version seems to be ok, but I dont quite understand how to use it to truncate a string. Any suggestions?
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Re: Re: Re: Efficienty truncating a long string
by PodMaster (Abbot) on Dec 18, 2003 at 09:39 UTC
    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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[reply]
[d/l]
      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.

[reply]
        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.
[reply]
      Ty, How embarrasing, that simple eh? Thanks again, sigh. :)
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