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Re: Re: Efficienty truncating a long string

by dino (Sexton)
on Dec 18, 2003 at 09:33 UTC ( [id://315471]=note: print w/replies, xml ) Need Help??

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

    MJD says "you can't just make shit up and expect the computer to know what you mean, retardo!"
    I run a Win32 PPM repository for perl 5.6.x and 5.8.x -- I take requests (README).
    ** The third rule of perl club is a statement of fact: pod is sexy.

      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.

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

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