in reply to Re^3: Out of Memory
in thread Out of Memory

Thanks... I didn see a reference to it in the thread that I got the while loop from but it noted that TR can only be used with single characters? which works in this case but maybe not others? I did try the TR thing and it does work for my data set without a memory error.

Are you sure this is the most efficient way to do this? Seems like to me that its creating a copy of the original string and trying to replace the matches before it outputs the count. (as far as i can tell given reading a quick page on teh TR function) I wouldn't think that would be as memory efficient as the while code... but I don't understand the internals of the while code either, if it instantiates a huge list the interates through them, I can see how that wouldn't be as efficient as teh TR code.

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Re^5: Out of Memory
by BrowserUk (Patriarch) on Mar 28, 2013 at 19:12 UTC
    Are you sure this is the most efficient way to do this?

    Yes, I am quite sure.

    1. tr doesn't make copies, it operates in-place.
    2. It is optimised internally to recognise that when the source and replacements are identical, it is in 'counting mode' only.

    For multi-character substring counting avoid creating the huge list:

    $s = 'ab'x 10e6;;

++$n while $s=~ m[ab]g;;
print $n;;
10000000

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    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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[reply]
[d/l]
      Ah that makes sense, I was wondering if it would not replace if both were the same.

      how does

      ++$n while $s=~ m[ab]g;;

      differ in execution from

      while($_=~s\0g) {count++}

      Thanks for sharing

[reply]
[d/l]
[select]
        how does ... differ in execution from ...

        Hm. "in execution"?

        1. The result -- the count -- is the same.
        2. The runtime is less than half:
          $s = 'ab'x 10e6;;

cmpthese -1, {
    a => q[ 
        my $n = 0; 
        while( $s =~ m[ab]g ){
            $n++
        };
    ],
    b=>q[
        my $n = 0; 
        ++$n while $s =~ m[ab]g;
    ],
};;

  s/iter    a    b
a   2.31   -- -37%
b   1.45  59%   --

          [download]
        3. It is easier to read.

        Don't use post increment if you aren't using the pre-increment value. Don't create scopes you don't need.

        Or, get into the habit of not doing anything you don't need to.

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]

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