Beefy Boxes and Bandwidth Generously Provided by pair Networks
No such thing as a small change
 
PerlMonks  

Comment on

( #3333=superdoc: print w/ replies, xml ) Need Help??

There's a bug... an off by one error on the last iteration: it should read length( $consensus ) - 1;

Actually they could be reduced to a single loop like this:

use strict; use warnings; my $consensus = 'abc*def*ghi'; my( @index_c, @index_n ); my $idx = 0; my $len = length( $consensus ); push( @{ substr( $consensus, $idx, 1 ) eq '*' ? \@index_c : \@index_n +}, $idx++ ) while $idx < $len; print "C: @index_c\n"; print "N: @index_n\n";

This trades three loops (two explicit -- grep, and one implicit -- the initialization of @indexes) for a single while loop. It also reduces the calls to substr in half.

Explaining the original (grep) method: Create an array that contains the indices of each position within the string $consensus. Then iterate over the array checking the character in $consensus at each position for the existence of the character '*'. If the condition is true ('*' exists in that position), place that index position in @index_c. Repeat the process testing for non-existence of '*', placing all indices where '*' doesn't exist into @index_n.

Explaining my refactor: Create some empty index holders (@index_c, @index_n). Create a counter variable and a variable that holds the length of $consensus ( $idx, $len ). Then we take the length of $consensus just one time and store it (rather than call length in the while loop conditional). Next iterate over the $idx values from 0 through the last index position of $consensus ($idx++ < $len). At each position use substr to access and test whether the character at position $idx contains a '*'. The ternary operator exposes either a reference to @index_c or a reference to @index_n to the @{...} dereference construct, creating an lvalue. We then push $idx (the current position) onto either @index_c or @index_n depending on the outcome of the conditional test. Repeat this process for each character position from 0 through $len-1.

That changes the algorithm efficiency from three O(n) loops to one O(n) loop. However, it does sacrifice coding clarity, which could be restored like this:

my $consensus = 'abc*def*ghi'; my( @index_c, @index_n ); my $idx = 0; my $len = length( $consensus ); while( $idx < $len ) { if( substr( $consensus, $idx, 1 ) eq '*' ) { push @index_c, $idx; } else { push @index_n, $idx; } $idx++; } print "C: @index_c\n"; print "N: @index_n\n";

Dave


In reply to Re^3: find index of specific array value that occurs multiple times by davido
in thread find index of specific array value that occurs multiple times by AWallBuilder

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • Outside of code tags, you may need to use entities for some characters:
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others avoiding work at the Monastery: (6)
    As of 2014-08-30 19:51 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      The best computer themed movie is:











      Results (293 votes), past polls