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Re^2: find index of specific array value that occurs multiple times

by AWallBuilder (Beadle)
on Mar 29, 2012 at 14:31 UTC ( #962417=note: print w/replies, xml ) Need Help??

in reply to Re: find index of specific array value that occurs multiple times
in thread find index of specific array value that occurs multiple times

Thank you. As you said this works. Can you please explain a few lines to me.

my @indexes = 0..length($consensus); my @index_c = grep { substr($consensus,$_,1) eq '*' } @indexes; my @index_n = grep { substr($consensus,$_,1) ne '*' } @indexes;
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Re^3: find index of specific array value that occurs multiple times
by davido (Archbishop) on Mar 29, 2012 at 15:51 UTC

    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";


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[Eily]: you could tie a variable into not having the same value each time, if you like to make people who try to debug your code facepalm
[Corion]: perl -wle 'package o; use overload q("") => sub {warn "str"; ""}, bool => sub{warn "bool"; 1}; package main; my $o={}; bless $o => o; print "Yay" if ($o && !length($o))'
[Corion]: But people writing such code should document the objects they construct and why it makes sense for an object to be invisible as string while being true in a boolean context
[hippo]: That's equal parts clever and horrendous.
[Eily]: the overload version wouldn't return true with "$x" && !length $x though, I guess
[hippo]: The more I look at this code, the more $x is a plain old scalar and the more this condition will never be true. I'm calling it a bug at this point.
[hippo]: Thanks for your input which has soothed my sanity (a little)
[Corion]: Eily: Sure - if you force both things into stringy things, then you break that magic. But that would also mean that you changed the expression, as now $x = 0.00 will be true instead of false as it were before
[Corion]: Ah no, at least in my feeble experiments that doesn't change the meaning
[Corion]: We sell sanity in small packages ;)

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