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RE: Generating a Pattern

by fundflow (Chaplain)
on Jul 25, 2000 at 01:29 UTC ( #24194=note: print w/ replies, xml ) Need Help??


in reply to Generating a Pattern

Here's my version:

@a = (1); for (1..10) { print "@a\n"; @na=(); for (@a) { $na[-1] == $_ ? $na[-2]++ : push @na, (1, $_) } @a=@na; }
It does not move elements around (assuming push/pop are doing the natural thing) and has only one temporary copy.

Update: Use [-1] instead of [$#na]. Cool, learned a new thing!
Update: use ?: instead of if/then


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RE: RE: Generating a Pattern
by eak (Monk) on Aug 06, 2000 at 07:58 UTC
    I first have to say that I really enjoyed this problem. It took me a little while to come up with a working algorithm, unfortunately it wasn't at elegant as yours. I thought I would play with your version to see if I could improve upon it a bit. The only thing I could do was add the use of references which reduces the memcopy time on larger lists. Below is a script that benchmarked the old 'non_ref' vs. the new 'ref'. Not necessarily staggering, but an improvement. I compacted the for loop for flipping it around and putting the for statement at the end of the conditional line.
    #!/usr/bin/perl use Benchmark; my $count = 100; my $end = 30; sub ref { my $a = [1]; for(1..$end){ # print "@$a\n"; my @na; ($na[-1] == $_ ? $na[-2]++ : push @na, (1, $_)) for @$a; $a = \@na; } } sub no_ref { my @a = (1); my @na; for (1..$end){ # print "@a\n"; @na = (); ($na[-1] == $_ ? $na[-2]++ : push @na, (1, $_)) for @a; @a = @na; } } timethese($count, { 'no_ref' => \&no_ref , 'ref' => \&ref} ); + +
    Benchmark: timing 100 iterations of no_ref, ref...
        no_ref: 11 wallclock secs (11.12 usr +  0.04 sys = 11.16 CPU)
           ref:  9 wallclock secs ( 8.34 usr +  0.06 sys =  8.40 CPU)  
    

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