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Re: using pop and shift to find the sum of an array

by johngg (Abbot)
on Sep 22, 2012 at 12:25 UTC ( #995098=note: print w/replies, xml ) Need Help??

in reply to using pop and shift to find the sum of an array

You can randomly use pop and shift to remove elements one at a time from either end of your array and add them to the sum, doing this in a loop while there are still elements in the array. However, this obviously destroys your array.

$ perl -Mstrict -Mwarnings -E ' > my @arr = ( 1 .. 10 ); > my $sum; > $sum += int rand 2 ? shift @arr : pop @arr while @arr; > say $sum; > say qq{-> @arr <-};' 55 -> <- $

If, on the other hand, you wish to preserve your array, do the same thing in an on-the-fly subroutine with your array as the argument, which is passed into the subroutine as the @_ array (see perlvar and perlsub). The pop and shift functions inside a subroutine operate on @_ by default, leaving your @arr array untouched.

$ perl -Mstrict -Mwarnings -E ' > my @arr = ( 1 .. 10 ); > my $sum = sub { > my $sum; > $sum += int rand 2 ? shift : pop while @_; > return $sum; > }->( @arr ); > say $sum; > say qq{-> @arr <-};' 55 -> 1 2 3 4 5 6 7 8 9 10 <- $

I hope this is helpful.

Update: Clarified the wording a little.



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[Corion]: Also I found that I can't conveniently weaken an array slot, which also is inconvenient, as I want my one-shots to disappear if the caller discards them
[Corion]: choroba: Currently two or three that my program handles (WWW::Mechanize:: Chrome), but there might be more that become interesting
[Corion]: But I don't expect more than 100 to be active at the same time, so I'm not really sure if there is a not-too-fancy data structure that is maintained with few lines of code where the performance is better than the linear scan ;)
[Corion]: But I should do a mock-up program so that others can see what I'm talking about ;)
[robby_dobby]: Corion: I hope you know all too well that passing around "fancy" datastructures is a recipe for disaster :-)
[robby_dobby]: As in, it's-too-fancy- that-it-will-be- messy-to-handle
[choroba]: bit vectors as keys?
[robby_dobby]: Hmm, I keep falling asleep at my desk, while maintaining an active appearance. Am I getting old?
[robby_dobby]: Every time I fall asleep, there's a small guy in the dreams, shouting "Whoo!" and it jolts me awake. :/
[Lady_Aleena]: robby_dobby, at least you aren't driving. I seem to always be driving somewhere in my dreams and end up at a weird house.

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