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The use of a hash in this application is positively dangerous unless the elements of the  @change array (which become the keys of the hash) can be guaranteed to be unique. E.g.:

>perl -wMstrict -le "my @change = (1.15, -0.1, 5.4, 1.03, 1.15, -0.241); my @numberofshares = (9999, 400, 200, 300, 500, 240); my %hash; @hash{@change} = @numberofshares; printf qq{%d keys in hash (oops...) \n}, scalar keys %hash; my $sum = 0; while (my($k, $v) = each %hash) { $sum += $v * $k; } print qq{sum == $sum (?)}; " 5 keys in hash (oops...) sum == 1866.16 (?)

Update: The algorithm can be expressed more concisely with some list-processing functions:

>perl -wMstrict -le "use List::Util qw(sum); use List::MoreUtils qw(pairwise); use vars qw($a $b); my @change = (1.15, -0.1, 5.4, 1.03, -0.241); my @numberofshares = ( 100, 400, 200, 300, 240); my $sum = sum pairwise { $a * $b } @change, @numberofshares; print qq{sum == $sum}; " sum == 1406.16

I think it could be expressed yet more concisely in Perl 6, but my 6-fu is not that great. Would it be something like:
    my $sum +=<<*<< @change, @numberofshares;


In reply to Re: simultaneously adding and multiplying numbers in a hash/ array by AnomalousMonk
in thread simultaneously adding and multiplying numbers in a hash/ array by derpp

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