>>>>> "XL" == Xi Liu <jason.li...@gmail.com> writes:
XL> I translated a program from c to perl.but the perl program cost 15 seconds
XL> compare to the original c one cost only less than 1 second, I guess this
XL> might be the result of I literally translated the program, using a lot of
XL> array subscripts. After I profile the perl program, it turned out my guess
XL> was true.
you need to learn real perl first and not code as in c. this is true for
most langs. each has idioms that work best and literal translations from
one to another are almost always bad.
XL> for($i = 0; $i < @lines; $i++)
first rule: don't use c style index loops when you can avoid them. that
line is much faster and more perlish as:
foreach my $i ( 0 .. $#lines )
XL> $sum += $lines[$_]->{foo_value} for ($i - $n + 1 .. $i);
XL> sorry I can't post the whole code, but the lines above is the big
XL> framework, I also omit all the boundary check. "$sum +=
XL> $lines[$_]->{foo_value} for ($i - $n2 + 1 .. $i)" this line
XL> consumes all the time.
XL> I got a array, for each element of the array I have to sum up the
XL> $n elements prior to the current one. I know I should use pop push
XL> shift unshift all the perl list operators instead of subscripting,
XL> but I just don't know how, would somebody tell me what is the perl
XL> way of doing this?
that doesn't seem to be what the code is doing. $sum isn't just a sum,
it is a sum of sums as you have a two dimensional loop. did you
translate the code incorrectly or can you more clearly specify what is
being summed here?
you can get speedups in other ways too. you can do a map or slice of the
array and call sum (in List::Util) on that list. it will be much faster
than the += loop you have there. also you are accessing {foo_value} over
and over for each line (since the loop is 2 levels deep). a better
algorithm would do a sum of sums where you sum one set and then add that
to the next set. i won't get into that since the second loop logic isn't
clear to me (which set of elements are being summed and resummed). that
is more your problem. but this can be sped up a great deal with better
coding.
uri
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