Alan M. Carroll wrote:
> Apparently I wrote it badly, but my question concerning scalar vs. $#array
> was whether there was a difference in run-time costs, not the functional
> difference.
>
> I don't have a specific issue, but code can always run faster. The question
> is whether I should use scalar() without concern, or whether I should try to
> cache the result. For instance, if scalar is O(n), then this loop:
>
> for ( my $i = 0 ; i < scalar(@objs) ; ++i ) { ... }
>
> becomes O(n^2) instead of O(n). That's exactly the kind of thing that adds up
> without raising an obvious red flag in profiling.
I would normally write that:
for (my $ii = 0; $ii < @objs; ++ii) { ... }
> One could then make it O(n) by doing
>
> my $limit = scalar(@objs);
> for ( my $i = 0 ; $i < $limit; ++$i ) { ... }
I'm not sure that this would gain you too much and if you don't need $i
inside the loop, the foreach form would be clearer.
> If $# is O(1) instead of O(n) then it would be good to have the habit of using
>
> for (my $i = 0 ; $i <= $#objs ; ++$i) { ... }
>
> Or, if one is building a list and needs to check later if the list is
> non-empty, should one set a flag during the list build or just use scalar()
> at the end?
I would normally use : if (@list) {
> The question is, should I try to make a habit of one of these in particular?
> Or does it matter?
I'd not bother with '<= $#list' unless you need the last element -
use '< @list' instead.
I'd also not bother with 'scalar @list' unless you need to force
context.
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