The for version creates a (potentially large) list, whereas the while version does not.
The cost of allocating the list is offset against repeated calls to each, with the result that for small lists, for iterates faster, but on larger lists the while version wins out. A quick test show that the break even point on my system is around 8000 elements. YMMV
use Benchmark qw[ cmpthese ];
$h{ $_ } = 0 for 1 .. 100;
cmpthese( -3, {
for => q[ $h{ $_ }++ for keys %h; ],
while => q[ $h{ $_ }++ while $_ = each %h; ]
});
Rate while for
while 11937/s -- -23%
for 15544/s 30% --
$h{ $_ } = 0 for 1 .. 1000;
cmpthese( -3, {
for => q[ $h{ $_ }++ for keys %h; ],
while => q[ $h{ $_ }++ while $_ = each %h; ]
});
Rate while for
while 1165/s -- -26%
for 1581/s 36% --
$h{ $_ } = 0 for 1 .. 10000;
cmpthese( -3, {
for => q[ $h{ $_ }++ for keys %h; ],
while => q[ $h{ $_ }++ while $_ = each %h; ] } );
Rate for while
for 80.0/s -- -8%
while 86.9/s 9% --
$h{ $_ } = 0 for 1 .. 5000;
cmpthese( -3, {
for => q[ $h{ $_ }++ for keys %h; ],
while => q[ $h{ $_ }++ while $_ = each %h; ]
});
Rate while for
while 207/s -- -15%
for 243/s 17% --
$h{ $_ } = 0 for 1 .. 8000;
cmpthese( -3, {
for => q[ $h{ $_ }++ for keys %h; ],
while => q[ $h{ $_ }++ while $_ = each %h; ]
});
Rate for while
for 119/s -- -0%
while 119/s 0% --
Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
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