Re^2: optimizing the miller-rabin algorithm

[punklrokk]

If I use your version of a For Loop, how do I tell it to iterate two steps instead of one?

JP Bourget (punklrokk) MS Information and Security Rochester Institute of Technology Rochester, NY

[GrandFather]

Either use the C style for loop in that case or, if appropriate, do something like for ($start/2..$end/2) with $start and $end both even (but that's rather clunky!).

The Perl style for is very good for iterating over lists so you can do something like for (1.5, 2..4, @moreNumbers). Generally you should think Perl style for first, then C style if you really need it.

[ikegami]

You could do

for (0..(5-1)/2) {  # 1..5, step 2
   my $i = $_*2 + 1;
   ...
}

but you're probably better off falling back to a C-style loop in that case.

As an aside, the above trick is particularly useful when dealing with floats.

>perl -le "for (my $i=0; $i<10; $i+=0.1) { print $i }"
...
8.79999999999998
8.89999999999998
8.99999999999998
9.09999999999998
9.19999999999998
9.29999999999998
9.39999999999998
9.49999999999998
9.59999999999998
9.69999999999998
9.79999999999998
9.89999999999998
9.99999999999998

>perl -le "for (0..99) { my $i = $_ / 10; print $i }"
...
8.8
8.9
9
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9