[GAP Forum] Iteration Over Lists of Lists

[Burkhard Höfling]

Dear Robert,

>> Thanks, this is what I wanted. 
>> However, I timed it this morning and found it to actually be slower than the method I previously mentioned (currently speed is of significant importance for what I'm doing). I've included the test script below. You can vary the values of `n` (number of lists) and `k` (size of lists). Note for `n=6` and `k=9` I am getting,

Iterators cause a lot of overhead per call. If you just modify your code a bit, you should get close to Python speed with GAP. On my machine, I get

teratorOfCartesianProduct Method: 915 ms
Your Mentioned Method                  : 493 ms
variant below:                                 : 125 ms

(didn’t try Python). 

Cheers

Burkhard.



size_rc := List([1..n], x -> k);
base := List([1..n], x -> 1); #We store the iteration step state in base
a := 1;

start := Runtime();
stop := false;
repeat

	#Perform whatever list[i][base[i]] calculations here
	#a := a + 1;
	#Print("Base: ", base, "\n");

	#Execute the incrementor
	base[1] := base[1] + 1;
	j := 1;
	while base[j] > size_rc[j] and j < n do
		base[j] := 1;
		j := j + 1;
		base[j] := base[j] + 1;
	od;
until j = n and base[n] > size_rc[n];

total2 := Runtime() - start;