[erlang-questions] Question about lists:filter

[Richard A. O'Keefe]

On 30/08/2013, at 1:52 AM, David Mercer wrote:

> 
> Curiously enough, though, timingwise the following 2 seem to do roughly the same:
> 
> is_palindrome1(X) ->
> 	integer_to_list(X) == lists:reverse(integer_to_list(X)).
> 
> is_palindrome2(X) ->
> 	Digits = integer_to_list(X),
> 	Digits == lists:reverse(Digits).
> 

Presumably integer_to_list/1 does the bulk of its work in C.
There are two kinds of cost involved in code like this: the
time it takes to build the garbage and the space the garbage
takes.  The two versions both create 3N list cells which
almost instantly become garbage.

The most space-efficient (and the most time-efficient) method,
at least for unboxed integers, is to do the palindrome test
with arithmetic, not using lists at all.

original code:	9.88 seconds (integer_to_list twice, reverse once)
revised list:	3.06 seconds (integer_to_list once, pattern match)
arithmetic:	2.14 seconds (no lists at all)