在公共Lisp中使用循环宏嵌套循环

Question

I am trying to implement a basic nested loop in CL, but the Loop macro is resisting this. Basically, I would like to find all possible products of 3-digit numbers and accumulate them into a list.

Here is my attempt:

 (loop for x downfrom 999 to 998 do (loop for y downfrom 999 to 998 collect (* x y)))

The code above returns NIL for some reason. By the way, I realize that I only run down to 998, but this is done for testing purposes.

What could I do to obtain a list like this:

(999*999 999*998 ... 998*998 998*997 ... 997*997 997*996 ... 100*100)

Answer 1

The COLLECT -clause in the inner loop doesn't affect the outer loop. So the inner loop returns a list of results, but the DO -clause in the outer loop just discards the result. You should use APPEND or NCONC instead of DO . Usually it's better to just stick with APPEND if there are no performance concerns, even if in this case NCONC would be safe.

(loop for x downfrom 999 to 900
      append (loop for y downfrom 999 to 900
                   collect (* x y)))