在 Common Lisp 中使用循环宏的嵌套循环

问题描述

我正在尝试在 CL 中实现一个基本的嵌套循环，但 Loop 宏拒绝这样做.基本上，我想找出所有可能的三位数的乘积并将它们累加到一个列表中.

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.

这是我的尝试:

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

由于某种原因，上面的代码返回NIL.顺便说一下，我意识到我只运行到 998，但这是为了测试目的.

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.

我该怎么做才能获得这样的列表:

What could I do to obtain a list like this:

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

推荐答案

内循环中的 COLLECT 子句不影响外循环.所以内循环返回一个结果列表，但外循环中的 DO 子句只是丢弃结果.您应该使用 APPEND 或 NCONC 而不是 DO.如果没有性能问题，通常最好坚持使用 APPEND，即使在这种情况下 NCONC 是安全的.

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)))

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