在公共Lisp中使用循环宏嵌套循环 [英] Nested Loops Using Loop Macro in Common Lisp

问题描述

我正在尝试在CL中实现一个基本的嵌套循环，但是Loop宏正在抵抗这种情况.基本上，我想找到所有可能的3位数字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)))

以上代码出于某种原因返回 NIL .顺便说一句，我意识到我只用到998，但这是出于测试目的.

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)

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

推荐答案

内部循环中的 COLLECT 子句不影响外部循环.因此，内部循环返回结果列表，但是外部循环中的 DO -子句只是丢弃结果.您应该使用 APPEND 或 NCONC 而不是 DO .通常，即使没有性能问题，最好还是坚持使用 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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