如何在嵌套循环中模拟收集

Question

我想创建一个 (a . b) 对 1 < a < b <= n 到 n 的列表，例如n = 5:

((2 . 3) (2 . 4) (3 . 4) (2 . 5) (3 . 5) (4 . 5))

（对的顺序并不重要。）

我想出了代码

(defun create-pairs (upper-bound)
  (loop for i from 3 to upper-bound
        for j from 2 to (1- upper-bound)
          collecting (cons j i)))

但这并不符合我的要求

* (create-pairs 5)
((2 . 3) (3 . 4) (4 . 5))

因为循环同时递增。

因此我尝试了这个

(defun create-pairs (upper-bound)
  (loop for i from 3 to upper-bound do
    (loop for j from 2 to (1- upper-bound)
      collecting (cons j i))))

结果：

* (create-pairs 5)

NIL

我不记得在哪里，但我读到不能像我第二次尝试那样在结构中使用 collecting。

那么如何得到我想要的结果呢？用loop for不就可以解决吗？

Answer 1

你快到了 - 你只需要 accumulate the results of the inner loop:

(defun create-pairs (upper-bound)
  (loop for i from 3 to upper-bound nconc
    (loop for j from 2 below i
      collect (cons j i))))
(create-pairs 5)
==> ((2 . 3) (2 . 4) (3 . 4) (2 . 5) (3 . 5) (4 . 5))

如何在嵌套循环中模拟收集

How to simulate collecting in nested loop for

lisp

common-lisp