Question

帮助！我正在尝试编写一个函数，它取两个输入中和之间所有数字的总和。

到目前为止，我有，

(define (sum-between x y)    
  (cond    
    [(= x y) x]
    [((- x y) 0) 0]
    [else (+ (+ x y) (sum-between (sub1 x) (sub1 y)))]))

这应该返回：

(check-expect (sum-between 0 2) 3)

(check-expect (sum-between -1 1) 0)

(check-expect (sum-between 7 7) 7)

(check-expect (sum-between 1 10) 55)

我不确定如何调用递归情况，因此它不会在无限循环中运行。有什么建议？谢谢！

Answer 1

使用内置程序怎么样？这更容易：

(define (sum-between x y)
  (apply + (range x (add1 y))))

但我想你想从头开始实现这个。这里的主要思想是其中一个值必须越来越接近另一个，你的错误是你正在递减它们的两个。试试这个：

(define (sum-between x y)    
  (cond    
    [(= x y) x]
    [else (+ x (sum-between (add1 x) y))]))

Answer 2

for / sum也可以在这里使用：

(define (sum-between x y)
  (for/sum ((i (in-range x (add1 y))))
    i))

(sum-between 0 2)
(sum-between -1 1)
(sum-between 7 7)
(sum-between 1 10)

输出：