Common Lisp - 如何求和用户输入

Question

我想采取一系列用户输入整数，然后求和输入。举例来说，如果用户输入：

1 <return> 
2 <return> 
3 <return> 
<return> 

6 

这是到目前为止我的代码：

(defun stuff() 
    (format t "Enter a number: ") 
    (let ((n (read))) 
    (+ n))) 

Answer 1

这个例子实际上是复杂得多，它应该是，因为它需要多个事情（循环，阅读输入和积累）。我将给你两个解决方案，一个是简单的方法，另一个是我亲自做的。首先最简单的方法：

(defun stuff (&optional (acc 0)) ; An optional argument for keeping track of the sum. 
    (if (y-or-n-p "Do you want to continue?") ; Ask if they want to continue 
     (progn (format t "Enter a number: ") ; If they say yes we need to ask them for the 
      (stuff (+ acc (read))))  ; number, read it, add it to the sum, and 
              ; continue. We need progn because we need to 
              ; execute two pieces of code (format and stuff) in the if 
     acc)) ; If they say no, then return the total sum 

更高级的版本，这是我会怎么做：

(defun stuff() 
    (loop while (y-or-n-p "Do you want to continue?") ; while they want to continue 
     do (format t "Enter a number: ")   ; print the prompt 
     sum (parse-integer (read-line))))   ; read the line, parse the integer, and sum it 

编辑：以前该停止在新行的版本。

(defun stuff (&optional (acc 0)) 
    (let ((line (read-line))) 
    (if (string= line "") ; If this line is the empty string 
     acc    ; return the sum 
     (stuff (+ acc (parse-integer line)))))) ; otherwise recur and sum the number on the line 

(defun stuff() 
    (loop for line = (read-line) 
     until (string= line "") 
     sum (parse-integer line)))