Common Lisp - 如何求和用戶輸入

Question

我想採取一系列用戶輸入整數，然後求和輸入。舉例來說，如果用戶輸入：

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

6 

這是到目前爲止我的代碼：

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

Answer 1

這個例子實際上是複雜得多，它應該是，因爲它需要多個事情（循環，閱讀輸入和積累）。我將給你兩個解決方案，一個是簡單的方法，另一個是我親自做的。首先最簡單的方法：

(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 

更高級的版本，這是我會怎麼做：

(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 

編輯：以前該停止在新行的版本。

(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)))