Question

我想让程序通过add和shift方法将两个数相乘。我已经在sbcl lisp中编写了这段代码。

(defun calculator (num1 num2)
  (write-line "In the function")
  (let ((res 0))
    (loop for lpr from 0 to 63
          do (let ((end-bit (logand num2 1)))
               (format t "res is : ~a. ~%" num2)
               (if (= end-bit 1)
                   (+ res num1))
               (ash num2 -1)
               (ash num1 1)
               (format t "after ash ~a ~%"num2)))
    (format t "result is ~a.~%" res)))

(let ((num1 (progn
              (write-line "Enter first number: ")
              (finish-output)
              (read)))
      (num2 (progn
              (write-line "Enter second number: ")
              (finish-output)
              (read))))
  (if (or (= num1 0) (= num2 0))
      (write-line "result is 0.0")
    (calculator num1 num2)))

但res，num2，num1，end-bit变量的值在整个程序中保持不变。我认为逻辑和按位操作没有发生。问题是什么。

Answer 1

所有函数+或ash都没有就地，这意味着你必须将结果设置回变量。

所以你想要更新一个var，就这样做：

(setf num2 (ash num2 -1))

对于增量和减量，有一个名为incf的就地变体：

(incf res num1)  ; (setf res (+ res num1))

sbcl lisp中没有发生逻辑运算

1 个答案: