lisp - sbcl lisp 中没有发生逻辑操作

Question

我想编写程序通过加法和移位法将两个数相乘。我用 sbcl lisp 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)))

但是res、num2、num1、end-bit变量的值在整个程序中保持不变。我认为逻辑和位运算没有发生。问题出在哪里。

Answer 1

函数+ 或ash 都不会就地 执行它们的操作，这意味着您必须将该结果设置回变量。

所以你想更新一个变量，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))