CODEWITHSUNDEEP
lispcommon-lispbit-manipulation
Amit Chahar
Amit Chahar

Reputation: 2619

logical operations not happening in sbcl lisp

I want to make program to multiply two numbers by add and shift method. I have written this code in sbcl lisp.

(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)))

but the value of res, num2, num1, end-bit variables remains same throughout the program.I think that the logical and bitwise operations are not happening.What's the problem.

Upvotes: 0

Views: 139

Answers (1)

mike3996
mike3996

Reputation: 17517

None of the functions + or ash do their thing in-place, meaning you have to set that result back to the variable.

So you want to update a var, do it like this:

(setf num2 (ash num2 -1))

For increments and decrements there's an in-place variant called incf:

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

Upvotes: 5

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