[编程交流] 电线/线路输入长度va

;; simple math string parser
;; arguments:
;; expr - the string in form: "A Space Symbol Space B"
;; where A and B the strings wich are represents a numeric values
;; i.e. "100 - 50", "100 / 50" etc
(defun expresseval  (expr / lst sym)
 (setq lst (read (strcat "(" expr ")"))
sym (cadr lst))
     (cond ((/= (length lst) 3)
     nil)
    ((equal sym '+)
     (+ (car lst) (caddr lst)))
    ((equal sym '-)
     (- (car lst) (caddr lst)))
    ((equal sym '*)
     (* (car lst) (caddr lst)))
    ((equal sym '/)
     (/ (car lst) (caddr lst)))
    (T nil)
    )
     )
Usage: (setq plwid (expresseval "25.64 - 15.45"))