我是Lisp编程的新手,我想比较一个变量,比如说op与字符+,-,*,/。但是我写的每一种方式都没有显示正确的结果。我该如何重写以下代码:
(defun evaluate(op o1 o2)
(cond
( (string= op `+) (+ o1 o2))
( (string= op `-) (- o1 o2))
( (string= op `*) (* o1 o2))
( (string= op `/) (/ o1 o2))
)
)
输入为:
(print (calculate `(- * + 4 3 2 5)))
因此,我在函数中加入了运算符。不要介意数字。可以肯定的是,这里的calculate函数。
(defun calculate (l)
(if (and (numberp (car l)) (not (null l)) )
(car l)
( evaluate (car l) (calculate (cdr l) )
(calculate (cdr l) ) )
)
)
calculate
编写方式是,这个值的(- (* (+ 4 4) (+ 4 4)) (* (+ 4 4) (+ 4 4)))
值为0。我不知道你期望什么,但是让我们看一下跟踪(SO>
是repl提示符):
SO> (trace evaluate calculate)
(EVALUATE CALCULATE)
SO> (calculate '(- * + 4 3 2 5))
0: (SO::CALCULATE (- * + 4 3 2 5))
1: (SO::CALCULATE (* + 4 3 2 5))
2: (SO::CALCULATE (+ 4 3 2 5))
3: (SO::CALCULATE (4 3 2 5))
3: CALCULATE returned 4
3: (SO::CALCULATE (4 3 2 5))
3: CALCULATE returned 4
3: (SO::EVALUATE + 4 4)
3: EVALUATE returned 8
2: CALCULATE returned 8
2: (SO::CALCULATE (+ 4 3 2 5))
3: (SO::CALCULATE (4 3 2 5))
3: CALCULATE returned 4
3: (SO::CALCULATE (4 3 2 5))
3: CALCULATE returned 4
3: (SO::EVALUATE + 4 4)
3: EVALUATE returned 8
2: CALCULATE returned 8
2: (SO::EVALUATE * 8 8)
2: EVALUATE returned 64
1: CALCULATE returned 64
1: (SO::CALCULATE (* + 4 3 2 5))
2: (SO::CALCULATE (+ 4 3 2 5))
3: (SO::CALCULATE (4 3 2 5))
3: CALCULATE returned 4
3: (SO::CALCULATE (4 3 2 5))
3: CALCULATE returned 4
3: (SO::EVALUATE + 4 4)
3: EVALUATE returned 8
2: CALCULATE returned 8
2: (SO::CALCULATE (+ 4 3 2 5))
3: (SO::CALCULATE (4 3 2 5))
3: CALCULATE returned 4
3: (SO::CALCULATE (4 3 2 5))
3: CALCULATE returned 4
3: (SO::EVALUATE + 4 4)
3: EVALUATE returned 8
2: CALCULATE returned 8
2: (SO::EVALUATE * 8 8)
2: EVALUATE returned 64
1: CALCULATE returned 64
1: (SO::EVALUATE - 64 64)
1: EVALUATE returned 0
0: CALCULATE returned 0
0
最后的0是结果。在这些数字中,只有第一个被触及,我想你也打算使用其他数字。
指出对(calculate (cdr l))
inside的两个调用calculate
始终会产生相同的结果,这将对你有所帮助。
旁注:
'
,而不是反引号。后者有不同的用途。
好的,谢谢您的跟踪。我希望它能够计算列表l中给出的前缀表示法算术表达式。我希望该列表可以通过引用传递,并且所有在第一个调用中完成的cdr都将在第二个调用中看到。