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some simple recursive lisp programs

2024-07-03 17:12:13   232人阅读

1. Write a procedure count-list to count the number of elements in a list

1 (defun count-list (numbers)
2        (if (null numbers) 0
3          (+ 1 (count-list (rest numbers)))))
4 (print (count-list ‘(1 2 3)))
5 
6 result: 3

 

2. Write a procedure reverse-list to reverse each word in a list of words

1 (defun reverse-list (numbers)
2        (if (null numbers) nil
3          (cons (reverse (first numbers)) (reverse-list (rest numbers)))))
4 (reverse-list ‘("dog" "pan" "tar" "tip" "net"))
5 
6 result: ("god" "nap" "rat" "pit" "ten")

 

3. Write a procedure evenp-list to process a list of numbers, replacing each number by t if it‘s even, and nil if it‘s odd

1 (defun evenp-list (numbers)
2        (if (null numbers) nil
3          (cons (if (evenp (first numbers)) t nil)
4                (evenp-list (rest numbers)))))

(evenp-list ‘(1 2 3 4 5 6 7 8))

 result:

(nil t nil t nil t nil t)

4. Write a procedure max-list to return the maximum element of a list.

1 (defun max-list (numbers)
2        (if (null numbers) 0
3          (if (> (first numbers) (max-list (rest numbers)))
4            (first numbers)
5            (max-list (rest numbers)))))

(max-list ‘(11 13 17 19 2 3 5 7))

should return 19.

 

These three small programs are all recursive. It is interesting to code in lisp. ;)

 

The fourth program is incorrect. There is bug: I assume that all numbers in the list are positive.

The following program is better:

 

1 (defun max-list (numbers)
2        (if (null (rest numbers)) (first numbers)
3          (if (> (first numbers) (max-list (rest numbers)))
4            (first numbers)
5            (max-list (rest numbers)))))

 

 

btw, max is a build-in procedure. so:

1 (defun max-list (numbers)
2        (if (null (rest numbers)) (first numbers)
3          (max (first numbers) (max-list (rest numbers))
4            )))

 

It becomes simpler. ;)

 


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