我正在尝试将用 Python 编写的递归问题(第四个问题 here 有关详细信息,请参阅其 repo 页面)转换为(通用)Lisp
这是我为了可读性稍微编辑过的 Python 代码:
def coin(target,coins,res):
# Default output to target
min_coins = target
# Base Case
if target in coins:
res[target] = 1
return 1
# Return a known result if it happens to be greater than 1
elif res[target] > 0:
return res[target]
else:
# for every coin value that is <= than target
for i in [c for c in coins if c <= target]:
num_coins = 1 + coin(target-i,coins,res)
# Reset Minimum if we have a new minimum
if num_coins < min_coins:
min_coins = num_coins
res[target] = min_coins
return min_coins
target = 14
coins = [1,3,5]
res = [0]*(target+1)
print(coin(target,coins,res))
# => returns 4 ( 1 x 1, 1 x 3, 2 x 5)
这是我写的 Lisp 代码:(defun coin (target coins res)
(let ((min_coins target))
(if (some (lambda (x) (= target x)) coins)
(progn
(setf (aref res target) 1)
1)
(if (> (aref res target) 0)
(aref res target)
(dolist (i (remove-if-not (lambda (x) (<= x target)) coins))
(let ((num_coins (1+ (coin (- target i) coins res))))
(when (< num_coins min_coins)
(setf min_coins num_coins)
(setf (aref res target) min_coins))
min_coins))))))
(coin 14 '(1 3 5) (make-array 15 :initial-element 0) )
当它被执行时,它会因为这个错误而停止:The value
NIL
is not of type
NUMBER
如何安排它才能正确运行?更新:
后
(trace coin)
这是输出:CL-USER> (coin 14 '(1 3 5) (make-array 15 :initial-element 0) )
0: (COIN 14 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
1: (COIN 13 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
2: (COIN 12 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
3: (COIN 11 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
4: (COIN 10 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
5: (COIN 9 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
6: (COIN 8 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
7: (COIN 7 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
8: (COIN 6 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
9: (COIN 5 (1 3 5) #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))
9: COIN returned 1
9: (COIN 3 (1 3 5) #(0 0 0 0 0 1 2 0 0 0 0 0 0 0 0))
9: COIN returned 1
9: (COIN 1 (1 3 5) #(0 0 0 1 0 1 2 0 0 0 0 0 0 0 0))
9: COIN returned 1
8: COIN returned NIL
; Evaluation aborted on #<TYPE-ERROR expected-type: NUMBER datum: NIL>.
最佳答案
正确性
您的代码失败,因为您的 dolist
返回 nil
.
你需要更换
(dolist (i (remove-if-not (lambda (x) (<= x target)) coins)) ...)
和(dolist (i (remove-if-not (lambda (x) (<= x target)) coins) min_coins) ...)
效率您正在使用
[]
在 Python 中创建免费列表在 for
循环并在 Lisp 中使用 remove-if-not
在 dolist
.众所周知的“足够聪明的编译器”应该能够消除它,但 Python 3 不够聪明,SBCL 也不够聪明。
风格
代码可读性很重要。我建议修改您的代码:
def coin(target,coins,res):
# Default output to target
# Base Case
if target in coins:
res[target] = 1
return 1
# Return a known result if it happens to be greater than 1
if res[target] > 0:
return res[target]
# for every coin value that is <= than target
min_coins = target
for i in target:
if i <= target:
num_coins = 1 + coin(target-i,coins,res)
# Reset Minimum if we have a new minimum
if num_coins < min_coins:
min_coins = num_coins
res[target] = min_coins
return min_coins
和(defun coin (target coins res)
(cond ((find target coins)
(setf (aref res target) 1))
((plusp (aref res target))
(aref res target))
(t
(let ((min-coins target))
(dolist (i coins min-coins)
(when (<= i target)
(let ((num-coins (1+ (coin (- target i) coins res))))
(when (< num-coins min-coins)
(setf min-coins num-coins)
(setf (aref res target) min-coins)))))))))
关于python - 将递归问题代码从 Python 转换为 Common Lisp,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/68137109/