[英]How to stop the stack from overflowing when generating permutations in python
I'm trying to generate permutations in python without using itertools
. 我试图在不使用
itertools
情况下在python中生成排列。 This is my code thus far: 到目前为止,这是我的代码:
def generatePermutations(minVal, maxVal, arrayLength, depth = -1, array = []):
if(depth == -1): # set all values to minVal initially
for i in range(arrayLength):
array.append(minVal)
depth += 1
generatePermutations(minVal, maxVal, arrayLength, depth, array) # recurse
elif depth < arrayLength:
a.append(array[:]) # a is a list declared above the function
current = 0
while current <= depth:
if array[current] < maxVal:
array[current] += 1
break
else:
if current < depth:
array[current] = minVal
else:
depth += 1
array[current] = minVal
if depth < arrayLength:
array[depth] += 1
break
current += 1
generatePermutations(minVal, maxVal, arrayLength, depth, array)
The function works for a small enough set of numbers. 该功能适用于足够小的数字。 For example,
generatePermutations(1,2,2)
populates list a
with the following: 例如,
generatePermutations(1,2,2)
使用以下内容填充列表a
:
[1, 1]
[2, 1]
[1, 2]
[2, 2]
But, when I try to create permutations of an array of length 9 ( generatePermutations(1,9,9)
), then I run into a stack overflow error long before the function is finished. 但是,当我尝试创建长度为9的数组(
generatePermutations(1,9,9)
)的排列时,我会在函数完成之前很久就遇到堆栈溢出错误。 Is there any way of preventing this? 有没有办法阻止这种情况?
I did a little testing, and I found that the way your function is set up, it calls itself for every single permutation . 我做了一些测试,我发现你的功能设置方式,它自称为每一个排列 。 As in, the recursion depth is the same as the number of permutations generated so far.
同样,递归深度与到目前为止生成的排列数相同。 When you try to do
generatePermutations(1,9,9)
, Python tries to recurse to 9!=362880
levels deep, which is far too deep (it's limited to 1000
). 当你尝试执行
generatePermutations(1,9,9)
,Python会尝试递归到9!=362880
深度,这太深了(它被限制为1000
)。
Instead, refactor your code so that you iterate over each element in a
, appending the current digit, and do this in a loop for each digit. 相反,重构您的代码,以便迭代
a
每个元素,附加当前数字,并在每个数字的循环中执行此操作。 This way, the recursion will only have to go 9 levels deep. 这样,递归只需要达到9级深度。
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