具有yield的递归函数不返回任何内容 [英] Recursive function with yield doesn't return anything

问题描述

我正在尝试创建用于置换目的的生成器.我知道还有其他方法可以在Python中执行此操作，但这是其他方法.不幸的是，我无法产生这些价值.你能帮忙吗?

I am trying to create a generator for permutation purpose. I know there are other ways to do that in Python but this is for something else. Unfortunately, I am not able to yield the values. Can you help?

def perm(s,p=0,ii=0):
    l=len(s)
    s=list(s)
    if(l==1):       
        print ''.join(s)
    elif((l-p)==2):
        yield ''.join(s)
        yield ''.join([''.join(s[:-2]),s[-1],s[-2]])
    else:
        for i in range(p,l):
            tmp=s[p]
            s[p]=s[i]
            s[i]=tmp        
            perm(s,p+1,ii)

推荐答案

您的行perm(s,p+1,ii)并没有执行任何操作，实际上:就像键入

Your line perm(s,p+1,ii) doesn't do anything, really: it's just like typing

>>> perm("fred")
<generator object perm at 0xb72b9cd4>

但是，如果您从该通话中获得收益，即

If you yield from that call, though, i.e.

        for subperm in perm(s, p+1, ii):
            yield subperm

那么你会得到

>>> list(perm("abc"))
['abc', 'acb', 'bac', 'bca', 'cab', 'cba']
>>> list(perm("abcd"))
['abcd', 'abdc', 'acbd', 'acdb', 'adbc', 'adcb', 'bacd', 'badc', 'bcad', 'bcda', 'bdac', 'bdca', 'cabd', 'cadb', 'cbad', 'cbda', 'cdab', 'cdba', 'dabc', 'dacb', 'dbac', 'dbca', 'dcab', 'dcba']

>>> len(_)
24
>>> len(set(perm("abcd")))
24

看起来还可以.除此之外，我还没有测试过代码.

which looks okay. I haven't tested the code beyond that.

顺便说一句，您可以将s[i]和s[p]与s[i], s[p] = s[p], s[i]交换；不需要tmp变量.

BTW, you can swap s[i] and s[p] with s[i], s[p] = s[p], s[i]; no need for a tmp variable.

PS:现在您不处理一个字符的情况.

PS: right now you don't handle the one-character case.

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