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Python recursive function calls

I am trying to implement a recursive function and ran into some difficulties, I will appreciate your thoughts. As an example, try creating a function named sliding

that does this

sliding("python", 2)
["py", "yt", "th", "ho", "on"]

That is, for the selected integer, we slide along the string, grabbing substrings of the corresponding length, and then returning them all in the list.

Now that I can (stupidly) try to define this recursively:

def sliding(string,k):
  return s if len(string)==k else [string[:k]].append(sliding(string[1:],k))

It will fail , mainly because it list.append()

happens in-place and returns None

. So my question is, is there a way to make such a recursive function even though there are many Python methods in practice?

Here's the best I have

def sliding(s,k):
    if len(s)==k:
        return s
    else:
        temp = [s[:k]]
        temp.append(sliding(s[1:],k) ) 
        return temp

The result is

sliding("python",k=2)
['py', ['yt', ['th', ['ho', 'on']]]]

which is obviously not exactly the desired way out, but is in the right direction. What other ways could there be? Thanks for your thoughts.

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5 answers


Use an operator +

to get a new concatenated list:



def sliding(s, k):
    if len(s) < k: return []
    else: return [s[:k]] + sliding(s[1:], k)
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Solution without recursion, just a little play on the slice syntax .



def sliding(s, i):
    return [s[n:n+i] for n in xrange(len(s)-i+1)]

assert sliding("python", 2) == ["py", "yt", "th", "ho", "on"]
assert sliding("python", 3) == ["pyt", "yth", "tho", "hon"]
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How about this?

def sliding(string, k):
    return [string[i:i+k] for i in range(len(string)-k+1)]
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Here are the iterative and recursive versions:

def sliding(s, window=2):
    for ind in range(len(s) - (window - 1)):
        yield s[ind:ind+window]


def sliding_recursive(s, window=2, ind=0):
    if ind > len(s) - window:
        return []
    strings = [s[ind: ind+window]] + sliding_recursive(s, window, ind+1)
    return strings


>>> list(sliding('python'))
['py', 'yt', 'th', 'ho', 'on']
>>> list(sliding('python', window=3))
['pyt', 'yth', 'tho', 'hon']

>>> sliding_recursive('python')
['py', 'yt', 'th', 'ho', 'on']
>>> sliding_recursive('python', window=3)
['pyt', 'yth', 'tho', 'hon']
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I would suggest:

temp.extend(sliding(s[1:],k) )

instead of adding as you will be getting a lot of objects with change.

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