print powerset of a string

Question

I'm trying to write python code to print the powerset of a string, but am running into some bugs. Here's what I've got:

def getperm (string):
    perm = []
    if len(string) == 0:
        perm.append("")
        return perm
    #if len(string) == 1:
    #   perm.append(string)
    #   perm.append("")
    first = string[0]
    print "first = " + str(first)
    rem = string[1:len(string)]
    print "rem = " + str(rem)
    words = getperm(rem)
    for word in words:
        for i in range(len(word)):
            temp = string[0:i] + first + string[i:len(string)]
            print "temp = " + str(temp)
            perm.append(temp)

    return perm

if __name__=="__main__":
    a = "ab"
    mag  = getperm(a)
    print mag

My expected output would be:

['', 'a', 'b', 'ab']

My actual output is:

[]

Can anyone help me figure out what's going on? Is this some nuance of python, or is there a bug in my code? I think my code should be ok -- I'm going off the fifth edition of Cracking the coding interview

Thank you!

You're looking to generate the power set, not permutations. The only permutations of the string `'ab'` are `'ab'` and `'ba'`. — Matt Ball, Sep 28 '12 at 01:34
Also, you really shouldn't name a string `string` (that's the name of a built-in module), and I'm not sure what you're trying to accomplish by calling `str` on objects that are already strings (`first`, `rem`, etc.). — abarnert, Sep 28 '12 at 01:36
@Matt Ball Yes -- good call. Even so, I'm not generating 'ab' or 'ba'. Do you know why? — mythander889, Sep 28 '12 at 01:37
@abarnert Very true -- I renamed string to something different, and removed the redundant str() but am still receiving the errors. Any idea why? — mythander889, Sep 28 '12 at 01:39
Well, obviously those weren't causing your problems; they were just making it harder to read… Meanwhile, you should update the question with the edited version of the code. — abarnert, Sep 28 '12 at 01:42
I changed the title of the question, since the output you are expecting is the powerset. permutations means something different — John La Rooy, Sep 28 '12 at 02:23

score 4 · Accepted Answer · answered Sep 28 '12 at 02:08

4

You're overthinking it

This part is trying to do too much

for word in words:
    for i in range(len(word)):
        temp = string[0:i] + first + string[i:len(string)]
        print "temp = " + str(temp)
        perm.append(temp)

See how simple it really should be

def get_powerset (string):
    perm = []
    if len(string) == 0:
        perm.append("")
        return perm
    #if len(string) == 1:
    #   perm.append(string)
    #   perm.append("")
    first = string[0]
    print "first = " + str(first)
    rem = string[1:len(string)]
    print "rem = " + str(rem)
    words = get_powerset(rem)
    perm.extend(words)
    for word in words:
        perm.append(first+word)

    return perm

if __name__=="__main__":
    a = "ab"
    mag  = get_powerset(a)
    print mag

Now you should be able to make the code look a lot nicer with a little refactoring

answered Sep 28 '12 at 02:08

John La Rooy

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That did it! Thank you. What does extend do that my code wasn't doing? – mythander889 Sep 28 '12 at 02:29
@mythander889, `.extend` just loads the result of the previous level into the current result. – John La Rooy Sep 28 '12 at 05:06
I tried running this code, it does not print the powers, it just prints all the alphabets of a string. – thenakulchawla Oct 12 '16 at 07:41
@nakulchawla09, I'm not sure what you mean by "powers", but it is probably different to a powerset – John La Rooy Oct 12 '16 at 23:11
1

I am sorry for the typing mistake, I meant power set only. For this program I was somehow getting the output `[" ", "a", "b"]` , I was thinking on the terms of getting an output like `[" ", "a", "b", "ab", "ba"]` – thenakulchawla Oct 12 '16 at 23:36

score 3 · Answer 2 · answered Sep 28 '12 at 01:38

3

Is this what you want?

import itertools as it

def func(s):
    for i in range(len(s)+1):
        for combo in it.combinations(s,i):
            yield "".join(combo)

print list(func("abc"))

answered Sep 28 '12 at 01:38

mgilson

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I believe that would do it, but I'm looking to implement the solution using recursion. Thank you! – mythander889 Sep 28 '12 at 01:39
was searching for combinations with replacement. Saw this, went to docs.. `combinations_with_replacement()` -- actually a method there! :') – Somjit Nov 07 '15 at 06:32
Is there a way to do what you do without using the package itertools? – thenakulchawla Oct 12 '16 at 07:42

score 2 · Answer 3 · answered Jul 01 '18 at 00:59

2

Here's a refactored iterative solution without the itertools module:

def powerset(s):
    a = ['']
    for i,c in enumerate(s):
        for k in range(2**i):
            a.append(a[k]+c)
    return a

answered Jul 01 '18 at 00:59

orwinmc

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score 1 · Answer 4 · answered Sep 28 '12 at 01:43

1

There are a method for permutations:

>>> import itertools
>>> chars = "ABCD"
>>> perms = list(itertools.permutations(chars))
>>> print(perms)
[('A', 'B', 'C'),
 ('A', 'C', 'B'),
 ('B', 'A', 'C'),
 ('B', 'C', 'A'),
 ('C', 'A', 'B'),
 ('C', 'B', 'A')]

answered Sep 28 '12 at 01:43

Pedro Lacerda

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score 0 · Answer 5 · answered Sep 28 '12 at 01:45

Have you tried tracing through what your algorithm actually does?

getperm('ab'):
  first, rem = 'a', 'b'
  words = getperm('b')
    first, rem = 'b', ''
    words = getperm('')
    words = ['']
    for word in words:
      for i in range(len(word)):
        pass # only called on '', so doesn't matter
    return []
  words = []
  for word in words:
    pass # only called on [], so doesn't matter

So, there's no nuance of Python here; your algorithm returns the empty list in O(N) steps, and you've coded that algorithm properly in Python.

(Instead of tracing it by hand, of course, you can add some more useful print statements and see what each step is actually doing.)

It probably wasn't the algorithm you wanted, but you'll need to tell us what you were trying to do. Are you, e.g., porting some pseudocode from Hoare into Python? If so, what's the pseudocode?

score 0 · Answer 6 · answered Dec 16 '16 at 01:07

0

Use powerset from more_itertools:

>>> import more_itertools

>>> ["".join(p) for p in list(more_itertools.powerset("ab"))]
['', 'a', 'b', 'ab']

This powerset is a convenience function directly implemented from the itertools recipes.

answered Dec 16 '16 at 01:07

pylang

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print powerset of a string

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