Combination iteration

[Glenton]

Hi All

I was writing a bit of code to solve Euler Project #90, and came across the need to iterate through all combinations of a particular list. Any improvements would be most welcome, but I thought a class with an iterator would be appropriate.

Here's the class with a bit of code.

class myComb(object):

    """my interable combinatorial object"""

    def __init__(self,elements,length):

        if len(elements)>length:

            self.length=length

            self.elements=elements

            self.noEls=len(elements)

    

    def comb(self):

        """return the combination for the current index"""

        c=[]

        for i in self.index:

            c.append(self.elements[i])

        return c

    

    def __iter__(self):

        self.index=range(self.length)

        self.index[self.length-1]+=-1

        return self

    

    def next(self):

        if self.index==range(self.noEls-self.length,self.noEls):

            raise StopIteration

        i=self.length-1

        while self.index[i]==self.noEls-self.length+i:

            i+=-1

        self.index[i]+=1

        for j in range(i+1,self.length):

            self.index[j]=self.index[j-1]+1

        return self.comb()

    

    

C=myComb(["a","b","c","d","e"],3)

for i in C:

    print i

This code will print out:
['a','b','c']
['a','b','d']
['a','b','e']
['a','c','d']
['a','c','e']
['a','d','e']
['b','c','d']
['b','c','e']
['b','d','e']
['c','d','e']

It's helpful for Euler Project type things!

[Glenton]

So I feel like a bit of a clown, but the exact same thing can be done with the built-in module itertools.

It has permutations, combinations and product:
Here are the docs

But basic usage is:

In [17]: P=itertools.permutations([1,2,3])

In [18]: for p in P:
   ....:     print p
   ....:     
   ....:     
(1, 2, 3)
(1, 3, 2)
(2, 1, 3)
(2, 3, 1)
(3, 1, 2)
(3, 2, 1)