combinations with two elements

问题

With python. It's possible to permute elements in a list with this method:

def perms(seq):
    if len(seq) <= 1:
        perms = [seq]
    else:
        perms = []
        for i in range(len(seq)):
            sub = perms(seq[:i]+seq[i+1:]) 
            for p in sub:    
                perms.append(seq[i:i+1]+p)

return perms

if a list is: seq = ['A', 'B', 'C'], the result will be..

[['A', 'B', 'C'], ['A', 'C', 'B'], ['B', 'A', 'C'], ['B', 'C', 'A'], ['C', 'A', 'B'], ['C', 'B', 'A']]

HOW TO modify this method, to make permutations two terms a time? I mean, if the list is: seq = ['A', 'B', 'C']. I wanna receive [['A', 'B'], ['A', 'C'], ['B', 'C'].

I can't do it. I'm trying but I can't. Thanks for the help.

回答1:

Consider using the combinations function in Python's itertools module:

>>> list(itertools.combinations('ABC', 2))
[('A', 'B'), ('A', 'C'), ('B', 'C')]

This does what it says: give me all combinations with two elements from the sequence 'ABC'.

回答2:

Building off your method: Pass in a counter (defaulted to 2) that keeps track of how many more letters to pick. Use this instead of length for the base case.

回答3:

def getCombinations(seq):
    combinations = list()
    for i in range(0,len(seq)):
        for j in range(i+1,len(seq)):
            combinations.append([seq[i],seq[j]])
    return combinations

>>> print(getCombinations(['A','B','C']))
[['A', 'B'], ['A', 'C'], ['B', 'C']]

来源：https://stackoverflow.com/questions/20762574/combinations-with-two-elements