unhashable type: 'list' when creating a combination function with Python-CodePudding

I'm currently trying to create a combination function that has the equal function to the itertools.combinations.

The following code is the code that I am working on:

def my_combs(s, num):
    if num == 0:
        return [[]]
    res = []
    for i in range(0, len(s)):
        m = s[i]
        rems = s[i   1:]
        for p in my_combs(rems, num-1):
            res.append([m] p)
    return res

from itertools import combinations
test = [1, 2, 3, 4]
print(set(combinations(test, 2)))
print(set(my_combs(test, 2)))

The result I want to get is

{(1, 2), (1, 3), (1, 4), (2, 3), (3, 4), (2, 4)}

But I keep getting an error that says unhashable type: 'list'

And then, when I change the print(set(my_combs(test, 2))) to print(my_combs(test, 2)), I get

[[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]

While the elements of the results seem to be the same,
I want to get the results in tuples (i.e { ( ) } ), instead of lists (i.e [ [ ] ]).

Is there any way for me to solve the error and get the results I want with the condition of unchanging any of the print statements?
(i.e I want to have the print statement as print(set(my_combs(test, 2))) and change only the my_combs() function)

CodePudding user response：

If you want tuples instead of lists - just use tuples instead of lists :)

def my_combinations(s, num):
    if num == 0:
        return [()]                              # HERE
    res = []
    for i in range(0, len(s)):
        m = s[i]
        rems = s[i   1:]
        for p in my_combinations(rems, num-1):
            res.append((m,) p)                   # HERE
    return res

from itertools import combinations
test = [1, 2, 3, 4]
print(set(combinations(test, 2)))
print(set(my_combinations(test, 2)))

Note that one-element tuple has to have a comma in it: (m,); without it, (m), it is just something in parentheses.

CodePudding user response：

Item in set should be hashable. List is mutable, Tuple is hashable, So change list to tuple.

Change last line to :

print(set(tuple(item) for item in my_combinations(test, 2)))

CodePudding user response：

You can change the local variable res into a tuple and add tuples to it recursively by changing res.append([m] p) to res = res ((m,) p,):

def my_combinations(s, num):
    if num == 0:
        return (tuple(),)
    res = tuple()
    for i in range(0, len(s)):
        m = s[i]
        rems = s[i   1:]
        for p in my_combinations(rems, num-1):
            res = res   ((m,) p,)
    return res

from itertools import combinations
test = [1, 2, 3, 4]
assert set(combinations(test, 2)) == set(my_combinations(test, 2))