Trying to generate subset of a string using recursion method

Question

Trying to implement an algorithm using recursion in python. looks like something is missing which I am not able to debug.

My approach is to have two branches of recursion and passing an element at every recursion. details below:

## input pattern : "ab"
## output pattern : ["", "a", "b", "ab"]

Recursion tree will look like following for this pattern

#           "ab" [ROOT]
#                   |
#           -a           +a
#           |             |
#       -b      +b    -b    +b
# =>    ""      "b"     "a"     "ab"

my existing code is below: it is not working as expected.

def gen_subset(slist):
    def helper(slist,i,temp,out):
        if len(slist) == i:
            out.append(temp)
            return()
        else:
            helper(slist,i+1,temp,out)
            temp.append(slist[i])
            helper(slist,i+1,temp,out)

    out = []
    helper(slist,0,[],out)
    return out

s = "ab"
print (gen_subset([c for c in s]))

this code produces the wrong result.

Output

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

am I missing anything here?

Answer 1

Change temp.append(slist[i]) to temp = temp + [slist[i]].

This is happening because temp.append() modifies the temp variable in-place.
Instead we need to pass a copy of temp to the next recursion call.