How to return a list of all possible numbers using recursion-CodePudding

I am learning about recursion in python currently but there is this one exercise that I simply cannot figure out in the following code I have two functions and using recursion i print all possible numbers from a given list, but for the life of me I cannot get all the possible numbers into a single list. Any tips on how to do this would really be appreciated.

def find_possible_strings(char_set, n):
    k = len(char_set)
    return possible_stringsRec(char_set, "", k, n)


def possible_stringsRec(char_set, prefix, k, n):
    if n == 0:
        print(prefix)
        return

    for i in range(k):
        newPrefix = prefix   char_set[i]
        possible_stringsRec(char_set, newPrefix, k, n - 1)

char_set = ['a','b']
possible_strings = find_possible_strings(char_set, 2)
print(possible_strings)

Just to specify I am not simply looking for a solution but rather an actual explanation of how this could be done in python if anyone would be so kind.

I am just getting the result as follows:

aa
ab
ba
bb
None

and what I want is for all those values to be stored inside a single list so essentially:

['aa', 'ab', 'ba', 'bb']

CodePudding user response：

Add a results=[] keyword argument to the possible_stringsRec function, and instead of printing out the values within the function, append to the results list:

def find_possible_strings(char_set, n):
    k = len(char_set)
    return possible_stringsRec(char_set, "", k, n)


def possible_stringsRec(char_set, prefix, k, n, results=[]):
    if n == 0:
        results.append(prefix)
        return

    for i in range(k):
        newPrefix = prefix   char_set[i]
        possible_stringsRec(char_set, newPrefix, k, n - 1)
    return results

char_set = ['a','b']
possible_strings = find_possible_strings(char_set, 2)
print(possible_strings)

Output:

['aa', 'ab', 'ba', 'bb']

The explanation is pretty straightforward: the results list starts out as empty when the first function call happens, and whenever a value get appended to it, the value remains inside the list for the next function call.

This demonstrates why:

def func(lst=[]):
    lst.append(1)
    print(lst)

func()
func()
func()

Output:

[1]
[1, 1]
[1, 1, 1]