set_permutations_combinations.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Tue Apr  6 14:44:50 2021

@author: mahan
"""

def factorial(number):
    def is_negative(number):     
        if number < 0:
            raise ValueError("factorial cannot be calculated for a negative number!")
    try:
        is_negative(number)
    except ValueError as err:
        print(err)
    else:
        return 1 if number <= 1 else number * factorial(number - 1)

def get_ith_pemutations(list_char = ['1','2','a','b'], i=0):
    l_char = list_char.copy()
    permutation = []
    while l_char:
        count = factorial(len(l_char)-1)
        select = i//count
        permutation.append(l_char[select])
        del(l_char[select])
        i -= count * select
    return permutation
                
def getElementFromEachList(l=[['1', '2'],['a', 'b']]):
    if len(l) == 1:
        return [[x] for x in l[0]]
    result = []
    for listElements in getElementFromEachList(l[1:]):
        for fElement in l[0]:
            result.append([fElement] + listElements)
    return result

def getPermutations(l=['a','b','c'], r=2):
    results = []
    def getPermutationsRecr(l, com = []):
        if len(com) == r:
            results.append(com)
            return
        for e in l:
            li = list(l)
            li.remove(e)
            getPermutationsRecr(li, com + [e])
    
    getPermutationsRecr(l)
    return results

def getCombinations(l=['a','b','c'], r=2):
    results = []
    def getCombinationsRecr(l, com = []):
        if len(com) == r:
            results.append(com)
            return

        for i, e in enumerate(l):
            getCombinationsRecr(l[i+1:], com + [e])
    
    getCombinationsRecr(l)
    return results

print(getCombinations())