import re
import pandas as pd


def list_comprehension(l1: list):
    """For input list l1 of integers, return list of elements
    whose square is less than sum of all elements"""

    sum_l1 = sum(l1)
    return [elem for elem in l1 if elem * elem < sum_l1]


def nested_list_comprehension(l1: list, l2: list):
    """For input lists l1, l2, return a list of elements common
    to both l1 and l2."""

    # result = []
    # for e1 in l1:
    #     for e2 in l2:
    #         if e1 == e2:
    #             result.append(e1)

    result = [e1 for e2 in l2 for e1 in l1 if e1 == e2]

    return result


def dict_comprehension(l1: list):
    """For input list l1 of integers, return a dictionary with
    keys from l1, and values are squares of keys"""

    result = {e: e * e for e in l1}

    return result


def generator(l1: list, l2: list):
    """For input lists l1 and l2 of integers, return a generator
    that gives all pairings (e1, e2) of element e1 from l1 and
    element e2 of l2."""

    for e1 in l1:
        for e2 in l2:
            yield (e1, e2)


if __name__ == "__main__":
    l1 = [1, 2, 3, 4]
    l2 = [2, 4, 6, 8]

    print("\n-------------------\n")
    print("Inputs:\n")
    print(f"l1: {l1}")
    print(f"l2: {l2}")
    print("\n-------------------\n")

    print("list_comprehension:\n")
    print(f"Desired output: [2, 4]")
    print(f"Output        : {list_comprehension(l2)}")
    print("\n-------------------\n")

    print("nested_list_comprehension:\n")
    print(f"Desired output: [2, 4]")
    print(f"Output        : {nested_list_comprehension(l1, l2)}")
    print("\n-------------------\n")

    print("dict_comprehension:\n")
    print("Desired output: {1: 1, 2: 4, 3: 9, 4: 16}")
    print(f"Output:       : {dict_comprehension(l1)}")
    print("\n-------------------\n")

    print("generator:\n")
    print("Desired output:")
    print("  (1, 2)")
    print("  (1, 4)")
    print("  (1, 6)")
    print("  (1, 8)")
    print("  (2, 2)")
    print("  (2, 4)")
    print("  (2, 6)")
    print("  (2, 8)")
    print("  (3, 2)")
    print("  (3, 4)")
    print("  (3, 6)")
    print("  (3, 8)")
    print("  (4, 2)")
    print("  (4, 4)")
    print("  (4, 6)")
    print("  (4, 8)")
    print("Output:       :")
    for pair in generator(l1, l2):
        print(f"  {pair}")