Combo challenge: my code works in workspaces but not in challenge

Question

I got from forum that this challenge should be solved like this:

def combo(my_list, my_string):
  new_list = []

  for index, item in enumerate(my_list):
    tup = item, my_string[index]
    new_list.append(tup)

  return new_list

But I didn't pay needed attention to enumerate func and solved it with more difficult code:

def combo(my_list, my_string):
  if len(my_list) == len(my_string):
    new_list = []
    count = 0
    for item in my_string:
      my_list.append(item)
    while True: 
      new_list.append(tuple(my_list[count::len(my_string)]))  
      count += 1
      if count == len(my_string):
        break
    print(new_list)

but it works only in workspaces and not in challenge. It says: "Didn't get the right output. For example, expected (10, 'T') as the first item, got (10, 'T') instead." Why? I overthinked the code to get it solved, so if anybody knows I will appreciate an answer.

p.s: also appriciate for any feedback of my code

Answer 1 · 2015-11-19T18:10:52Z

November 19, 2015 6:10pm

You are not crazy: your code returns the correct list of tuples results... but with side-effects.

By using the statement my_list.append() the code is modifying one of the input parameters. It is rarely a good practice to modify an input unless that is the purpose of the function. Python passes arguments by reference so my_list is equivalent to the global list passed in.

One fix for your code would be to operate on a copy of my_list:

def combo(my_list, my_string):
    if len(my_list) == len(my_string):
        new_list = []
    my_list_copy = my_list[:] #<-- make a copy
    count = 0
    for item in my_string:
        my_list_copy.append(item) #<-- operate on copy
    while True: 
        new_list.append(tuple(my_list_copy[count::len(my_string)]))          
        count += 1
        if count == len(my_string):
            break
    return new_list

How I would improve your code:

def combo(my_list, my_string):
    # commented out conditional  init. Challenge assumes equal length
    #if len(my_list) == len(my_string):
    #    new_list = []
    new_list = []
    my_list_copy = my_list[:] #<-- make a copy
    for item in my_string:
        my_list_copy.append(item) #<-- operate on copy

    # replace 'while True / count / break' with 'for range'
    # count = 0
    # while True: 
    for count in range(len(my_string)):
        new_list.append(tuple(my_list_copy[count::len(my_string)]))      
        # count += 1
        # if count == len(my_string):
        #     break
    return new_list

Ultimately, the challenge can be solved without loops by using zip:

def combo(my_list, my_string):
    new_list = list(zip(my_list, my_string))
    return new_list

Answer 2 · 2015-11-20T12:18:52Z

November 20, 2015 12:18pm

Thank's Chris. Using a 'for range' loop is a nice idea, I have not thought about this before. This is why I should get into basics of python more carefully and find out what exactly iterables and iterators are and how they can be used with loops. And thank's a lot about 'side-effects', now I know more how to handle with arguments , if they vary inside of a function.

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Iskander Ismagilov

Iskander Ismagilov

Combo challenge: my code works in workspaces but not in challenge

2 Answers

Chris Freeman

Chris Freeman

Chris Freeman

Chris Freeman

Iskander Ismagilov

Iskander Ismagilov