How do I get my nested loop function to iterate through the entire (at least 2) list variables and append to a new list output?

Question

I'm trying to write a function that takes 2 list variables i.e. first_names and last_names.

I'm using a nested for loop in my function to iterate through both lists and append the values to return a new 'combined' variable list.

The function takes the two list parameters but iterates through just the first index value [0] of each list and outputs that - the loop ends.

first_names = ["Dave", "James", "Steve"]

last_names = ["Smith", "Jones", "Jackson"]

def NameCombine(first_names,last_names):
    combined = []
        for first in first_names:
            for last in last_names:
                combined.append(first+last)
                return combined

print(NameCombine(first_names,last_names))

Expected output: DaveSmith, JamesJones, SteveJackson

Actual output: DaveSmith

I'm expecting a new combined list of both the first and last name at each index.

But it's returning the first two values of each list and then the loop ends.

Think about where you are returning from the function. How many elements have you iterated over at that point? — molbdnilo, Jul 04 '19 at 13:04
Possible duplicate of [How to concatenate element-wise two lists in Python?](https://stackoverflow.com/questions/19560044/how-to-concatenate-element-wise-two-lists-in-python) — Georgy, Jul 04 '19 at 13:04
Apologies all - left the while condition in the when i was experimenting and forgot to delete before copying over. Edited and corrected now. — DJoseph1, Jul 04 '19 at 13:06

score 2 · Accepted Answer · answered Jul 04 '19 at 13:20

2

You can combine them with zip within comprehension:

def NameCombine(first_names,last_names):
    return [a+b for a, b in zip(first_names, last_names)]

answered Jul 04 '19 at 13:20

zipa

27,316
6
40
58

This achieves the target output and props to @Pasan Chamikara for his answer which includes formatting too. I'm still trying to figure out if my original code can achieve the same output using a nested loop. – DJoseph1 Jul 04 '19 at 13:39
You got the logic right but the indentation was missing. Although the answer from @Pasan will work, but what this solution is more elegant and pythonic way to achieve the same result in a single line of code – Varun Verma Jul 04 '19 at 16:06

score 1 · Answer 2 · answered Jul 11 '19 at 17:54

Since most of them here answer specifically to your question. I would like to add the other way of achieving the solution to this problem as below,

here I used map and add from operator module

list(map(operator.add, first_names, last_names))
# ['DaveSmith', 'JamesJones', 'SteveJackson']

score 0 · Answer 3 · answered Jul 04 '19 at 13:08

0

You may try with this code.

first_names = ["Dave", "James", "Steve"]

last_names = ["Smith", "Jones", "Jackson"]

def NameCombine(first_names, last_names):
    combined = []
    for i in range(0, len(first_names)):
        if last_names[i] != None:
            combined.append(first_names[i] + " " + last_names[i])
        else:
            combined.append(first_names[i])
    return combined

print(NameCombine(first_names,last_names))

answered Jul 04 '19 at 13:08

Pasan Chamikara

715
9
21

Thanks @Pasan Chamikara. While this does achieve my target output, I'm still trying to figure out how to achieve the same output using a nested loop. – DJoseph1 Jul 04 '19 at 13:20
@DJoseph1 - A loop cycling three times nested in a loop also cycling three times yields a total of nine cycles, i. e. nine resulting values, while you only want three resulting values. Therefore, a nested loop cannot achieve what you want. – Armali Jul 04 '19 at 13:26

score 0 · Answer 4 · answered Jul 04 '19 at 16:09

The pythonic and list comprehension is my preferred way. But if you really wanted a solution using a nested loop, this would work for you. Just used your code and added an additional condition to only print the desired result.

The results from this solution are: ['DaveSmith', 'JamesJones', 'SteveJackson']

first_names = ["Dave", "James", "Steve"]

last_names = ["Smith", "Jones", "Jackson"]

def NameCombine(first_names,last_names):
  combined = []
  for first in first_names:
    for last in last_names:
      if first_names.index(first) == last_names.index(last):
        combined.append(first+last)
  return combined

print(NameCombine(first_names,last_names))

How do I get my nested loop function to iterate through the entire (at least 2) list variables and append to a new list output?

4 Answers4