How to create new list by adding existing lists using for loop

Question

I would like to join word from one nested list together with the word "and" in between. For example:

l1 = [Patrick, Peter, Jerome], [Lisa, Karen, Alice]

l2 should be: [Patrick and Lisa, Peter and Karen, Jerome and Alice]

The code should not make a difference whether there are 2,3 or whatever amount of lists in the nested list.

I have tried iterating over the range of lists in the variable and then append the result to a new variable. However, the outcome is just the first combination or the results end after the second combination.

def names(one):
    newlist = []
    x = 0
    for y in range(len(one)):
        newlist.append(one[x][y] + " and " + one[x+1][y])
        x += 1
        return newlist

['Patrick and Lisa']

is what i got untill now. I know it has something to do with the iteration but since i am pretty new to python i got stuck.

lmiguelvargasf · Accepted Answer · 2019-09-30 18:37:25Z

2

It seems that l1 is a tuple of two elements. Use the following:

l2 = []

for name_a, name_b in zip(l1[0], l1[1]):
    l2.append(name_a + ' and ' + name_b)

If you are using Python 3.6+ you can use an f-string:

l2 = []

for name_a, name_b in zip(l1[0], l1[1]):
    l2.append(f'{name_a} and {name_b}')

Also keep in mind that you can also use a list-comprehension.

Python 3.6+

l2 = [f'{name_a} and {name_b}' for name_a, name_b in zip(l1[0], l1[1])]

l2 = [name_a + ' and ' + name_b for name_a, name_b in zip(l1[0], l1[1])]

Note: If you are not allowed to use zip use range and use index notation as follows:

l2 = []

for i in range(len(l1[0])):
    name_a = l1[0][i]
    name_b = l1[1][i]
    l2.append(name_a + ' and ' + name_b)

Update: based on the previous comments, you should use the following:

l2 = []

for i in range(len(l1[0])):
    l2.append(' and '.join(e[i] for e in l1))

edited Sep 30, 2019 at 18:37

answered Sep 30, 2019 at 18:17

lmiguelvargasf

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8 Comments

Koen Wijnen Over a year ago

Since this is an exercise i am limited to using for loops and indices, unfortunatly i can't make use of zip

Koen Wijnen Over a year ago

This answer helps me a lot but leaves 1 problem. If i now have more than 2 lists in my nested list, it wont take the ones after position [1] into account. Can you help me with that problem?

lmiguelvargasf Over a year ago

@Koenwijnen, can you provide an example of that input and the example of the expected output?

Koen Wijnen Over a year ago

Yes I can. Lets say the input nested list = [Banana, Apple, Pear], [Elephant, Monkey, Snail], [Blue, Red, Yellow]. In your example the third lists won't be taken into account. However i need my code to access all lists in the nested lists regardless of how many those are. So the expected outcome would be: [Banana and Elephant and Snail, etc..]

lmiguelvargasf Over a year ago

@KoenWijnen, what is your expected output in that case?

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Amardeep Bhowmick · Accepted Answer · 2019-09-30 18:34:48Z

1

You can use zip to get a combination of names from sub lists and the list having "and" repeated the number of times elements are present in the first list one[0] :

one = [['Patrick', 'Peter', 'Jerome'], ['Lisa', 'Karen', 'Alice']]
name_tup = zip(one[0], ["and"] * len(one[0]), one[1])

Then use a list comprehension to generate the out list from the tuple returned by zip. Using unpacking the three elements are joined by a whitespace in between using the join function:

list = [ " ".join([x, y, z]) for x, y, z in name_tup]

>>> list
['Patrick and Lisa', 'Peter and Karen', 'Jerome and Alice']

edited Sep 30, 2019 at 18:34

answered Sep 30, 2019 at 18:19

Amardeep Bhowmick

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