How do I combine two lists into one, while changing some of the values?

Question

I am working on my very first coding project, which is to make a text encoder. Once complete, I plan to make the decoder that will pair with it. For now, I am having trouble getting two lists to combine/overlap. I apologize if what I am about to show has an actual name that references it, I am new to coding and still learning many things.

list1 = [20.0, 'X', 'X', 46.0, 0.0, 18.0, 'X', 40.0]
list2 = ['Y', 31.0, 45.0, 'Y', 'Y', 'Y', 47.0, 'Y']

I need the output to be:

list3 = [20.0, 31.0, 45.0, 46.0, 0.0, 18.0, 47.0, 40.0]

Both lists have an equal number of values, and I need to combine them into one list, keep the numbers in their current order, and eliminate the "X"s and "Y"s entirely.

Welcome to Stack Overflow! Please take the tour, look around, and read through the Help Center, in particular How do I ask a good question? If you run into a specific problem, research it thoroughly, search thoroughly here, and if you're still stuck post your code and a description of the problem. Also, remember to include Minimum, Complete, Verifiable Example. People will be glad to help — Andreas
– Andreas, Commented Jul 3, 2019 at 2:20
Here is one way to iterate over the two lists simultaneously: stackoverflow.com/questions/21098350/…. You can use isintance(s, str) to test if s is a string. You can use list3.append(x) to append x to list3. Combine those and you will have a solution. — jarmod
– jarmod, Commented Jul 3, 2019 at 2:32

Austin · Accepted Answer · 2019-07-03 02:34:13Z

2

Use zip with isinstance in a list-comprehension:

list1 = [20.0, 'X', 'X', 46.0, 0.0, 18.0, 'X', 40.0]
list2 = ['Y', 31.0, 45.0, 'Y', 'Y', 'Y', 47.0, 'Y']

list3 = [x if isinstance(x, float) else y for x, y in zip(list1, list2)]
# [20.0, 31.0, 45.0, 46.0, 0.0, 18.0, 47.0, 40.0]

answered Jul 3, 2019 at 2:34

Austin

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2 Comments

Xero Smith Over a year ago

Comprehension expressions, zip and isinntance are too advanced for a complete beginner. Simple loops and an accumulator are more appropriate.

BrysonMG Over a year ago

@Xero I have a little bit of familiarity with zip, even though I am still learning all of the different ways to implement it. but this is the first I am hearing of isinstance.

Vasu Deo.S · Accepted Answer · 2019-07-03 02:34:57Z

1

list1 = [20.0, 'X', 'X', 46.0, 0.0, 18.0, 'X', 40.0]
list2 = ['Y', 31.0, 45.0, 'Y', 'Y', 'Y', 47.0, 'Y']

list3 = []

for x in range(len(list1)):
    if isinstance(list1[x], float):
        list3.append(list1[x])

    else:
        list3.append(list2[x])

print(list3)

OUTPUT:-

[20.0, 31.0, 45.0, 46.0, 0.0, 18.0, 47.0, 40.0]

answered Jul 3, 2019 at 2:34

Vasu Deo.S

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3 Comments

Vasu Deo.S Over a year ago

@jarmod can you please tell me why?

jarmod Over a year ago

Your answer works, but is unpythonic. See python-forum.io/…

Vasu Deo.S Over a year ago

@jarmod Thanks for your suggestion. I would look forward to do as you mentioned in the later future.

edilio · Accepted Answer · 2019-07-03 02:45:38Z

1

As @Austin said use zip to combine several lists of the same size.

Here is a version a little easier to understand if your new to development

def get_number(item1, item2):
    if item1 in ['X', 'Y']:
        return item2
    else:
        return item1

[get_number(x, y) for x, y in zip(list1, list2)]

OUTPUT:-

[20.0, 31.0, 45.0, 46.0, 0.0, 18.0, 47.0, 40.0]

edited Jul 3, 2019 at 2:45

answered Jul 3, 2019 at 2:39

edilio

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1 Comment

edilio Over a year ago

True we shouldn't be hardcoding values but in this case the problem is not clear. The problem says we need to eliminate Xs and Ys, it doesn't really states that it should be float values. So what happens if is really an int for example.

U13-Forward · Accepted Answer · 2019-07-03 02:46:13Z

0

Why not max with isinstance:

print([max(i, key=lambda x: isinstance(x, float)) for i in zip(list1, list2)])

Output:

[20.0, 31.0, 45.0, 46.0, 0.0, 18.0, 47.0, 40.0]

answered Jul 3, 2019 at 2:46

U13-Forward

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1 Comment

jarmod Over a year ago

This is not an obfuscation competition ;-) We're trying to teach basic Python to a beginner.

MEOWWWWWWWW · Accepted Answer · 2019-07-03 02:37:34Z

-2

list3 = [x if not str(x).isalpha() else list2[i] for i, x in enumerate(list1)]

verified

edited Jul 3, 2019 at 2:37

answered Jul 3, 2019 at 2:27

MEOWWWWWWWW

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3 Comments

jarmod Over a year ago

Er, no. Did you test this?

doron Over a year ago

Your code results in the list [0, 1, 2, 3, 4, 5, 6, 7].

MEOWWWWWWWW Over a year ago

sorry for previous answer... I edited code and it works

Lê Tư Thành · Accepted Answer · 2019-07-03 02:44:26Z

-2

This is a suggestion if you don't seriously care about the order of comnbined list

list1 = [20.0, 'X', 'X', 46.0, 0.0, 18.0, 'X', 40.0]
list2 = ['Y', 31.0, 45.0, 'Y', 'Y', 'Y', 47.0, 'Y']
a = list1 + list2
a = [x for x in a if not isinstance(x, str)]

>>>a
[20.0, 46.0, 0.0, 18.0, 40.0, 31.0, 45.0, 47.0]

edited Jul 3, 2019 at 2:44

answered Jul 3, 2019 at 2:35

Lê Tư Thành

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4 Comments

Austin Over a year ago

It does not output in the expected form.

Lê Tư Thành Over a year ago

@Austin, you mean the order of list?

doron Over a year ago

That's not what the OP asked for. The output should be [20.0, 31.0, 45.0, 46.0, 0.0, 18.0, 47.0, 40.0].

Lê Tư Thành Over a year ago

@Austin, thanks for reminding. I edited my answer as a suggestion.

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