Add two lists in Python

Question

I am trying to add together two lists so the first item of one list is added to the first item of the other list, second to second and so on to form a new list.

Currently I have:

def zipper(a,b):
    list = [a[i] + b[i] for i in range(len(a))]
    print 'The combined list of a and b is'
    print list

a = input("\n\nInsert a list:")
b = input("\n\nInsert another list of equal length:")

zipper(a,b)

Upon entering two lists where one is a list of integers and one a list of strings I get the Type Error 'Can not cocanenate 'str' and 'int' objects.

I have tried converting both lists to strings using:

list = [str(a[i]) + str(b[i]) for i in range(len(a))]

however upon entering:

a = ['a','b','c','d']
b = [1,2,3,4]

I got the output as:

['a1','b2','c3','d4']

instead of what I wanted which was:

['a+1','b+2','c+3','d+4']

Does anyone have any suggestions as to what I am doing wrong?

N.B. I have to write a function that will essentially perform the same as zip(a,b) but I'm not allowed to use zip() anywhere in the function.

Ignacio Vazquez-Abrams · Accepted Answer · 2011-10-31 00:37:18Z

9

Zip first, then add (only not).

['%s+%s' % x for x in zip(a, b)]

answered Oct 31, 2011 at 0:37

Ignacio Vazquez-Abrams

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2 Comments

George Burrows Over a year ago

Sorry I should have mentioned, I have to write a function that will essentially perform the same as zip(a,b) but I'm not allowed to use zip() anywehere in the function.

Ignacio Vazquez-Abrams Over a year ago

So then adapt what you already have and use that in place of zip().

lc2817 · Accepted Answer · 2011-10-31 00:43:36Z

3

What you should do

You should use

list = [str(a[i]) +"+"+ str(b[i]) for i in range(len(a))]

instead of

list = [str(a[i]) + str(b[i]) for i in range(len(a))]

In your version, you never say that you want the plus character in the output between the two elements. This is your error.

Sample output:

>>> a = [1,2,3]
>>> b = ['a','b','c']
>>> list = [str(a[i]) +"+"+ str(b[i]) for i in range(len(a))]
>>> list
['1+a', '2+b', '3+c']

edited Oct 31, 2011 at 0:43

answered Oct 31, 2011 at 0:36

lc2817

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4 Comments

George Burrows Over a year ago

I still get the same 'cannot cocatenate 'str' and 'int' objects.'

lc2817 Over a year ago

@GeorgeBurrows How did you get that ['a1','b2','c3','d4'] without error so, could your detail?

George Burrows Over a year ago

By using: list = [str(a[i]) + str(b[i]) for i in range(len(a))]

lc2817 Over a year ago

@GeorgeBurrows What about the snipped I just typed in my python log with exactly the same line I gave you?

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