Comparing lists in Python

Question

Say I have list1 = [1,2,3,4] and list2 = [5,6,7,8]. How would I compare the first element, 1, in list1 with the first el

Answer 1

If your result is going to be a new list, then you can use a list comprehension:

new_list = [ some_function(i, j) for i, j in zip(list1, list2) ]

Here's a real example of the above code:

>>> list1 = [1, 2, 3, 4]
>>> list2 = [1, 3, 4, 4]
>>> like_nums = [ i == j for i, j in zip(list1, list2) ]
>>> print like_nums
[True, False, False, True]

This will make a list of bools that show whether items of the same index in two lists are equal to each other.

Furthermore, if you use the zip function, there is a way to unzip the result when you are done operating on it. Here's how:

>>> list1 = [1, 2, 3, 4]
>>> list2 = [1, 3, 4, 4]
>>> new_list = zip(list1, list2)         # zip
>>> print new_list
[(1, 1), (2, 3), (3, 4), (4, 4)]
>>> newlist1, newlist2 = zip(*new_list)  # unzip
>>> print list(newlist1)
[1, 2, 3, 4]
>>> print list(newlist2)
[1, 3, 4, 5]

This could be useful if you need to modify the original lists, while also comparing the elements of the same index in some way.