Find matching values in a list of lists

Question

I'm trying to iterate over a list of lists in python 2.7.5 and return those where the first value is found in a second list, something like this:

#python 2.7.5
list1 = ['aa', 'ab', 'bb', 'bc', 'cc']
list2 = [['aa', 1, 3, 7],['de', 2, 2, 1],['bc', 3, 4, 4]]

list3 = []
for x in list1:
    for y in list2:
        if x == y:
            list3.append(y)

So I would want list3 to contain [['aa',1,3,7],['bc', 3, 4, 4]] but instead I just get the whole of list2.

Answer 1

Try a more simple approach that is closer to what you want:

for e in list2:
    if e[0] in list1:
        list3.append(e)

You need e[0] since list2 is a list of lists. You can also write this in a single line using the filter() function:

list3 = filter(lambda e: e[0] in list1, list2)

or using list comprehension:

list3 = [e for e in list2 if e[0] in list1]

Answer 2

There's just a bug in your code. You need x == y[0] instead of x == y. The latter is comparing a string to a list.

You can also use list comprehensions

>>> [x for x in list2 if x[0] in list1]
[['aa', 1, 3, 7],['bc', 3, 4, 4]]

Answer 3

It looks like your issue is because you are attempting to compare a list of elements against a list of a list of elements.

If you step through the for loop, you'll see that on your first iteration you are comparing 'aa' against ['aa', 1, 3, 7], which is probably not what you would like to do.

If you're just comparing the first element, you will need to change your for loop to:

for x in list1:
    for y in list2:
        if x == y[0]:
            list3.append(y)