Finding elements in two lists within certain range

Question

So I have two lists L1 is fomatted like this:

L1 = ['12:55:35.87', '12:55:35.70', ...]
L2 = ['12:55:35.53', '12:55:35.30', ...]

I am trying to find pairs in both list that start with the same 4 characters ie xx:x and then return the indexes of the pairs for each list

So far I have:

for pair1 in L1:
    for pair2 in L2:
        if pair1[:4] in pair2:
            print(L1.index(pair1))

This doesn't seem to return the correct indexes and it obviously doesn't return the index of the second list. Any help would be greatly appreciated.

Answer 1

Here's how to make your code work. Keep in mind this is a naive solution, there are faster way to solve this if your lists are big. Runtime here is O(n^2) but this could be solved in linear time.

for i,pair1 in enumerate(L1):
    for j,pair2 in enumerate(L2):
        if pair1[:4] == pair2[:4]:
            print("list1: %s , list2: %s" % (i,j))

Update: for future visitors here's an average linear time solution:

from collections import defaultdict
l1_map = defaultdict([])

for i,val in enumerate(L1):
    prefix = val[:4]
    l1_map[prefix].append(i)


for j,val in enumerate(L2):
     prefix = val[:4]
     for l1 in l1_map[prefix]:
        print("list1: %s , list2: %s" % (l1,j))

Answer 2

Because OP lists seem to have lots of repeated "firsts 4 characters", I would do something like the following:

indices = {}
for i, entry in enumerate(L1):
    indices.setdefault(entry[:4], [])
    indices[entry[:4]].append("L1-{}".format(i))
    if L2[i][:4] in indices:
        indices[L2[i][:4]].append("L2-{}".format(i))

Then you can access your repeated entries as:

for key in indices:
    print(key, indices[key])

This is better than O(n^2).

edit : as someone pointed out in the comments this is assuming that the lists do share the same length.

In case they don't, assume L2 is larger than L1 , then after doing the above you can do:

for j, entry in enumerate(L2[i+1:]):
    indices.setdefault(entry[:4], [])
    indices[entry[:4]].append("L2-{}".format(j))

If L2 is shorter than L1 just change the variables names in the code shown.

Answer 3

You can use itertools.product to loop the Cartesian product.

from itertools import product

L1 = ['12:55:35.87', '12:55:35.70']
L2 = ['12:55:35.53', '12:45:35.30']

res = [(i, j) for (i, x), (j, y) in 
       product(enumerate(L1), enumerate(L2)) 
       if x[:4] == y[:4]]

# [(0, 0), (1, 0)]

Answer 4

Use the range() or enumerate() function in the for-loops to provide you loop index.

For example, using the range() function:

for x in range(len(L1)):
   for y in range(len(L2)):
       if L1[x][:4] == L2[y][:4]:
           print(x, y)

Answer 5

enumerate is great for things like this.

indexes = []
for index1, pair1 in enumerate(L1):
    pair1_slice = pair1[:4] 
    for index2, pair2 in enumerate(L2):        
        if pair1_slice == pair2[:4]:
            indexes.append([index1, index2])
            print(index1, index2)

Answer 6

I think the enumerate function is what you're looking for!

L1 = ['12:55:35.87', '12:55:35.70', 'spam']
L2 = ['12:55:35.53', 'eggs', '12:55:35.30']

idxs = []

for idx1, pair1 in enumerate(L1):
    for idx2, pair2 in enumerate(L2):
        if pair1[:4] == pair2[:4]:
            idxs.append((idx1, idx2))

print(idxs)

Output

[(0, 0), (0, 2), (1, 0), (1, 2)]