Python how do you get the index of a list in a list, by knowing the first element of the list in lists?

Question

Imagine the list [[0, 1],[2, 3][4, 5]],

how can I get the index of the list [2,3] (which is supposed to be 1), by knowing the number 2. In code something like:

in a list of lists, find the index of the list where list[0] == 2.

This should return 1.

And that is a very poorly chosen data storage paradigm. It's very inneficient to find this information due to the form of the data container. Maybe try to rethink how you want to store the information based on how you are going to access it. — Mathieu, Oct 16 '20 at 13:24

score 1 · Accepted Answer · answered Oct 16 '20 at 13:23

1

You can do this using a for loop. So for example:

nums = [[0, 1],[2, 3],[4, 5]]

for index, num_list in enumerate(nums):
   if num_list[0] == 2:
      print(index)

answered Oct 16 '20 at 13:23

abhigyanj

2,355
2
9
29

Alain T. · Answer 2 · 2020-11-30T20:00:38.273

You could use the next function on an enumeration of the list that would return the index of matching items

aList =  [[0, 1],[2, 3],[4, 5]]

index = next(i for i,s in enumerate(aList) if s[0]==2)

print(index) # 1

or, if you're not concerned with performance, using a more compact way by building a list of the first elements of each sublist and using the index() method on that:

index = [*zip(*aList)][0].index(2)

or

index = [i for i,*_ in aList].index(2)

score 0 · Answer 3 · answered Oct 16 '20 at 13:23

0

Iterate through the list and check the value of the first element. Use a variable to track which index you're looking at.

answered Oct 16 '20 at 13:23

Tyler

143
9

score 0 · Answer 4 · answered Oct 16 '20 at 13:25

0

for i in list:
    if i[0]==2:
        print(list.index(i))

Check Kaggle python course for such practical exercises https://www.kaggle.com/learn/python

answered Oct 16 '20 at 13:25

Gvantsa

69
6

score 0 · Answer 5 · answered Oct 16 '20 at 13:27

0

Since index method only supports exact value comparison, you need to iterate. See this question: Python: return the index of the first element of a list which makes a passed function true

answered Oct 16 '20 at 13:27

young padawan

76
5

score 0 · Answer 6 · answered Oct 16 '20 at 13:30

0

array  = [[0, 1],[2, 3],[4, 5]] 

def get_index(array):
  i = 0

  for element in array:
    if(element[0]==2):
      break
    i+=1

  return i 

print(str(get_index(array)))

answered Oct 16 '20 at 13:30

erwanlfrt

111
11

score 0 · Answer 7 · answered Oct 16 '20 at 13:33

If you have lots of such queries to make, it might be more efficient to build a dict first, with the first value of each sublist as key and the sublist as value:

data =  [[0, 1], [2, 3], [4, 5]]

data_by_first_value = {lst[0]: lst for lst in data}

This dict will look like:

print(data_by_first_value)
# {0: [0, 1], 2: [2, 3], 4: [4, 5]}

It is then an O(1) operation to get the sublist you're looking for:

print(data_by_first_value[2])
# [2, 3]

Python how do you get the index of a list in a list, by knowing the first element of the list in lists?

7 Answers7