Python如何通过知道列表中列表的第一个元素来获取列表中列表的索引？

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.

Answer 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)

Answer 2

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)

Answer 3

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

Answer 4

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

Answer 5

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

Answer 6

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)))

Answer 7

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]