How can I index a single value from a list containing sublists in python?

Question

As an example, right now I have a list of:

list = [[1276, 'c'], [910, 'b'], [819, 'o'], [759, 'z'], [699, 'l']]

and I would like to index this list for only a specific letter, for instance:

index = list.index('c')

As is, I realize that I'm going to get a ValueError, and I've learned that to make this work, I would need to index for the entire sublist.

index = list.index([1276,'c'])

Is there anyway I can work around having to index for the list as a whole, given that I don't currently know the integer value?

I hope this all makes sense, and if needed, I can certainly provide further clarification.

Thank you

You could iterate over the list of lists and build a dictionary of indices for each item then use the dictionary. enumerate() might be helpfull. — wwii
– wwii, Commented Mar 23, 2019 at 1:21

RoadRunner · Accepted Answer · 2019-03-23 02:36:42Z

1

If you know that the inner lists will only have two values, you could create your own function to get the first item that matches with next() and enumerate():

def get_index(lst, item):
    return next((i for i, (_, x) in enumerate(lst) if x == item), None)

Which works like this:

print(get_index(lst, 'c')) # 0
print(get_index(lst, 'b')) # 1
print(get_index(lst, 'o')) # 2
print(get_index(lst, 'z')) # 3
print(get_index(lst, 'l')) # 4
print(get_index(lst, 'g')) # None

And returns None if an item was not found.

As suggested in the comments, a nicer solution would be to create a dictionary of index mappings:

def get_index(lst, item):
    indices = {x: i for i, (_, x) in enumerate(lst)}
    return indices.get(item)

edited Mar 23, 2019 at 2:36

answered Mar 23, 2019 at 2:31

RoadRunner

26.4k6 gold badges46 silver badges77 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

ant-Korn · Accepted Answer · 2019-03-23 01:46:18Z

1

Declaring a class whose instances are "equal" to any value can help.

class Any:
    def __eq__(self, other):
        return True

list = [[1276, 'c'], [910, 'b'], [819, 'o'], [759, 'z'], [699, 'l']]
index = list.index([Any(), 'c'])
print(index)

answered Mar 23, 2019 at 1:46

ant-Korn

264 bronze badges

Comments

Alain T. · Accepted Answer · 2019-03-23 02:48:42Z

1

You could do it with a list comprehension that only returns the letter part of the tuple. The index will be the same.

index = [c for _,c in list].index('c')

answered Mar 23, 2019 at 2:48

Alain T.

42.2k4 gold badges36 silver badges57 bronze badges

Comments

Andy Ng · Accepted Answer · 2019-03-23 12:27:09Z

1

Instead of directly iterating over the items in a list AKA for i in list, you can iterate an integer value over the length of the list AKA for i in range(len(list)).

Here is an example of how to obtain the index of the correct sublist.

list = [[1276, 'c'], [910, 'b'], [819, 'o'], [759, 'z'], [699, 'l']]
for i in range(len(list)):
    if 'c' in list[i]:
        index = i
        break

edited Mar 23, 2019 at 12:27

Andy Ng

191 bronze badge

answered Mar 23, 2019 at 1:28

Remy

1609 bronze badges

3 Comments

wwii Over a year ago

Wouldn't you need two numbers to identify the location of an item in a list of lists?

Remy Over a year ago

My example only shows how to get the index of the correct sublist yeah. To get the correct index within the sublist, the solution is already provided in the question itself: index2 = list[i].index('c')

Sheldore Over a year ago

@wwii: The index of the character is always [-1] or [1] within the sublist. So here the main task is just to find the index of the sublist which contains the character always at position -1 or 1

iGian · Accepted Answer · 2019-03-23 11:24:45Z

0

Lookups using dictionaries are often faster, so you can build a dictionary, for example like this:

list_dct  = dict([ [y, [i,x,y]] for i, (x, y) in enumerate(mylist)])
#=> {'c': [0, 1276, 'c'], 'b': [1, 910, 'b'], 'o': [2, 819, 'o'], 'z': [3, 759, 'z'], 'l': [4, 699, 'l']}

In this way you can access the index as:

list_dct['o'][0] #=> 2

If you can get rid of the index you can simply convert your list to a dict, then access by the letter to get the integer:

list_dct = dict([ [y,x] for i, (x, y) in enumerate(mylist)])

list_dct #=> {'c': 1276, 'b': 910, 'o': 819, 'z': 759, 'l': 699}

list_dct['c'] #=> 1276

Remember that duplicated keys in dictionaries are not allowed.

answered Mar 23, 2019 at 11:24

iGian

11.2k3 gold badges24 silver badges38 bronze badges

Collectives™ on Stack Overflow

How can I index a single value from a list containing sublists in python?

5 Answers 5

Comments

Comments

Comments

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

Comments

Comments

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related