Extract First Item of Each Sublist

Extract first item of each sublist

Using list comprehension:

>>> lst = [['a','b','c'], [1,2,3], ['x','y','z']]
>>> lst2 = [item[0] for item in lst]
>>> lst2
['a', 1, 'x']

Extracting first and last element from sublists in nested list

You have the right idea. If your list inside list is only one layer deep, you can access the first and last elements with a list comprehension and -1 indexing like you were trying to do:

a = [[sublist[0],sublist[-1]] for sublist in x]

Output:

>>> a
[[1, 3], [4, 6], [7, 9]]

Get first item of every sublist of list in Python

To capture the first elements into a variable, you can use a list comprehension like:

Code:

data = {
    'ID': [[310.5, 1.1612828],
           [310.0, 15.0],
           [310.0, 2.9755309],
           [309.5, 30.0]]
}

variable = [x[0] for x in data['ID']]

print(variable)

Results:

[310.5, 310.0, 310.0, 309.5]

How to extract a sublist by the value of the first element from a nested list

my_list = [
    ['nom', 'N', 'eye'],
    ['acc', 'E', 'computer'],
    ['dat', 'C', 'screen']
]
my_input = input("Enter first string to find: ")

for lis in my_list:
    if lis[0] == my_input:
        print(lis)
        break

Output:

Enter variable name: nom
['nom', 'N', 'eye']

Select first two elements of each sublist in a list

Use a list comprehension to apply the same slice to each sublist:

b = [l[:2] for l in a]

C# linq how to get all the first elements of a nested dynamic list?

Proper and efficient way of doing this is as follows:

mainList.Select(subList => subList?.First()).OfType<dynamic>();

This will take care of null lists and null elements

If you want to select all the elements of sublists try following

mainlist.Select(subList => subList).OfType<List<dynamic>>();

This will only take care of null lists

Extract the first, second, third , … sub-element from a list in R and store as list

Yes

lapply(1:15, function(i) sapply(list_data, '[', i))

Counting the frequency of the first element of a list within a list

Just split your task into a two parts:

1. Retrieve first element from each inner list;
2. Sort elements in descending order by number of occurrences.

You can easily find an answer of each of this problem on Stack Overflow:

1. Extract first item of each sublist;
2. Sorting a List by frequency of occurrence in a list.

Even question you've additionally answered in comments already have been answered: How to access the first element or number in counter using Python.

You should make some research efforts before asking question otherwise you waste both yours time and time of users who will answer your question.

Anyway, let's go back to your problem.

1. To get first value of inner list you should iterate over outer list and retrieve first element from every item. With simple for loop it will look like this:

   source_list = [[4, 2, 1, 3], [4, 3, 1, 2], [4, 3, 1, 2], 
                  [1, 3, 4, 2], [2, 3, 4, 1], [2, 1, 3, 4]]
   first_items_list = []
   for inner_list in source_list:
       first_items_list.append(inner_list[0])
   

   You can also use list comprehension:

   source_list = [[4, 2, 1, 3], [4, 3, 1, 2], [4, 3, 1, 2], 
                  [1, 3, 4, 2], [2, 3, 4, 1], [2, 1, 3, 4]]
   first_items_list = [inner_list[0] for inner_list in source_list]
   

2. Now we have to find max element in list of first items by frequency. To not reinvent the wheel you can apply collections.Counter and specifically it's Counter.most_common() method.

   To initialize Counter you need to pass first_items_list generated in code above to Counter() constructor. Then you need to call Counter.most_common() and pass 1 to arguments as you need just most common element:

   from collections import Counter
   ...
   counter = Counter(first_items_list)
   item, number_of_occurrences = counter.most_common(1)[0]
   

To simplify the code you can change list comprehension to generator expression and pass it directly into Counter constructor:

source = [[4, 2, 1, 3], [4, 3, 1, 2], [4, 3, 1, 2], 
          [1, 3, 4, 2], [2, 3, 4, 1], [2, 1, 3, 4]]
print("Most common:", Counter(inner[0] for inner in source).most_common(1)[0][0])

Select first element of nested list

Not much of a shortcut, but you can do this:

lapply(x, `[[`, 1)
# [[1]]
# [1] 1
#
# [[2]]
# [1] 3
#
# [[3]]
# [1] 5

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