create a sublist from a python list

Question

I have a python list as follows:

list_ = [-100, 1, 3, 5, 7, 100]

I want to create another list such as below; (Note that the last 100 is not included!!)

new_list = [[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

I tried as follows:

new_list = []

for i in list_:
    new_list.append(list([i]))

It generates [[-100], [1], [3], [5], [7], [100]] , which is not the one I want.

Answer 1

Is this want you want?

l = [-100, 1, 3, 5, 7, 100]
print([l[:i+1] for i in range(0, len(l) - 1)])

Output:

[[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

Or even simpler:

print([l[:i] for i in range(1, len(l))])

Answer 2

This is probably what you meant:

list_ = [-100, 1, 3, 5, 7, 100]

new_list = []

for i in range(1, len(list_)):
    new_list.append(list_[:i])

print(new_list)
# outputs [[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

In each iteration, we take the list slice up to index i , starting at index 0 up until len(list_) - 1 , using range to generate the indexes.

Answer 3

What you probably want to do here is work with list slices like so:

new_list = []

for i in range(1,len(list_)):
    new_list.append(list_[:i])
    

printing the list produced by this loop yields:

[[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

For more details on how list slicing works, see the slicing section in the informal introduction to Python here: https://docs.python.org/3/tutorial/introduction.html

Answer 4

What about a recursive function ?

list_ = [-100, 1, 3, 5, 7, 100]
matrix_ = []


def matrix(lst):
    if not lst:
        return
    length = len(lst)-1
    if length:
        matrix_.append(lst[:length])
        matrix(lst[:length])

x = matrix(list_)
print(matrix_)

output

[[-100, 1, 3, 5, 7], [-100, 1, 3, 5], [-100, 1, 3], [-100, 1], [-100]]

Then reverse it if you want:

matrix_.reverse()
print(matrix_)
# [[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

Additional

1. Using a While loop

list_ = [-100, 1, 3, 5, 7, 100]


def matrix(lst):

    matrix_ = []
    count = 0
    prev = []
    while count < len(lst)-1:
        prev.append(lst[count])
        matrix_.append(prev.copy())
        count += 1
    return matrix_


x = matrix(list_)
print(x)
# [[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

2. Using Generator Function

list_ = [-100, 1, 3, 5, 7, 100]


def matrix(lst):
    matrix_ = []
    count = 0
    prev = []
    while count < len(lst)-1:
        prev.append(lst[count])
        matrix_.append(prev.copy())
        count += 1
    yield matrix_


x = list(matrix(list_))
print(x[0])
# [[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

3. Enumerate

list_ = [-100, 1, 3, 5, 7, 100]


def matrix(lst):
    result = []
    for idx, _ in enumerate(lst):
        if idx:
            list_block = lst[:idx]
            result.append(list_block)
    return result

x = matrix(list_)
print(x)
# [[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

4. range function

list_ = [-100, 1, 3, 5, 7, 100]


def matrix(lst):
    result = []
    for i in range(len(lst)):
        if i:
            list_block = lst[:i]
            result.append(list_block)
    return result

x = matrix(list_)
print(x)
# [[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

5. Using itertools takewhile just for fun

from itertools import takewhile

list_ = [-100, 1, 3, 5, 7, 100]


def make_list(item):

    matrix = []
    for i in range(1, len(item)):
        wanted_items = list(takewhile(lambda x: x != item[-i], item))
        matrix.append(wanted_items)
    matrix.reverse()
    return matrix

result = make_list(list_)
print(result)
# [[-100], [-100, 1], [-100, 1, 3], [-100, 1, 3, 5], [-100, 1, 3, 5, 7]]

Answer 5

try this

lst=[-100,1,3,5,7,100]
new_list = []

for i in range(len(lst):
    new_list.append(lst[:i])