How do you extract the first and last elements from each sublist in a nested list?
For example:
x=[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print(x)
#[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
The desired result-
[[1, 3], [4, 6], [7, 9]]
This is the closest I've seen-
a = [x[i][i] for i in (0, -1)]
print(a)
#[1, 9]
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]]
Here is how you can use list slices:
x = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
x = [l[::len(l)-1] for l in x]
print(x)
Output:
[[1, 3], [4, 6], [7, 9]]
x = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> [a[:1] + a[-1:] for a in x]
[[1, 3], [4, 6], [7, 9]]
I extract 2 slices, one with the first element, one with the last, and concatenate them.
It will works even when sublist are of different lengths.
This works:
x = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for item in x:
print(item[0], item[2])
For your exact desired result, use this:
x = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
new_list = []
for item in x:
new_list.append([item[0], item[2]])
print(new_list)
I would recommend doing it like this. The list[-1] syntax is meant to get the last element in a list which is what you want.
x=[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for elem in x:
print(elem[0], elem[-1])
This just prints them in a standard format but to put them back into a new list in this order would be simple.
You can loop through the list and access them from there:
x=[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
out = []
for arr in x:
out.append([arr[0], arr[-1]])
print(out)
Output:
[[1, 3], [4, 6], [7, 9]]
Furthermore, you can use list comprehension:
out = [[arr[0], arr[-1]] for arr in x]