Python: Display index in a 2D array of all elements

Question

If I have a list/array such as:

B = [[1,2,3,4],[2,3,4,5],[4,3,2,4]]

(there are 50 elements in total)

I only want to show the 2nd value of each element. e.g. 3,4,2

I have tried something like B([:,2]) but I keep getting an error "TypeError: list indices must be integers or slices, not tuple"

I was thinking I might have to use some sort of loop?

Tuple indexing works for numpy arrays. Lists don't take those. Instead, you can index each sublist in a list comprehension. — Moses Koledoye
– Moses Koledoye, Commented Oct 27, 2017 at 16:40
I have done that and it is showing all values in column 3, but If I want to change all the values in column 3 to '0' how would I do this? — Atmosworld
– Atmosworld, Commented Oct 27, 2017 at 19:32

Mureinik · Accepted Answer · 2017-10-27 16:42:56Z

3

List comprehensions for the rescue:

result = [i[2] for i in B]

answered Oct 27, 2017 at 16:42

Mureinik

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Atmosworld Over a year ago

I now have it showing me the 2nd value in each element but do you know how I would replace that value (say with 0) and then write the data back to my original array/list B?

Transhuman · Accepted Answer · 2017-10-27 16:49:28Z

1

In Python 3.x

list(zip(*B))[2]

In Python 2.x

zip(*B)[2]

Output:

(3, 4, 2)

answered Oct 27, 2017 at 16:49

Transhuman

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Comments

Kaushik NP · Accepted Answer · 2017-10-27 16:50:35Z

0

Using List Comprehension :

#`n` is the row
>>> [row[n] for row in B]

#driver value :

IN : B = [[1,2,3,4],[2,3,4,5],[4,3,2,4]]
IN : n = 2
OUT : [3, 4, 2]

answered Oct 27, 2017 at 16:50

Kaushik NP

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