Printing A Column Of A 2-D List In Python

Suppose if A = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] Then A[0][:] prints [1, 2, 3] But why does A[:][0] print [1, 2, 3] again ? It should print the column [1, 4, 7], shouldn't it?

Solution 1:

[:] is equivalent to copy.

A[:][0] is the first row of a copy of A. A[0][:] is a copy of the first row of A.

The two are the same.

To get the first column: [a[0] for a in A] Or use numpy and np.array(A)[:,0]

Solution 2:

When you don't specify a start or end index Python returns the entire array:

A[:] = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Solution 3:

[:] matches the entire list.

So A[:] is the same as A. So A[0][:] is the same as A[0].

And A[0][:] is the same as A[0].

Solution 4:

A[:] returns a copy of the entire list. which is A = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] A[:][0] Thus selects [1, 2, 3]. If you want the first column, do a loop:

col = []
for row in A:
    col.append(row[0])

Solution 5:

A is actually a list of list, not a matrix. With A[:][0] You are accessing the first element (the list [1,2,3]) of the full slice of the list A. The [:] is Python slice notation (explained in the relevant Stack Overflow question).

To get [1,4,7] you would have to use something like [sublist[0] for sublist in A], which is a list comprehension, a vital element of the Python language.