Creating specific matrix in pure Python

Question

I have little problem here. As I am not good in math I can't figure this out tho.

I need to create function that takes n as a parameter which is matrix size and creates matrix of this type

 [[1, -8, 9, -16]
 [2, -7, 10, -15]
 [3, -6, 11, -14]
 [4, -5, 12, -13]]

All I achieved was this

def arrange_matrix2(n):
    matrix = []
    row = []
    num = 1
    for i in range(n):
        for j in range(n):
            row.append(num)
            num += 1
        matrix.append(row)
        row = []

    final_matrix = map(list, zip(*matrix))
    print(*final_matrix)

It creates matrix that looks like this

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

Also. Is there a way to create matrix that is arranged by column without using zip() function?

Thank you for your time.

actually you are moving from top->bottom->top-->bottom this is the pattern — Pygirl
– Pygirl, Commented Mar 26, 2020 at 18:15
If you will search by snake pattern you will get lot of resources :) — Pygirl
– Pygirl, Commented Mar 26, 2020 at 18:21

Mad Physicist · Accepted Answer · 2020-03-26 18:30:46Z

2

You don't need to do anything fancy here. First, preallocate your output:

final_matrix = [[0] * n for _ in range(n)]

Now you can do whatever you want with the indices. For example, here is a very simple approach:

num = 1
for j in range(n):
    for i in range(n):
        if j % 2:
            final_matrix[-(i + 1)][j] = -num
        else:
            final_matrix[i][j] = num
        num += 1

The modulo operator returns the remainder, and can therefore be used to check for divisibility. j % 2 will be 1 if j is odd, 0 if even. So odd columns get reversed and negated. The row index -(i + 1) is a Python shorthand for n - (i + 1). As the actual i goes 0->n-1, the index of odd columns goes n-1->0.

answered Mar 26, 2020 at 18:30

Mad Physicist

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2 Comments

Mileta Dulovic Over a year ago

that is what I needed. Thank you a lot. And for explanation too :)

Mad Physicist Over a year ago

Start small. If you understand the simple things, you understand everything. If you only know complicated things, you're just fooling yourself.

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