Filling a 2D matrix in numpy using a for loop

First you have to install numpy using

$ pip install numpy

Then the following should work

import numpy as np    
n = 100
matrix = np.zeros((n,2)) # Pre-allocate matrix
for i in range(1,n):
    matrix[i,:] = [3*i, i**2]

A faster alternative:

col1 = np.arange(3,3*n,3)
col2 = np.arange(1,n)
matrix = np.hstack((col1.reshape(n-1,1), col2.reshape(n-1,1)))

Even faster, as Divakar suggested

I = np.arange(n)
matrix = np.column_stack((3*I, I**2))

This is very pythonic form to produce a list, which you can easily swap e.g. for np.array, set, generator etc.

n = 10

[[i*3, i**2] for i, i in zip(range(0,n), range(0,n))]

If you want to add another column it's no problem. Simply

[[i*3, i**2, i**(0.5)] for i, i in zip(range(0,n), range(0,n))]

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