Creating a Matrix in Python

Question

I am trying to create a 100 by 100 matrix that will be populated with 1's and -1's(at random), and the diagonal of the matrix should be all zeros.

I am new to python and numpy.

• mgilson: @That1Guy -- Try to be a little more constructive with you comment. Bare "what have you tried" type comments are deprecated
• mgilson: @user2509830 -- That's a good start. Can you post the code? We can do a much better job showing you how to use the libraries if we can see how you've chosen to attack the problem.
• user2509830: so far, I have created a matrix with all 1's and -1's, but I can't get it to have zeros along the diagonal. Also, when a matrix is created with numpy, how do i access the values( if i wanted to sum up values in a matrix)?
• Daniel: In addition to the answers below you should probably read this page: http://wiki.scipy.org/Cookbook/BuildingArrays

Answer 1

To create the matrix with ones:

a = numpy.ones( (100,100) )

To create the random matrix:

a = numpy.random.random( (100,100) ) # by default between zero and one

To set all the diagonals to zero:

numpy.fill_diagonal(a, 0)

• user2509830: Thanks for the help. After the matrix is created, how can i access values inside the matrix? for instance, is the value in the 2nd row, 3rd column just example_matrix[2][3]?
• RussellStewart: essentially, but python uses a 0 to index the first element, so the 2nd row, 3rd column, so it would be example_matrix[1][2]
• SethMMorton: @user2237635 That is true for python's lists of lists, but a multidimensional numpy array is indexed like [x,y], not [x][y]
• mgilson: Isn't there a numpy.fill_diagonal?
• Daniel: http://docs.scipy.org/doc/numpy/reference/generated/numpy.fill_diagonal.html New in 1.4.0 and likely a lot faster since the indices are never constructed. Just uses fancy slicing.

Answer 2

As an alternative, just using list comprehension and random:

from random import randint
x = [1,-1]
my_matrix = [[x[randint(0,1)] if i!=j else 0 for i in range(100)] for j in range(100)]

This will give you the random choice between -1 and 1 with 0 for the diagonal. You can embed this in a function to give you a matrix of NxN size as follows:

from random import randint
def make_matrix(n):
 x = [-1,1]
 return [[x[randint(0,1)] if i!=j else 0 for i in range(n)] for j in range(n)]

Answer 3

Here is a simple python line to create a 2D matrix - 100X100:

 yourmatrix = [[0 for x in xrange(100)] for x in xrange(100)]