Selecting random position in numpy matrix

Question

I have a numpy matrix that is randomly populated with zeros and ones:

grid = np.random.binomial(1, 0.2, size = (3,3))

Now I need to pick a random position inside this matrix and turn it to 2

I tried:

pos = int(np.random.randint(0,len(grid),1))

But then I get an entire row filled with 2s. How do I pick only one random position? Thank you

Select any random position or any random position that is also 1 or any random position that is also 0? For the former simply do : np.put(grid,np.random.choice(grid.size),2). — Divakar
– Divakar, Commented Oct 13, 2017 at 16:52

eapetcho · Accepted Answer · 2017-10-15 20:41:53Z

3

The issue with your code is that you're just requesting only one random value for the indices instead of two random numbers (random). This one way of achieving your goal:

# Here is your grid
grid = np.random.binomial(1, 0.2, size=(3,3))

# Request two random integers between 0 and 3 (exclusive)
indices =  np.random.randint(0, high=3, size=2)

# Extract the row and column indices
i = indices[0]
j = indices[1]

# Put 2 at the random position
grid[i,j] = 2

edited Oct 15, 2017 at 20:41

answered Oct 15, 2017 at 20:32

eapetcho

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1 Comment

nattys Over a year ago

Your solution helped me solve another problem in my code that I had no idea was related to the lack of indices. Thank you so much

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