Assume I have the following 8x8 2D list:
[[0, 1, 0, 1, 0, 1, 0, 1], [1, 0, 1, 0, 1, 0, 1, 0], [0, 1, 0, 1, 0, 1, 0, 1], [0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0], [-1, 0, -1, 0, -1, 0, -1, 0], [0, -1, 0, -1, 0, -1, 0, -1], [-1, 0, -1, 0, -1, 0, -1, 0]] How do I get a random index for the value "1" for example ?
Here's a nice one-liner with a nested list comprehension:
import random random.choice([(i, j) for i, row in enumerate(x) for j, val in enumerate(row) if val == 1]) where x is your list. You just gather a list of indices (i, j) where val == 1, then randomly pick one.
If the list is rectangular (all elements are lists and these lists have the same length and the elements of these lists are numbers), we can boost the filtering process by using numpy instead:
from numpy import array, where from random import choice choice(array(where(a == 1)).T) In case a is not an numpy array yet, we can convert it with:
choice(array(where(array(a) == 1)).T) This then for instance returns:
>>> choice(array(where(a == 1)).T) array([1, 2]) In case we want the result to be a list or tuple, we can call the constructor, like:
>>> tuple(choice(array(where(a == 1)).T)) (1, 6) 
You could do something like:
indices = [] for row_idx in range(len(a)): for col_idx in range(len(a[row_idx])): num = a[row_idx][col_idx] if num == 1: indices.append((row_idx, col_idx)) import random rand_idx = random.randrange(0, len(indices)) print indices[rand_idx] 