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pythonarraysfor-loopnumpy-ndarray
Jose Luis Teran Soto
Jose Luis Teran Soto

Reputation: 1

How do I insert a value into a two-dimensional array in python

I have been trying to get a personal homework to work properly but I can't seem to get it right. I need to insert a random value into a 2D array using two For cycles and print the array. This is what I have but it's not finished and it's the barebones of my struggle.

import numpy as np

import random

arr = np.array([], [])

for i in range(0, 6):
    
    for j in range(0, 6):
        value = random.randint(0, 6)
        arr[i][j] = value
        print(arr(i, j))
print('')

I want to know how to insert the random value into the array's position, like if the for cycle is telling me I'm at the position 0,0 , then I want to insert a number into that position.

Upvotes: 0

Views: 3490

Answers (3)

Bill the Lizard
Bill the Lizard

Reputation: 405715

Creating an array with arr = np.array([], []) gives you an array with no rows and no columns, so you'll be using indexes that are out of bounds as soon as you try to set a value. You need to give some dimension to the array when you create it, even if you don't provided initial values. There are several ways to do that, but try this:

import numpy as np
import random

arr = np.empty(shape=(6, 6)) # np.zeroes would also work

print(arr)

for i in range(0, 6):
    for j in range(0, 6):
        value = random.randint(0, 6)
        arr[i][j] = value

print(arr)

Upvotes: 0

Mark
Mark

Reputation: 92440

In general, you should do everything you can to avoid loops with Numpy. It defeats the purpose. For almost anything you want to do, there is a Numpy way that doesn't require a loop.

Here you can use numpy.random.randint and built the array directly without the loop. This will be much faster:

import numpy as np

# arguments are low, high, shape
arr = np.random.randint(0, 6, (6, 6))

# array([[0, 4, 0, 5, 3, 4],
#        [2, 2, 0, 0, 3, 1],
#        [1, 3, 5, 0, 4, 2],
#        [3, 1, 1, 4, 5, 2],
#        [2, 5, 4, 3, 2, 3],
#        [2, 3, 0, 2, 0, 0]])

Upvotes: 2

Niv Dudovitch
Niv Dudovitch

Reputation: 1658

Some changes:

  1. Define the array with np.zeros
  2. print(arr(i, j)) To print(arr[i, j])
arr = np.zeros((6, 6))
import random
for i in range(0, 6):
    for j in range(0, 6):
        value = random.randint(0, 6)
        arr[i][j] = value
        print(arr[i, j])

If you want to print the entire array, you can do this:

arr = np.zeros((6, 6))
import random
for i in range(0, 6):
    for j in range(0, 6):
        value = random.randint(0, 6)
        arr[i][j] = value
print(arr)

Upvotes: 0

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