New to Python – Why does this program produce an array of length 1000, instead of 50000, and why are the first 50 values of the array equal to 0?

Question

Why does this program produce an array of length 1000, instead of 50000, and why are the first 50 values of the array equal to 0?

import numpy as np
arr_growthrates = np.linspace(0, 4, 1000)
arr_initial_x = np.random.random((1, 50))

l = []
for k in arr_growthrates:
    for m in arr_initial_x:
        iteration = 100

        v = []
        v.append(m)

        for i in range(iteration):
            v.append(k * v[i] * (1 - v[i]))


        l.append(v[100])

print(len(l))
print(l)

Because this involves a loop within a loop, and the first loop has 1000 iterations, and the second loop has 50, and because I append values to l after the second loop, I expected l to have a length of 50000, but instead it has a length of 1000. Also why are the first 50 values in l equal to 0?

Asked By: cookiecainsy

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Source

Answer 1

As discussed in the comments, the problem was with the shape of np.random.random((1, 50)). It results in an array of the shape (1, 50), which is equivalent of a list [[a1, a2, ...]] from the iteration point of view. Instead, use just:

 arr_initial_x = np.random.random(50)

Answered By: bereal

Answer 2

Because arr_initial_x = np.random.random((1, 50)) returns an array with this shape (1, 50).

So, inside the loop for m in arr_initial_x: there is only 1 round done because only one object read (e.g. the whole array of 50 items).

I guess you would have a shape equal to (50,), and you can have this result with this command arr_initial_x[0].shape.

Answered By: C L

New to Python – Why does this program produce an array of length 1000, instead of 50000, and why are the first 50 values of the array equal to 0?

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