Value assigned to array element comes out as zero

Question: Why does a list element to which I assign a non-zero value become zero?

Have a list called eta, which contains a lot of different values. With this list want to make a new list a, in which each element a[i], is equal to eta[i+1]-eta[i]. The value for eta[i+1]-eta[i] can be calculated and can check that it gives me a non-zero value, but when I make a for loop and run it for i in some range, it sets a[i] = 0 for some reason.

Here is the code:

import numpy as np

a = np.arange(5)
for i in range(5):
    a[i] = eta[i+1]-eta[i]

Tried printing the values for eta[i+1]-eta[i] for each step and it comes out as a non-zero number, but if print a[i] it comes out as zero. Screenshot of code with prints.

Solution

You need to specify the data type when calling np.arange() or else it will infer that the indexes should be ints as the parameter passed in is 5 (an int).

Something like this will work.

a = np.arange(5, dtype='float')