Python/Numpy: How do you assign the end+1 element of an array similar to how it's done in Matlab?

Question

For example, for a 1D array with n elements, if I want to do this in Matlab I can do:

A(end+1) = 1

that assigns the value of 1 to the last element of array A which is now n+1 in length.

Is there an equivalent in Python/Numpy?

Answer 1

You can just append a value to the end of an array/list using append or numpy.append:

# Python list
a = [1, 2, 3]
a.append(1)
# => [1, 2, 3, 1]

# Numpy array
import numpy as np
a = np.array([1, 2, 3])
a = np.append(a, 1)
# => [1, 2, 3, 1]

Note, as pointed out by @BrenBarn, that the numpy.append approach creates a whole new array each time it is executed, which makes it inefficient.

Answer 2

I bet the Matlab/Octave operation does the same - create a new object. But I don't know if there is something like the Python id(a) to verify that.

A crude timing test in Octave supports this - creating a large array by appending is slower than stepping through the full array. Both are much slower than direct assignment like A=1:N

octave:36> t=time; N=1000000; A=[]; A(N)=1; for i=1:N A(i)=i; end; t-time
ans = -4.0374
octave:37> t=time; N=1000000; A=[]; for i=1:N A(end+1)=i; end; t-time
ans = -15.218

Extending an array with (end+1) is more idiomatic in Javascript than Matlab.