Array size difference Python

Question

My question is about Python array shape.

What is the difference between array size (2, ) and (2, 1)?

I tried to add those two arrays together. However, I got an error as follows:

Non-broadcastable output operant with shape (2, ) doesn't match the broadcast shape (2, 2)

Answer 1

There is no difference in the raw memory. But logically, one is a one-dimensional array of two values, the other is a 2D array (where one of the dimensions just happens to be size 1).

The logical distinction is important to numpy; when you try to add them, it wants to make a new 2x2 array where the top row is the sum of the (2, 1) array's top "row" with each value in the (2,) array. If you use += to do that though, you're indicating that you expect to be able to modify the (2,) array in place, which is not possible without resizing (which numpy won't do). If you change your code from:

arr1 += arr2

to:

arr1 = arr1 + arr2

it will happily create a new (2, 2) array. Or if the goal was that the 2x1 array should act like a flat 1D array, you can flatten it:

alreadyflatarray += twodarray.flatten()