mrgloom mrgloom - 2 months ago 22
Python Question

Numpy array assignment with copy

For example, if we have a numpy array

A
, and we want a numpy array
B
with the same elements.

What is difference between these methods? When is additional memory allocated, and when is it not?


  1. B = A

  2. B[:] = A
    (same as
    B[:]=A[:]
    ?)

  3. numpy.copy(B, A)


Answer

All three versions do different things.

  1. This binds a new name B to the existing object already named A. Afterwards they refer to the same object, so if you modify one in place, you'll see the change through the other one too.

  2. This copies the values from A into an existing array B. The two arrays must have the same shape for this to work. B[:] = A[:] does the same thing (but B = A[:] would do something more like 1).

  3. This is not legal syntax. You probably meant B = numpy.copy(A). This is almost the same as 2, but it creates a new array, rather than reusing the B array. If there were no other references to the previous B value, the end result would be the same as 2, but it will use more memory temporarily during the copy.

    Edit: or maybe you meant numpy.copyto(B, A), which is legal, and is equivalent to 2?