Selecting NumPy array range, including the last element

Question

If I want to select all elements of a NumPy array, up to index N, I can write:

x = my_array[:N]

For example, if I want to select all elements, up to index 5, I can write:

N = 5
x = my_array[:N]

Or, if I want to select all elements, up to and including the penultimate element, I can write:

N = -1
x = my_array[:N]

But what if I want to select all elements up to and including the final element? How can I do this using the above notation?

I tried:

N = -0
x = my_array[:N]

But this returns a blank array.

p.s. Yes, I could just write out x = my_array[:], but I need it to be in the format my_array[:N], where N is defined dynamically.

Just leave out that value, i.e. array[:] is going to give all. Or do array[:len(array)]. — a_guest
– a_guest, Commented Jan 28, 2019 at 20:08
I need to have it in the format my_array[:N], such that N can be dynamically defined — Karnivaurus
– Karnivaurus, Commented Jan 28, 2019 at 20:09
Then use N = len(array). Actually you can use any N >= len(array) for that purpose. Also N = None will do. — a_guest
– a_guest, Commented Jan 28, 2019 at 20:10
either N == len(array) or N == None should work, I'm pretty sure — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jan 28, 2019 at 22:21

Matt Whitehead · Accepted Answer · 2019-01-29 06:44:09Z

2

Using your method:

N = len(my_array)
x = my_array[:N]

You could then specify any arbitrary value of N if you only wish to slice up to that index. You could also specify the length of your array directly, if known.

To illustrate this...

my_array = [1, 2, 3, 4, 5]
N = len(my_array)
x = my_array[:N]
my_array == x

...returns True.

answered Jan 29, 2019 at 6:44

Matt Whitehead

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