Let's say have an array a and I want every other element. With numpy, I would use a[::2]. How can I do the same in julia?
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1 Answer
It is similar to python where elements are selected using start:stop[:step] but in julia it's start:[step:]stop, so if all three arguments are given, step and stop have opposite meaning. See the docs on : or colon
For example
julia> a = randn(20);
julia> a[1:2:end]
10-element Vector{Float64}:
...
julia> a[begin:2:end] # equivalent for default one-based indexing
10-element Vector{Float64}:
...
julia> a[1:5:end]
4-element Vector{Float64}:
...
But ignoring the bounds won't work as in python because : has several meanings in julia
julia> a[::2]
ERROR: syntax: invalid "::" syntax
julia> a[:2:]
ERROR: syntax: missing last argument in ":(2):" range expression
julia> a[2::]
ERROR: syntax: unexpected "]"
julia> a[:2:end] # `:2` is a `Symbol` and evaluates to `2`, so start from 2nd element
19-element Vector{Float64}:
...
4 Comments
Chris Rackauckas
Should mentioned that this is the same as MATLAB's notation.
Olsgaard
I think the link to the
:/colon-documentation has moved to here: docs.julialang.org/en/v1/base/math/#Base.::
Fred Schoen
fixed in the answer
Oscar Smith
This answer should probably highlight
begin as well.