Replace Vector at indices and return a copy

General Usage
,
1

What’s the easiest way to return a copy of a Vector with replaced elements at some indices with some specific values ?
I can start with one approach:

julia> v = [10,20,30,40,50];

julia> inds = [2,5];

julia> vals = [200, 500];

julia> newv = let v2=deepcopy(v); v2[inds] = vals; v2; end
5-element Vector{Int64}:
  10
 200
  30
  40
 500

julia> v #still hasn't changed
5-element Vector{Int64}:
 10
 20
 30
 40
 50

I was expecting that there would be a replaceat and replaceat! function to do this.
The implementation would be maybe something similar along the way:

function replaceat(v, inds, vals)
  v2 = deepcopy(v)
  v2[inds] = vals
  v2
end

function replaceat!(v, inds, vals)
  v[inds] = vals
  v
end
2 
v[inds].=vals

in place replacement

for a copy

v[inds]=vals
3

Why would you need a replaceat function, when the current solution is even simpler:

v[inds] = vals

?

Also, I would use copy.

4

No, those two are the same, both are in-place.

5

I probably meant not allocating vs allocating :flushed:

6

They are actually indistinguishable. There’s no observable difference between them, it’s just that the dotted version involves broadcasting, but in the end it amounts to the same thing.

1 Like
7

This is what Accessors.jl is for.

using Accessors

julia> x = [10,20,30]
3-element Vector{Int64}:
 10
 20
 30

julia> @set x[2] = 99
3-element Vector{Int64}:
 10
 99
 30

julia> x
3-element Vector{Int64}:
 10
 20
 30
1 Like
8

Both expressions are not what I need.
Probably my bad.
The original v vector must not be mutated.
So first make a copy and then mutate the copy.
Have a look at my working example.

Plus v[inds]=vals return vals which is bad for one-liners. I would like it to return the mutated copy of v.

I think the scenarios could be quite a few.
I just have an original vector and I want get another one with just some replaced values based on index.
There is replace and replace! but these work in the value level.

9

Yes, this was clear. You were requesting both an in-place and an out-of-place version, so it was implied that you had to copy for the out-of-place version.

Then it seems like Accessors.@set is the way to go, or making your own.

10

that’s it. Also works for multiple replacements with @set v[inds] = vals .
perfect!