# Lazy subranges or \`1:end\` as entity
**URL:** https://discourse.julialang.org/t/lazy-subranges-or-1-end-as-entity/11159
**Category:** General Usage
**Created:** [May 26, 2018, 6:27am UTC](https://discourse.julialang.org/t/lazy-subranges-or-1-end-as-entity/11159 "2018-05-26T06:27:07Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@mschauer](https://discourse.julialang.org/u/mschauer)
#### Post date: [May 26, 2018, 6:27am UTC](https://discourse.julialang.org/t/lazy-subranges-or-1-end-as-entity/11159/1 "2018-05-26T06:27:07Z")
Sometimes I’d like to be able to write
```julia
r = 1:End()-1
x[r] .= y[r]
```
Is this a good idea and how to achieve this?
---
### Author:  [@yakir12](https://discourse.julialang.org/u/yakir12)
#### Post date: [May 26, 2018, 6:41am UTC](https://discourse.julialang.org/t/lazy-subranges-or-1-end-as-entity/11159/2 "2018-05-26T06:41:43Z")
Are you concerned by the cost of building the `1:end - 1` range in:
```julia
x[1:end - 1] .= y[1:end - 1]
```
?
---
### Author:  [@mschauer](https://discourse.julialang.org/u/mschauer)
#### Post date: [May 26, 2018, 7:18am UTC](https://discourse.julialang.org/t/lazy-subranges-or-1-end-as-entity/11159/3 "2018-05-26T07:18:02Z")
No, not at all, just for readability in expressions like
```julia
sk=10; plot([first.(y[1:sk:end]) for y in ys[1:sk:end]],[last.(y[1:sk:end,end]) for y in ys[1:sk:end]])
```
vs
```julia
r=1:10:End()
plot([first.(y[r, end]) for y in ys[r]], [last.(y[r,end]) for y in ys[r]])
```
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [May 26, 2018, 7:41am UTC](https://discourse.julialang.org/t/lazy-subranges-or-1-end-as-entity/11159/4 "2018-05-26T07:41:14Z")
You could define
1. an `End` type, eg describing `end + a`,
2. arithmetic on it,
3. a `Base.colon` method, that emits another custom type which can contains ranges with this kind of endpoints,
4. `Base.getindex` and `Base.setindex` method for this custom type.
I don’t think it is worth it though. If in the above example you don’t want `End` to resolve differently for `x` and `y` and they have the same indexes, you could use something like
```julia
r = indices(x, 1)[1:(end-1)]
x[r] .= y[r]
```
Otherwise, I think that `x[1:(end-1)] .= y[1:(end-1)]` is just more readable.
---
### Author:  [@mauro3](https://discourse.julialang.org/u/mauro3)
#### Post date: [May 26, 2018, 7:45am UTC](https://discourse.julialang.org/t/lazy-subranges-or-1-end-as-entity/11159/5 "2018-05-26T07:45:03Z")
I once (in Julia 0.2 or 0.3) made such an `End` type: [Bitbucket](https://bitbucket.org/maurow/ragged.jl/src/ebbd1cdfd796ef626e9c5193dc75b6494fe8b21f/src/Ragged.jl#lines-13)
But I’m sure there are better ways to go about it. But it’s not trivial, that is why Julia opted for a syntax approach to the problem.
---
### Author:  [@mschauer](https://discourse.julialang.org/u/mschauer)
#### Post date: [May 26, 2018, 10:40am UTC](https://discourse.julialang.org/t/lazy-subranges-or-1-end-as-entity/11159/6 "2018-05-26T10:40:06Z")
I could also image to define something along the lines
```julia
struct Formula{T} f::T end
farg(args, symb) = last(args[findlast(x->first(x)==symb, args)])
import Base.getindex
(f::Formula)(;args...) = f.f(;args...)
getindex(a::AbstractArray, f::Formula) = a[f(END=length(a))]
```
so that for
```julia
r = Formula((;args...)->1:farg(args, :END)-1)
```
```julia
julia> rand(10)[r]
9-element Array{Float64,1}:
0.355413
0.26395
0.260973
0.0454235
0.860611
0.958574
0.912541
0.812007
0.500342
```
and then define a macro `@formula` giving syntactic sugar to `r = Formula((;args...)->1:farg(args, :END)-1)`