# Shorthand for ((i-1)\*k+1):(i\*k)

**URL:** <https://discourse.julialang.org/t/shorthand-for-i-1-k-1-i-k/133968>\
**Category:** General Usage\
**Tags:** question, indexing\
**Created:** [November 18, 2025, 2:35pm UTC](https://discourse.julialang.org/t/shorthand-for-i-1-k-1-i-k/133968 "2025-11-18T14:35:08Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [November 18, 2025, 2:35pm UTC](https://discourse.julialang.org/t/shorthand-for-i-1-k-1-i-k/133968/1 "2025-11-18T14:35:08Z")

</div>

I often run into the problem of partitioning data to into equal “chunks”, eg a vector of `N*K` elements into `N` chunks of `K`, then doing something to each chunk. As an MWE, consider

```julia
z = zeros(N*K)
for i in 1:N
    z[((i-1)*k+1):(i*k)] = some_calculation(i)
end

```

Typing this up every time is prone to various coding errors.

In some contexts, I can use `reshape`, as in

```julia
_z = reshape(z, K, N)
for i in 1:N
    _z[:, i] = some_calculation(i)
end
# subsequent calculations use z as a result

```

but this only works if `z` is accepted by `reshape` as a view, and it relies on the structure and representation of matrices.

I also considered making a calculator for this, as in

```julia
struct Chunks
    K::Int
end

Base.getindex(c::Chunks, i::Int) = (e = c.K * i; (e - c.K + 1):e)

```

that I can reuse:

```julia
julia> for i in 1:3
       @show Chunks(7)[i]
       end
(Chunks(7))[i] = 1:7
(Chunks(7))[i] = 8:14
(Chunks(7))[i] = 15:21

```

but it is too tiny to make into a package (does any package have this already?)

How do others deal with this kind of problem?

---

<div class="post-metadata">

**Author:** ![baggepinnen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/baggepinnen/32/693_2.png) [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)\
**Post date:** [November 18, 2025, 2:39pm UTC](https://discourse.julialang.org/t/shorthand-for-i-1-k-1-i-k/133968/2 "2025-11-18T14:39:02Z")

</div>

I often do

```julia-auto
inds = 1:K
for i = ...
   z[inds] = ...
   inds = inds .+ K
end

```

this is allocation free since adding a constant to a range keeps `inds` a range. It’s also easy to handle the case when `inds` doesn’t cover the entire chunk, that is, `inds` is shorter than the step length between iterations

---

<div class="post-metadata">

**Author:** ![gdalle](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gdalle/32/27854_2.png) [@gdalle](https://discourse.julialang.org/u/gdalle)\
**Post date:** [November 18, 2025, 2:44pm UTC](https://discourse.julialang.org/t/shorthand-for-i-1-k-1-i-k/133968/3 "2025-11-18T14:44:38Z")

</div>

Isn’t that what ChunkSplitters.jl is for?

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<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [November 18, 2025, 2:49pm UTC](https://discourse.julialang.org/t/shorthand-for-i-1-k-1-i-k/133968/4 "2025-11-18T14:49:02Z")

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Exactly! I just didn’t know about it.

(The Julia community is spoiling me. Every time I run into a problem, it turns out that someone made a high-quality package that solves it.)

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<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [November 18, 2025, 2:49pm UTC](https://discourse.julialang.org/t/shorthand-for-i-1-k-1-i-k/133968/5 "2025-11-18T14:49:20Z")

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Or even `Iterators.partition`: that’ll efficiently spit back out ranges. I often find its API to feel backwards — you specify the length of each inner chunk rather than the total number of chunks, but in this case it looks like it’s exactly what you want!

Just grab the axis, get your list of `k`-length partitions, and iterate over that (or `enumerate` it)

---

<div class="post-metadata">

**Author:** ![baggepinnen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/baggepinnen/32/693_2.png) [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)\
**Post date:** [November 18, 2025, 2:49pm UTC](https://discourse.julialang.org/t/shorthand-for-i-1-k-1-i-k/133968/6 "2025-11-18T14:49:35Z")

</div>

You also have `partition`

```julia-auto
julia> K = 3; N = 5;

julia> Iterators.partition(1:K*N, K) |> collect
5-element Vector{UnitRange{Int64}}:
 1:3
 4:6
 7:9
 10:12
 13:15

```

* * *

Looks like Matt beat me to it 🙂