# Divide a matrix into 4 submatrices
**URL:** https://discourse.julialang.org/t/divide-a-matrix-into-4-submatrices/83856
**Category:** General Usage
**Created:** [July 6, 2022, 8:24pm UTC](https://discourse.julialang.org/t/divide-a-matrix-into-4-submatrices/83856 "2022-07-06T20:24:48Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@JosieG](https://discourse.julialang.org/u/JosieG)
#### Post date: [July 6, 2022, 8:24pm UTC](https://discourse.julialang.org/t/divide-a-matrix-into-4-submatrices/83856/1 "2022-07-06T20:24:48Z")
Hi, I’m given a 2\*38 matrix.
> -7.002 -6.94768 -6.92167 -6.85372 … -3.67384 -3.61184 -3.51171
> -1.00187 -1.15017 -1.08179 -1.09073 -0.389809 -0.730911 -0.813239
And I want to get four submatrices. The first three ones would be 2_10 matrices. And the last one would be a 2_8 matrix.
I tried
> matrix[1:10]
> But didn’t work.
Is there anyway to get that?
---
### Author:  [@Christopher\_Fisher](https://discourse.julialang.org/u/Christopher_Fisher)
#### Post date: [July 6, 2022, 8:34pm UTC](https://discourse.julialang.org/t/divide-a-matrix-into-4-submatrices/83856/2 "2022-07-06T20:34:14Z")
There appears to be typos in your description. Assuming you need a 2 X 10 sub-matrix, you need to specify the range of the first dimension:
```julia
matrix[1:2,1:10]
```
The size of the first dimension is 2, which means you can use
```julia
matrix[:, 1:10]
```
for simplicity.
---
### Author:  [@aplavin](https://discourse.julialang.org/u/aplavin)
#### Post date: [July 6, 2022, 8:51pm UTC](https://discourse.julialang.org/t/divide-a-matrix-into-4-submatrices/83856/3 "2022-07-06T20:51:17Z")
```julia
using SplitApplyCombine
A = your matrix
submatrices = combinedims.(Iterators.partition(splitdims(A), 10))
```
should give the four submatrices.
---
### Author:  [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)
#### Post date: [July 7, 2022, 8:58am UTC](https://discourse.julialang.org/t/divide-a-matrix-into-4-submatrices/83856/4 "2022-07-07T08:58:28Z")
A packageless option:
```julia
x = rand(2,38)
y = reduce.(hcat, Iterators.partition(eachcol(x), 10))
```
---
### Author:  [@jishnub](https://discourse.julialang.org/u/jishnub)
#### Post date: [July 7, 2022, 10:12am UTC](https://discourse.julialang.org/t/divide-a-matrix-into-4-submatrices/83856/5 "2022-07-07T10:12:56Z")
Same result, but instead of partitioning the array we partition the indices, which is faster:
```julia
julia> x = rand(2,38);
julia> m = [x[:, ax2] for ax2 in Iterators.partition(axes(x,2), 10)]
4-element Vector{Matrix{Float64}}:
[0.904705750749821 0.89098630758476 … 0.0858392997086902 0.5148424692100146; 0.4269731549291972 0.6437199317364444 … 0.09762794727478441 0.2914246434802973]
[0.24813277171941672 0.7528059317416134 … 0.138920960237655 0.37400443614388357; 0.9669781731841847 0.017065106733320068 … 0.7448520415962544 0.13803101847512966]
[0.42658976306258567 0.3065657481592433 … 0.7828512257915194 0.3755535656510294; 0.29531362851265053 0.4489025029925201 … 0.6823570780073742 0.710501907163102]
[0.7784440666703072 0.7317438098094589 … 0.6476143013715214 0.23107949964776875; 0.12731524958584162 0.9071662545609575 … 0.8694040829128722 0.3029390510048475]
julia> y = @btime reduce.(hcat, Iterators.partition(eachcol($x), 10));
9.596 μs (272 allocations: 18.52 KiB)
julia> m = @btime [$x[:, ax2] for ax2 in Iterators.partition(axes($x,2), 10)];
309.531 ns (5 allocations: 944 bytes)
julia> m == y
true
```
Of course, we may wrap views instead of slices, which is even faster:
```julia
julia> v = [@view(x[:, ax2]) for ax2 in Iterators.partition(axes(x,2), 10)]
4-element Vector{SubArray{Float64, 2, Matrix{Float64}, Tuple{Base.Slice{Base.OneTo{Int64}}, UnitRange{Int64}}, true}}:
[0.904705750749821 0.89098630758476 … 0.0858392997086902 0.5148424692100146; 0.4269731549291972 0.6437199317364444 … 0.09762794727478441 0.2914246434802973]
[0.24813277171941672 0.7528059317416134 … 0.138920960237655 0.37400443614388357; 0.9669781731841847 0.017065106733320068 … 0.7448520415962544 0.13803101847512966]
[0.42658976306258567 0.3065657481592433 … 0.7828512257915194 0.3755535656510294; 0.29531362851265053 0.4489025029925201 … 0.6823570780073742 0.710501907163102]
[0.7784440666703072 0.7317438098094589 … 0.6476143013715214 0.23107949964776875; 0.12731524958584162 0.9071662545609575 … 0.8694040829128722 0.3029390510048475]
julia> v = @btime [@view($x[:, ax2]) for ax2 in Iterators.partition(axes($x,2), 10)];
70.397 ns (1 allocation: 256 bytes)
julia> v == y
true
```
Each of these produces a `Vector` of matrices, where the elements are sub-matrices of the sizes that you want.
```julia
julia> size.(v)
4-element Vector{Tuple{Int64, Int64}}:
(2, 10)
(2, 10)
(2, 10)
(2, 8)
```
---
### Author:  [@HerAdri](https://discourse.julialang.org/u/HerAdri)
#### Post date: [July 8, 2022, 10:52am UTC](https://discourse.julialang.org/t/divide-a-matrix-into-4-submatrices/83856/6 "2022-07-08T10:52:28Z")
> [@jishnub](#):
>
> `@view`
```julia
julia> m=[$x[:, ax2] for ax2 in Iterators.partition(axes($x,2), 10)]
ERROR: syntax: "$" expression outside quote around REPL[4]:1
Stacktrace:
[1] top-level scope
@ REPL[4]:1
julia> [@view($x[:, ax2]) for ax2 in Iterators.partition(axes($x,2), 10)];
ERROR: syntax: "$" expression outside quote around REPL[8]:1
Stacktrace:
[1] top-level scope
@ REPL[8]:1
julia> 70.397 ns (1 allocation: 256 bytes)
```
> **VERSION**
>
> julia\> versioninfo()
> Julia Version 1.7.3
> Commit 742b9abb4d (2022-05-06 12:58 UTC)
> Platform Info:
> OS: Windows (x86\_64-w64-mingw32)
> CPU: Intel(R) Core™ i5-4570 CPU @ 3.20GHz
> WORD\_SIZE: 64
> LIBM: libopenlibm
> LLVM: libLLVM-12.0.1 (ORCJIT, haswell)
---
### Author:  [@jishnub](https://discourse.julialang.org/u/jishnub)
#### Post date: [July 8, 2022, 11:20am UTC](https://discourse.julialang.org/t/divide-a-matrix-into-4-submatrices/83856/7 "2022-07-08T11:20:54Z")
You need to run these without the dollar signs, those are for interpolation while benchmarking