# Optimal way to make a line and column permutation copy of an array

**URL:** <https://discourse.julialang.org/t/optimal-way-to-make-a-line-and-column-permutation-copy-of-an-array/29331>\
**Category:** Performance\
**Created:** [September 30, 2019, 3:26pm UTC](https://discourse.julialang.org/t/optimal-way-to-make-a-line-and-column-permutation-copy-of-an-array/29331 "2019-09-30T15:26:24Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![pedrohnv](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pedrohnv/32/212503_2.png) [@pedrohnv](https://discourse.julialang.org/u/pedrohnv)\
**Post date:** [September 30, 2019, 3:26pm UTC](https://discourse.julialang.org/t/optimal-way-to-make-a-line-and-column-permutation-copy-of-an-array/29331/1 "2019-09-30T15:26:24Z")

</div>

How can I optimize the following functions that copy a line-, column- or both- permutation (`pl`, `pc`, `pcl`, respectively) of a `src` array into a `dest` array?

In particular, can the `temp` array be avoided in `pcl`?

```julia
using BenchmarkTools

function pl!(dest, src, lines)
    n = lines
    @views for i = 1:fld(n, 2)
        dest[i,:] = src[n-i+1,:];
        dest[n-i+1,:] = src[i,:];
    end
end

function pc!(dest, src, columns)
    n = columns
    @views for i = 1:fld(n, 2)
        dest[:,i] = src[:,n-i+1];
        dest[:,n-i+1] = src[:,i];
    end
end

function pcl!(dest, src, lines, columns)
    temp = Array(dest)
    pc!(temp, src, columns)
    pl!(dest, temp, lines)
end

n = 1000;
m = 900;
a = zeros(n,m);
b = rand(n,m);

@btime pl!(a, b, n); # 14.170 ms (1000 allocations: 46.88 KiB)
@btime pc!(a, b, m); # 1.410 ms (900 allocations: 42.19 KiB)
@btime pcl!(a, b, n, m); # 16.194 ms (1902 allocations: 6.95 MiB)

```