# Iterating over elements of upper triangular matrix, but cartesian indices are needed

**URL:** <https://discourse.julialang.org/t/iterating-over-elements-of-upper-triangular-matrix-but-cartesian-indices-are-needed/65498>\
**Category:** New to Julia\
**Tags:** indexing, matrices\
**Created:** [July 29, 2021, 3:00pm UTC](https://discourse.julialang.org/t/iterating-over-elements-of-upper-triangular-matrix-but-cartesian-indices-are-needed/65498 "2021-07-29T15:00:42Z")\
**Posts on this page:** 1\
**Showing post:** 3

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [July 29, 2021, 6:26pm UTC](https://discourse.julialang.org/t/iterating-over-elements-of-upper-triangular-matrix-but-cartesian-indices-are-needed/65498/3 "2021-07-29T18:26:19Z")

</div>

Thanks. The custom function resulted to be the most practical alternative for now. For the records, it is this one:

```julia
@inline function iuppert(k::Integer,n::Integer)
  i = n - 1 - floor(Int,sqrt(-8*k + 4*n*(n-1) + 1)/2 - 0.5)
  j = k + i + ( (n-i+1)*(n-i) - n*(n-1) )÷2
  return i, j
end

```

```julia-repl
julia> iuppert(1,3)
(1,2)

julia> iuppert(3,3)
(2,2)

julia> iuppert(3,4)
(1,4)

```

---

_[View the full topic](https://discourse.julialang.org/t/iterating-over-elements-of-upper-triangular-matrix-but-cartesian-indices-are-needed/65498)._