# How to check if there are at least one true Cartesian index?

**URL:** <https://discourse.julialang.org/t/how-to-check-if-there-are-at-least-one-true-cartesian-index/72961>\
**Category:** General Usage\
**Tags:** question\
**Created:** [December 12, 2021, 1:25am UTC](https://discourse.julialang.org/t/how-to-check-if-there-are-at-least-one-true-cartesian-index/72961 "2021-12-12T01:25:36Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![leon](https://avatars.discourse-cdn.com/v4/letter/l/dc4da7/32.png) [@leon](https://discourse.julialang.org/u/leon)\
**Post date:** [December 12, 2021, 1:25am UTC](https://discourse.julialang.org/t/how-to-check-if-there-are-at-least-one-true-cartesian-index/72961/1 "2021-12-12T01:25:36Z")

</div>

I love Cartesian indices because of its performance. One issue I’m not clear is how to check if there are at least one true Cartesian index.

For example, if I have a vector as the below:  
`A = [1, 2, 3, 4, 5];`

My regular Index will work like this:

```julia
Ind1 = A .>3.5
5-element BitVector:
 0
 0
 0
 1
 1

```

By using the below syntax, I can check if there is at least one valid value:

```julia
if sum(Ind1) > 0
...
end

```

My question is how do I check whether a group of Cartesian indices have at least one valid value?

Many thanks!

---

<div class="post-metadata">

**Author:** ![aramirezreyes](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aramirezreyes/32/42573_2.png) [@aramirezreyes](https://discourse.julialang.org/u/aramirezreyes)\
**Post date:** [December 12, 2021, 2:04am UTC](https://discourse.julialang.org/t/how-to-check-if-there-are-at-least-one-true-cartesian-index/72961/2 "2021-12-12T02:04:28Z")

</div>

Can you give an example of what you need this for?  
In particular, I don’t understand what so you mean by “index” being different than zero. In your example you don’t have a list of indices.

Could you use the findall function and check if the result is empty?

```julia
julia> A = [1, 2, 3, 4, 5];

julia> findall(>(3.5),A)
2-element Vector{Int64}:
 4
 5

julia> isempty(findall(>(3.5),A))
false

julia> B = [1 2 ; 3 4]
2×2 Matrix{Int64}:
 1 2
 3 4

julia> findall(>(3.5),B)
1-element Vector{CartesianIndex{2}}:
 CartesianIndex(2, 2)

julia> isempty(findall(>(3.5),B))
false

```

---

<div class="post-metadata">

**Author:** ![leon](https://avatars.discourse-cdn.com/v4/letter/l/dc4da7/32.png) [@leon](https://discourse.julialang.org/u/leon)\
**Post date:** [December 12, 2021, 2:27am UTC](https://discourse.julialang.org/t/how-to-check-if-there-are-at-least-one-true-cartesian-index/72961/3 "2021-12-12T02:27:34Z")

</div>

Thank you for the reply!

For example, I have a function like the below to check if a column data has at least real value, i.e., non-NaN and non -999 (missing value indicator).

`f(A) = [idx for idx in CartesianIndices(A) if @inbounds (!isnan(A[idx]) && A[idx] != -999)];`

If my column data is as below:  
B = [1.0, 2.0, NaN, 5.0, -999.0, 8.0];

`Ind2 = f(B)` would give me these Cartesian indices:  
`Ind2 = CartesianIndex{1}[CartesianIndex(1,), CartesianIndex(2,), CartesianIndex(4,), CartesianIndex(6,)]`

`C[Ind2] = [1.0, 2.0, 5.0, 8.0]`

What I’m trying to learn is how to check if Ind2 contains at least one true values.

I think I found the answer:  
`isempty(Ind2) = true`. # when there are no true values.

---

<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:** [December 12, 2021, 8:11am UTC](https://discourse.julialang.org/t/how-to-check-if-there-are-at-least-one-true-cartesian-index/72961/4 "2021-12-12T08:11:05Z")

</div>

`any(>(3.5),B)` is what you’re looking for

---

<div class="post-metadata">

**Author:** ![gustaphe](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gustaphe/32/18174_2.png) [@gustaphe](https://discourse.julialang.org/u/gustaphe)\
**Post date:** [December 12, 2021, 8:40am UTC](https://discourse.julialang.org/t/how-to-check-if-there-are-at-least-one-true-cartesian-index/72961/5 "2021-12-12T08:40:42Z")

</div>

This is also how you should be doing the linear index comparison in the beginning. Why? Because `any` short-circuits. If the first element of `A` is `>0`, only one comparison is needed to definitively say that there is at least one element `>0`. Compare

```julia
julia> function ispositive(x)
    println("$x > 0? $(x > 0)")
    return x > 0
end;

julia> x = [-1, 1, -2, 2];

julia> sum(ispositive.(x)) > 0
-1 > 0 ? false
1 > 0 ? true
-2 > 0 ? false
2 > 0 ? true
true

julia> any(ispositive, x)
-1 > 0 ? false
1 > 0 ? true
true

julia> all(ispositive, x)
-1 > 0 ? false
false

```