# Improvement for slicing 2d arrays?
**URL:** https://discourse.julialang.org/t/improvement-for-slicing-2d-arrays/52775
**Category:** New to Julia
**Created:** [January 3, 2021, 3:09pm UTC](https://discourse.julialang.org/t/improvement-for-slicing-2d-arrays/52775 "2021-01-03T15:09:42Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@freeman](https://discourse.julialang.org/u/freeman)
#### Post date: [January 3, 2021, 3:09pm UTC](https://discourse.julialang.org/t/improvement-for-slicing-2d-arrays/52775/1 "2021-01-03T15:09:42Z")
Consider the following 2d array:
```julia
X = Array{Float64, 2}(undef, 4, 2)
X[:,1] = [1,NaN,3,4]
X[:,2] = [2,3,4,5]
```
I want to filter out all rows which include at least one `NaN`. This could be done by instantiating a “mask” vector:
```julia
idx = .!any(isnan.(X), dims=2);
X[idx, :]
```
However the second line errs:
```julia
ERROR: BoundsError: attempt to access 4×2 Array{Float64,2} at index [Bool[1; 0; 1; 1], 1:1]
```
I think what is happening is that `X[idx, :]` expects that `idx` is either one-dimensional or two-dimensional with the same dimensions as `X`. Since the type is `BitArray{2}` then Julia concludes that we’re in the second scenario. To get around this I had to do:
```julia
idx_1d = reshape(idx, size(idx)[1])
X[idx_1d, :]
```
Now `idx_1d` is type `BitArray{1}` and this works, but is ugly. Given that this is such a normal use case for slicing, my suggestion is that slicing should consider the first scenario if `BitArray{2}` and the size is 1 in the second dimension. Admittedly I haven’t considered what side-effects this could cause elsewhere, but I thought I’d share this observation.
---
### Author:  [@mcabbott](https://discourse.julialang.org/u/mcabbott)
#### Post date: [January 3, 2021, 3:18pm UTC](https://discourse.julialang.org/t/improvement-for-slicing-2d-arrays/52775/2 "2021-01-03T15:18:50Z")
I think the indexing question is tricky. But you might also consider writing `idx = map(r -> !any(isnan, r), eachrow(X))` or more compactly `idx = .!any.(isnan, eachrow(X))`.
---
### Author:  [@yha](https://discourse.julialang.org/u/yha)
#### Post date: [January 3, 2021, 3:41pm UTC](https://discourse.julialang.org/t/improvement-for-slicing-2d-arrays/52775/3 "2021-01-03T15:41:30Z")
I think this is more about reductions with `dims` keyword arg not dropping the reduced dimension. There’s a somewhat long discussion about it in the relevant github issue: [https://github.com/JuliaLang/julia/issues/16606](https://github.com/JuliaLang/julia/issues/16606).
In this specific case, I think rewriting it as @mcabbott has suggested is the nicest solution. But more generally, you can just drop the extra dimension yourself:
```julia
idx = .!any(isnan.(X), dims=2)[:,1]
```
or
```julia
idx = dropdims(.!any(isnan.(X), dims=2), dims=2)
```
a bit ugly too, but clearer than `reshape`, I think.