# Byrow with user defined function
**URL:** https://discourse.julialang.org/t/byrow-with-user-defined-function/79132
**Category:** Data
**Tags:** question, inmemorydatasets
**Created:** [April 6, 2022, 11:55pm UTC](https://discourse.julialang.org/t/byrow-with-user-defined-function/79132 "2022-04-06T23:55:43Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@monopolynomial](https://discourse.julialang.org/u/monopolynomial)
#### Post date: [April 6, 2022, 11:55pm UTC](https://discourse.julialang.org/t/byrow-with-user-defined-function/79132/1 "2022-04-06T23:55:44Z")
for data like
```julia
ds=Dataset(x1=[1,2,3],x2=[3,2,1])
```
if I want to filter rows where x1 is less than x2 then this works
```julia
filter(ds, :x1, type=isless, with = :x2)
```
but when I define a function for `byrow` like
```julia
f(x)=x[1]< x[2]
byrow(ds, f, :) # works
```
I cannot use it with `filter`
```julia
filter(ds, :, type=f)
ERROR: BoundsError: attempt to access 3×2 Dataset at index [Union{Missing, Bool}[true, false, false], :]
Stacktrace:
```
---
### Author:  [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)
#### Post date: [April 7, 2022, 10:13am UTC](https://discourse.julialang.org/t/byrow-with-user-defined-function/79132/2 "2022-04-07T10:13:48Z")
after reading that
" … `filter(ds, cols; [view = false, type = all,...])` is the shortcut for `ds[byrow(ds, type, cols; ...), :]` …"
```julia
julia> byrow(ds, f, :)# works
3-element Vector{Union{Missing, Bool}}:
true
false
false
```
I have tried the following expressions, which I would expect to give the same result.
but it is not so, as rightly observed by @monopolynomial
```julia
julia> ds[[true,false,false],:]
1×2 Dataset
Row │ x1 x2
│ identity identity
│ Int64? Int64?
─────┼────────────────────
1 │ 1 3
julia> ds[byrow(ds, f, :),:]
ERROR: BoundsError: attempt to access 3×2 Dataset at index [Union{Missing, Bool}[true, false, false], :]
```
" Naturally, other `fun` s supported by `byrow` which return a `Vector{Bool}` or `BitVector` can be used to filter observations, too."
the problem therefore seems to be that `byrow (...)` only works with `Vector{Bool}` but not with `Vector{Union{Missing, Bool}}`
this “works”
```julia
julia> ds[Bool.(byrow(ds, f, :)),:]
1×2 Dataset
Row │ x1 x2
│ identity identity
│ Int64? Int64?
─────┼────────────────────
1 │ 1 3
```
the issue seems to be the handling of `Union {Missing, T}` types
---
### Author:  [@monopolynomial](https://discourse.julialang.org/u/monopolynomial)
#### Post date: [April 9, 2022, 1:43am UTC](https://discourse.julialang.org/t/byrow-with-user-defined-function/79132/3 "2022-04-09T01:43:55Z")
```julia
julia> ds[Bool.(byrow(ds, f, :)),:]
```
This doesn’t work if `f` returns missing.
---
### Author:  [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)
#### Post date: [April 9, 2022, 10:28am UTC](https://discourse.julialang.org/t/byrow-with-user-defined-function/79132/4 "2022-04-09T10:28:20Z")
Yes.
I have seen and, in part, I had even foreseen it.
But I didn’t want to linger.
Perhaps, in the general case, a function would be needed to establish on a case-by-case basis whether missing is to be evaluated as true or false (or simply remain missing)…
Maybe a kwarg that can be set by the user as needed could do the trick.
---
### Author:  [@Kia\_Kia](https://discourse.julialang.org/u/Kia_Kia)
#### Post date: [May 1, 2022, 10:32am UTC](https://discourse.julialang.org/t/byrow-with-user-defined-function/79132/5 "2022-05-01T10:32:55Z")
findall is good for this.
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [August 25, 2022, 4:41pm UTC](https://discourse.julialang.org/t/byrow-with-user-defined-function/79132/6 "2022-08-25T16:41:36Z")
A post was split to a new topic: [Help with understanding Julia types](https://discourse.julialang.org/t/help-with-understanding-julia-types/86345)