# Test a function modifies argument
**URL:** https://discourse.julialang.org/t/test-a-function-modifies-argument/29688
**Category:** New to Julia
**Created:** [October 9, 2019, 5:54pm UTC](https://discourse.julialang.org/t/test-a-function-modifies-argument/29688 "2019-10-09T17:54:21Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@squirrel](https://discourse.julialang.org/u/squirrel)
#### Post date: [October 9, 2019, 5:54pm UTC](https://discourse.julialang.org/t/test-a-function-modifies-argument/29688/1 "2019-10-09T17:54:21Z")
How should I write a `@testset` to verify that a function modifies its argument?
For example, here is a function that is passed a vector (of Rationals) `v` and a row index `row`:
```julia
function divideBelow!(v, row)
f = v[row]
for i in row:length(v)
v[i]=v[i]//f
end
return v
end
```
Do I just write multiple lines? Like this:
```julia
@testset "DivideBelow Tests" begin
v = [3//1, 6//5]
divideBelow(v, 2)
@test v == [3//1, 1//1]
end
```
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 9, 2019, 6:26pm UTC](https://discourse.julialang.org/t/test-a-function-modifies-argument/29688/2 "2019-10-09T18:26:15Z")
It is common to return the modified argument (like you do), so you could check that directly, eg
```julia
@test divideBelow!([3//1, 6//5]) == [3//1, 1//1]
```
However, if you explicitly want to check that `v` was modified (instead of copied), you could make a copy and then compare, eg
```julia
w = copy(v)
do_something!(v)
@test w !== v
```
---
### Author:  [@squirrel](https://discourse.julialang.org/u/squirrel)
#### Post date: [October 9, 2019, 8:24pm UTC](https://discourse.julialang.org/t/test-a-function-modifies-argument/29688/3 "2019-10-09T20:24:49Z")
Copying `v` would necessarily make it different than `w` and so `w!==v` would always be true. But I think I see what you mean. I could do something like this:
```julia
v = [1//1, 2//1, 3//1, 4//1]
w = do_something!(v)
@test w === v
```
---
### Author:  [@greg\_plowman](https://discourse.julialang.org/u/greg_plowman)
#### Post date: [October 9, 2019, 11:08pm UTC](https://discourse.julialang.org/t/test-a-function-modifies-argument/29688/4 "2019-10-09T23:08:50Z")
> [@squirrel](#):
>
> Copying `v` would necessarily make it different than `w` and so `w!==v` would always be true.
I think Tamas meant `@test w != v`
---
### Author:  [@greg\_plowman](https://discourse.julialang.org/u/greg_plowman)
#### Post date: [October 10, 2019, 12:18am UTC](https://discourse.julialang.org/t/test-a-function-modifies-argument/29688/5 "2019-10-10T00:18:56Z")
> [@squirrel](#):
>
> I could do something like this:
>
> ```julia
> v = [1//1, 2//1, 3//1, 4//1]
> w = do_something!(v)
> @test w === v
>
> ```
This test will pass for `do_nothing!(v)` too! (assuming the function returns its argument)
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 10, 2019, 5:30am UTC](https://discourse.julialang.org/t/test-a-function-modifies-argument/29688/6 "2019-10-10T05:30:14Z")
Yes, thanks for the correction.