# Error when combining single row with multiple row CSV file into DataFrames
**URL:** https://discourse.julialang.org/t/error-when-combining-single-row-with-multiple-row-csv-file-into-dataframes/111664
**Category:** Data
**Tags:** dataframes, csv
**Created:** [March 15, 2024, 1:37pm UTC](https://discourse.julialang.org/t/error-when-combining-single-row-with-multiple-row-csv-file-into-dataframes/111664 "2024-03-15T13:37:24Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@phantom](https://discourse.julialang.org/u/phantom)
#### Post date: [March 15, 2024, 1:37pm UTC](https://discourse.julialang.org/t/error-when-combining-single-row-with-multiple-row-csv-file-into-dataframes/111664/1 "2024-03-15T13:37:24Z")
Hi! I am combining several `.csv` files into DataFrames. Each file has uniform column numbers and data types. Everything works but when I try to combine a `.csv` file that contains a single row with one that contains multiple rows I get the following error.
```julia
CSV.read(["singlerowfilepath","multiplerowfilepath"], DataFrame)
```
returns
```julia
ERROR: UndefVarError: `A` not defined
Stacktrace:
[1] (::CSV.var"#3#4")(x::PooledArrays.PooledVector{String31, UInt32, Vector{UInt32}})
@ CSV ./none:0
[2] iterate
@ ./generator.jl:47 [inlined]
[3] collect(itr::Base.Generator{Vector{PooledArrays.PooledVector{String31, UInt32, Vector{UInt32}}}, CSV.var"#3#4"})
@ Base ./array.jl:834
[4] chaincolumns!(a::Any, b::Any)
@ CSV ~/.julia/packages/CSV/tmZyn/src/utils.jl:240
[5] CSV.File(sources::Vector{String}; source::Nothing, kw::@Kwargs{})
@ CSV ~/.julia/packages/CSV/tmZyn/src/file.jl:930
[6] File
@ ~/.julia/packages/CSV/tmZyn/src/file.jl:901 [inlined]
[7] read(source::Vector{String}, sink::Type; copycols::Bool, kwargs::@Kwargs{})
@ CSV ~/.julia/packages/CSV/tmZyn/src/CSV.jl:117
[8] read(source::Vector{String}, sink::Type)
@ CSV ~/.julia/packages/CSV/tmZyn/src/CSV.jl:113
[9] top-level scope
@ REPL[134]:1
```
and
```julia
DataFrame!(CSV.File(["singlerowfilepath","multiplerowfilepath"]))
or
DataFrame(CSV.File(["singlerowfilepath","multiplerowfilepath"]))
```
each return
```julia
ERROR: UndefVarError: `A` not defined
Stacktrace:
[1] (::CSV.var"#3#4")(x::PooledArrays.PooledVector{String31, UInt32, Vector{UInt32}})
@ CSV ./none:0
[2] iterate
@ ./generator.jl:47 [inlined]
[3] collect(itr::Base.Generator{Vector{PooledArrays.PooledVector{String31, UInt32, Vector{UInt32}}}, CSV.var"#3#4"})
@ Base ./array.jl:834
[4] chaincolumns!(a::Any, b::Any)
@ CSV ~/.julia/packages/CSV/tmZyn/src/utils.jl:240
[5] CSV.File(sources::Vector{String}; source::Nothing, kw::@Kwargs{})
@ CSV ~/.julia/packages/CSV/tmZyn/src/file.jl:930
[6] CSV.File(sources::Vector{String})
@ CSV ~/.julia/packages/CSV/tmZyn/src/file.jl:901
[7] top-level scope
@ REPL[138]:1
```
Whereas reversing the order of the files i.e.
```julia
CSV.read(["multiplerowfilepath","singlerowfilepath"], DataFrame)
```
returns a Dataframe with uniform column number and data type.
Just wondering if anyone had any insight on why this is happening and what I might do to fix it without having to worry about the order of the files. Thanks!
---
### Author:  [@nilshg](https://discourse.julialang.org/u/nilshg)
#### Post date: [March 15, 2024, 2:02pm UTC](https://discourse.julialang.org/t/error-when-combining-single-row-with-multiple-row-csv-file-into-dataframes/111664/2 "2024-03-15T14:02:38Z")
This is a pretty annoying error and I think the error message should be improved (or maybe the behaviour).
You can roughly guess from here:
```julia
[1] (::CSV.var"#3#4")(x::PooledArrays.PooledVector{String31, UInt32, Vector{UInt32}})
@ CSV ./none:0
```
That it comes from `PooledArrays`. What’s happening here is that CSV is trying to stitch together one table from the two CSVs, but if it decides to pool the values in the first one this fails if there are additional values in the second table. In your case where you have only one row there’s only one value in it and the pool can’t capture the other values, while if you go the other way around the pool will have seen the whole range of values and can accomodate the additional value.
You can turn pooling off with the `pool = false` kwarg.
---
### Author:  [@phantom](https://discourse.julialang.org/u/phantom)
#### Post date: [March 15, 2024, 2:14pm UTC](https://discourse.julialang.org/t/error-when-combining-single-row-with-multiple-row-csv-file-into-dataframes/111664/3 "2024-03-15T14:14:28Z")
got it thanks! Is there a downside to defaulting to pool = false other than memory usage?
---
### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [March 15, 2024, 2:19pm UTC](https://discourse.julialang.org/t/error-when-combining-single-row-with-multiple-row-csv-file-into-dataframes/111664/4 "2024-03-15T14:19:39Z")
Hopefully not! `PooledVector{String}` should be the exact same as `Vector{String}` for all intents and purposes.
---
### Author:  [@phantom](https://discourse.julialang.org/u/phantom)
#### Post date: [March 15, 2024, 2:22pm UTC](https://discourse.julialang.org/t/error-when-combining-single-row-with-multiple-row-csv-file-into-dataframes/111664/5 "2024-03-15T14:22:40Z")
got it, thanks!
---
### Author:  [@nalimilan](https://discourse.julialang.org/u/nalimilan)
#### Post date: [March 15, 2024, 2:36pm UTC](https://discourse.julialang.org/t/error-when-combining-single-row-with-multiple-row-csv-file-into-dataframes/111664/6 "2024-03-15T14:36:54Z")
Could you file a bug against CSV.jl on GitHub? If you can provide toy files to reproduce the problem that would be even better.
---
### Author:  [@phantom](https://discourse.julialang.org/u/phantom)
#### Post date: [March 15, 2024, 8:48pm UTC](https://discourse.julialang.org/t/error-when-combining-single-row-with-multiple-row-csv-file-into-dataframes/111664/7 "2024-03-15T20:48:29Z")
Sure thing. I submitted an issue with a dummy example [here](https://github.com/JuliaData/CSV.jl/issues/1130).