# Dataframe population can't figure out why one approach works and the other does not

**URL:** <https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253>\
**Category:** New to Julia\
**Created:** [March 1, 2022, 6:43pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253 "2022-03-01T18:43:17Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![anon69491625](https://avatars.discourse-cdn.com/v4/letter/a/aeb1de/32.png) [@anon69491625](https://discourse.julialang.org/u/anon69491625)\
**Post date:** [March 1, 2022, 6:43pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253/1 "2022-03-01T18:43:17Z")

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Hi all  
julia noob here. I’m trying to get to grips with changing some functionality in my code based on some excellent guidance from wonderful members of discourse. I have the code “working” but one approach SHOULD work and the other DOES work. I want to know what I am missing. Here is the situation.  
thank you for any help.

I have the following dataframe ( from a csv)

```julia
Type Symbol
 STK XLU       
 STK XLV     
 STK XOP       
 STK XRT       
 CASH EUR       
 CASH GBP       
 CASH AUD       
 CASH JPY       
 CASH CHF     
 CASH CAD       
 IND VIX      
 IND RVX       
 IND VXN       
 IND VXD      
 FUT ES      
 FUT CL  

```

I want just the STK rows so I filter

```julia
symbol_list = symbol_data[symbol_data.Type .== "STK", :]

```

and get

```julia
Type Symbol
 STK XLU       
 STK XLV     
 STK XOP       
 STK XRT       

```

so far so good. NOW comes the tricky bit

I want to build a Dataframe like this

```julia
Symbols price ticksize
XLU 0.0 0.0
XLV 0.0 0.0
XOP 0.0 0.0
XRT 0.0 0.0

```

someone was kind enough to send me this code

```julia
df = DataFrame(symbols = symbol_list, price = zeros(length(symbol_list)), tickSize = zeros(length(symbol_list)))

```

which gives me this error

```julia
RROR: MethodError: no method matching length(::DataFrame)
Closest candidates are:
  length(::Union{Base.KeySet, Base.ValueIterator}) at ~/julia-1.7.0/share/julia/base/abstractdict.jl:58
  length(::Union{LinearAlgebra.Adjoint{T, S}, LinearAlgebra.Transpose{T, S}} where {T, S}) at ~/julia-1.7.0/share/julia/stdlib/v1.7/LinearAlgebra/src/adjtrans.jl:171
  length(::Union{DataStructures.OrderedRobinDict, DataStructures.RobinDict}) at ~/.julia/packages/DataStructures/vSp4s/src/ordered_robin_dict.jl:86
  ...

```

as does

```julia
df = DataFrame(symbols = symbol_list.Symbol, price = zeros(length(symbol_list)), tickSize = zeros(length(symbol_list)))
ERROR: MethodError: no method matching length(::DataFrame)
Closest candidates are:
  length(::Union{Base.KeySet, Base.ValueIterator}) at ~/julia-1.7.0/share/julia/base/abstractdict.jl:58
  length(::Union{LinearAlgebra.Adjoint{T, S}, LinearAlgebra.Transpose{T, S}} where {T, S}) at ~/julia-1.7.0/share/julia/stdlib/v1.7/LinearAlgebra/src/adjtrans.jl:171
  length(::Union{DataStructures.OrderedRobinDict, DataStructures.RobinDict}) at ~/.julia/packages/DataStructures/vSp4s/src/ordered_robin_dict.jl:86
  ...

```

YET

```julia
symbol_list2 = symbol_list.Symbol
df = DataFrame(symbols = symbol_list2, price = zeros(length(symbol_list2)), tickSize = zeros(length(symbol_list2)))

```

works fine???

```julia
symbols price ticksize
XLU 0.0 0.0
XLV 0.0 0.0
XOP 0.0 0.0
 XRT 0.0 0.0

```

what’s the difference between

```julia
typeof(symbol_list2)
Vector{Union{Missing, String7}} (alias for Array{Union{Missing, String7}, 1})

```

and

```julia
typeof(symbol_list.Symbol)
Vector{Union{Missing, String7}} (alias for Array{Union{Missing, String7}, 1})

```

---

<div class="post-metadata">

**Author:** ![nilshg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nilshg/32/2283_2.png) [@nilshg](https://discourse.julialang.org/u/nilshg)\
**Post date:** [March 1, 2022, 7:23pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253/2 "2022-03-01T19:23:22Z")

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Isn’t the issue here the difference between `symbol_list` and `symbol_list2`?

The errors just tell you the `length` is not defined for `DataFrame`s (you would want `nrow` in that case).

`symbol_list` in your code is a DataFrame, while `symbol_list.Symbol` is a vector (for which `length` is defined). You’re not showing how `symbol_list2` is defined, but it appears to be a vector as well (maybe you did `symbol_list2 = symbol_list.Symbol` somehwere?)

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<div class="post-metadata">

**Author:** ![anon69491625](https://avatars.discourse-cdn.com/v4/letter/a/aeb1de/32.png) [@anon69491625](https://discourse.julialang.org/u/anon69491625)\
**Post date:** [March 1, 2022, 7:38pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253/3 "2022-03-01T19:38:08Z")

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hi @nilshg  
thanks for the code and I am listening to you two 🙂

If you look at the code snippets you’ll see that I did include how symbol\_list2 was formed

```julia
symbol_list2 = symbol_list.Symbol

```

my fault for multiline, sorry.

so I would expect

```julia
df = DataFrame(symbols = symbol_list.Symbol, price = zeros(length(symbol_list)), tickSize = zeros(length(symbol_list)))

```

BUT as I cut and pasted it I THINK I see the issue. I didn’t change the zeros values to get the length of symbol\_list.Symbol but left them at symbol\_list. Ho hum. I’ll try later tonight to go back to the drawing board 🙂  
sorry to have bothered you all.

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<div class="post-metadata">

**Author:** ![pdeffebach](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pdeffebach/32/10320_2.png) [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)\
**Post date:** [March 1, 2022, 7:39pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253/4 "2022-03-01T19:39:24Z")

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Nils gave you the answer above. All you need to do is instead of write `length(symbol_list)`, do `nrow(symbol_list)`.

---

<div class="post-metadata">

**Author:** ![anon69491625](https://avatars.discourse-cdn.com/v4/letter/a/aeb1de/32.png) [@anon69491625](https://discourse.julialang.org/u/anon69491625)\
**Post date:** [March 1, 2022, 7:43pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253/5 "2022-03-01T19:43:18Z")

</div>

which is cleaner? remember I am learning here.

```julia
df = DataFrame(symbols = symbol_list.Symbol, price = zeros(length(symbol_list.Symbol)), tickSize = zeros(length(symbol_list.Symbol)))

```

OR

```julia
df = DataFrame(symbols = symbol_list.Symbol, price = zeros(nrow(symbol_list)), tickSize = zeros(nrow(symbol_list)))

```

and why please?

I want to make sure I set a firm foundation.

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<div class="post-metadata">

**Author:** ![junder873](https://avatars.discourse-cdn.com/v4/letter/j/e95f7d/32.png) [@junder873](https://discourse.julialang.org/u/junder873)\
**Post date:** [March 1, 2022, 8:40pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253/6 "2022-03-01T20:40:59Z")

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I would probably say the second is “cleaner”, mainly because if you change the name of the dataframe column from “Symbol” to something else it is less of an issue.

However, if your goal is to form a new DataFrame based on part of another, I would actually not do either. [DataFramesMeta](https://github.com/JuliaData/DataFramesMeta.jl) makes these operations a lot easier. I would do something more like this:

```julia
df_symbols = @rsubset(df, :Type == "STK") # r in rsubset stands for a by row operation
@rselect!(
    df_symbols,
    :Type,
    :price = 0,# change this and the next one to 0.0 if you actually need float values
    :tickSize = 0
)

```

---

<div class="post-metadata">

**Author:** ![anon69491625](https://avatars.discourse-cdn.com/v4/letter/a/aeb1de/32.png) [@anon69491625](https://discourse.julialang.org/u/anon69491625)\
**Post date:** [March 1, 2022, 9:04pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253/7 "2022-03-01T21:04:48Z")

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thank you for pointing out DataFramesMeta I wasn’t aware of that package. I’ll look into it and thanks for the guidance on the coding issue.

---

<div class="post-metadata">

**Author:** ![anon69491625](https://avatars.discourse-cdn.com/v4/letter/a/aeb1de/32.png) [@anon69491625](https://discourse.julialang.org/u/anon69491625)\
**Post date:** [March 1, 2022, 9:58pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253/8 "2022-03-01T21:58:46Z")

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thanks @nilshg as always. I’m getting there slowly but surely 🙂

---

<div class="post-metadata">

**Author:** ![anon69491625](https://avatars.discourse-cdn.com/v4/letter/a/aeb1de/32.png) [@anon69491625](https://discourse.julialang.org/u/anon69491625)\
**Post date:** [March 1, 2022, 11:32pm UTC](https://discourse.julialang.org/t/dataframe-population-cant-figure-out-why-one-approach-works-and-the-other-does-not/77253/9 "2022-03-01T23:32:13Z")

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Hi there  
just in case you thought I was ignoring @nilshg I wouldn’t. I respect his opinion and he’s done nothing but be supportive of my attempts to get my julia adventure off on the the right track. What happened here wa that I realized my mistake and had sent my reply BEFORE I read his solution. Hope this clarifies the situation. I have nothing but respect for @nilshg