Re-posted from: https://bkamins.github.io/julialang/2023/07/07/dfbroadcast.html
Introduction
Broadcasting, a.k.a. the . operator, is a powerful feature of Julia that allows you to
easily vectorize any function. Today I want to write about some common broadcasting
patterns that can be used in DataFrames.jl.
The post was written under Julia 1.9.2 and DataFrames.jl 1.5.0.
Conditional replacement of values in a data frame
Let us create a data frame with some random contents:
julia> using DataFrames
julia> using Random
julia> df = DataFrame(rand(5, 6), :auto)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼─────────────────────────────────────────────────────────────────
1 │ 0.364225 0.690894 0.240867 0.720774 0.331573 0.549766
2 │ 0.225226 0.241412 0.0793279 0.418206 0.775367 0.35275
3 │ 0.19913 0.0633375 0.767805 0.280096 0.721995 0.917259
4 │ 0.708132 0.230088 0.702677 0.947402 0.928979 0.66101
5 │ 0.0267573 0.0122425 0.549734 0.331788 0.32658 0.00476749
Assume we want to get a new data frame that has true when the value stored
in the cell is greater than 0.5 and false otherwise. This is easy.
We just broadcast the > operator:
julia> df .> 0.5
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Bool Bool Bool Bool Bool Bool
─────┼──────────────────────────────────────────
1 │ false true false true false true
2 │ false false false false true false
3 │ false false true false true true
4 │ true false true true true true
5 │ false false true false false false
Now assume we want to replace all values greater than 0.5 with 0.5 and
keep the lower values untouched. This can be done with ifelse:
julia> ifelse.(df .> 0.5, 0.5, df)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼─────────────────────────────────────────────────────────────────
1 │ 0.364225 0.5 0.240867 0.5 0.331573 0.5
2 │ 0.225226 0.241412 0.0793279 0.418206 0.5 0.35275
3 │ 0.19913 0.0633375 0.5 0.280096 0.5 0.5
4 │ 0.5 0.230088 0.5 0.5 0.5 0.5
5 │ 0.0267573 0.0122425 0.5 0.331788 0.32658 0.00476749
Or with clamp:
julia> clamp.(df, -Inf, 0.5)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼─────────────────────────────────────────────────────────────────
1 │ 0.364225 0.5 0.240867 0.5 0.331573 0.5
2 │ 0.225226 0.241412 0.0793279 0.418206 0.5 0.35275
3 │ 0.19913 0.0633375 0.5 0.280096 0.5 0.5
4 │ 0.5 0.230088 0.5 0.5 0.5 0.5
5 │ 0.0267573 0.0122425 0.5 0.331788 0.32658 0.00476749
Similarly we could clamp values to the [0.1, 0.9] interval:
julia> clamp.(df, 0.1, 0.9)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼────────────────────────────────────────────────────────────
1 │ 0.364225 0.690894 0.240867 0.720774 0.331573 0.549766
2 │ 0.225226 0.241412 0.1 0.418206 0.775367 0.35275
3 │ 0.19913 0.1 0.767805 0.280096 0.721995 0.9
4 │ 0.708132 0.230088 0.702677 0.9 0.9 0.66101
5 │ 0.1 0.1 0.549734 0.331788 0.32658 0.1
Importantly, we do not need to keep the element type of the source column fixed.
Assume that we want to set values greater than 0.5 to missing:
julia> ifelse.(df .> 0.5, missing, df)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64? Float64? Float64? Float64? Float64? Float64?
─────┼─────────────────────────────────────────────────────────────────────────────────────────────────────
1 │ 0.364225 missing 0.240867 missing 0.331573 missing
2 │ 0.225226 0.241412 0.0793279 0.418206 missing 0.35275
3 │ 0.19913 0.0633375 missing 0.280096 missing missing
4 │ missing 0.230088 missing missing missing missing
5 │ 0.0267573 0.0122425 missing 0.331788 0.32658 0.00476749
Note that the operation performed an automatic promotion of column element types.
As a final operation consider taking sign(log(x) + 1) on our data frame:
julia> sign.(log.(df) .+ 1)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼──────────────────────────────────────────────────────
1 │ -1.0 1.0 -1.0 1.0 -1.0 1.0
2 │ -1.0 -1.0 -1.0 1.0 1.0 -1.0
3 │ -1.0 -1.0 1.0 -1.0 1.0 1.0
4 │ 1.0 -1.0 1.0 1.0 1.0 1.0
5 │ -1.0 -1.0 1.0 -1.0 -1.0 -1.0
Again – things are easy and intuitive. Data frame behaves just like a matrix in all operations.
I hope now you are comfortable with creation of a new data frame using broadcasting.
We can turn to in-place operations on a data frame.
In-place update of values in a data frame
In general it is enough to just put data frame on a right hand side of a broadcasted assignment
operator to update it in-place:
julia> df2 = copy(df)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼─────────────────────────────────────────────────────────────────
1 │ 0.364225 0.690894 0.240867 0.720774 0.331573 0.549766
2 │ 0.225226 0.241412 0.0793279 0.418206 0.775367 0.35275
3 │ 0.19913 0.0633375 0.767805 0.280096 0.721995 0.917259
4 │ 0.708132 0.230088 0.702677 0.947402 0.928979 0.66101
5 │ 0.0267573 0.0122425 0.549734 0.331788 0.32658 0.00476749
julia> df3 = df2
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼─────────────────────────────────────────────────────────────────
1 │ 0.364225 0.690894 0.240867 0.720774 0.331573 0.549766
2 │ 0.225226 0.241412 0.0793279 0.418206 0.775367 0.35275
3 │ 0.19913 0.0633375 0.767805 0.280096 0.721995 0.917259
4 │ 0.708132 0.230088 0.702677 0.947402 0.928979 0.66101
5 │ 0.0267573 0.0122425 0.549734 0.331788 0.32658 0.00476749
julia> df2 .= log.(df)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼─────────────────────────────────────────────────────────────────────
1 │ -1.00998 -0.369769 -1.42351 -0.32743 -1.10391 -0.598262
2 │ -1.49065 -1.42125 -2.53417 -0.87178 -0.254419 -1.04199
3 │ -1.6138 -2.75928 -0.26422 -1.27262 -0.325737 -0.0863653
4 │ -0.345124 -1.46929 -0.352858 -0.0540319 -0.0736689 -0.413987
5 │ -3.62095 -4.40285 -0.598321 -1.10326 -1.11908 -5.34594
julia> df2 === df3
true
Note that with the last check I made sure that indeed the df2 .= log.(df) was in-place.
We updated the contents of df2, and not created a new object.
However, sometimes things are more tricky. Consider the df .> 0.5 operation we did above:
julia> df2 .= df .> 0.5
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼──────────────────────────────────────────────────────
1 │ 0.0 1.0 0.0 1.0 0.0 1.0
2 │ 0.0 0.0 0.0 0.0 1.0 0.0
3 │ 0.0 0.0 1.0 0.0 1.0 1.0
4 │ 1.0 0.0 1.0 1.0 1.0 1.0
5 │ 0.0 0.0 1.0 0.0 0.0 0.0
Note that there is a difference from creating a new data frame with df .> 0.5.
The issue is that columns of df2 keep their original types. This is expected, as we
wanted a fully in-place operation. However, sometimes you might want to change the
element type of a column when doing broadcasting. This is possible, however,
then you need to use data frame indexing with a special ! row selector which signals
that column replacement is requested:
julia> df2[!, :] .= df .> 0.5
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Bool Bool Bool Bool Bool Bool
─────┼──────────────────────────────────────────
1 │ false true false true false true
2 │ false false false false true false
3 │ false false true false true true
4 │ true false true true true true
5 │ false false true false false false
julia> df3
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Bool Bool Bool Bool Bool Bool
─────┼──────────────────────────────────────────
1 │ false true false true false true
2 │ false false false false true false
3 │ false false true false true true
4 │ true false true true true true
5 │ false false true false false false
Indeed we got what we wanted. I showed df3 variable to convince you that
still all operations were done on the same data frame object and df2 and df3
are still pointing to it.
Let me give an example where the difference between in-place and column replace
operations particularly matters and is a common surprise for new users.
It is a case when we want to introduce missing values to a column that initially does not allow them.
julia> df2 = copy(df)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64 Float64 Float64 Float64 Float64 Float64
─────┼─────────────────────────────────────────────────────────────────
1 │ 0.364225 0.690894 0.240867 0.720774 0.331573 0.549766
2 │ 0.225226 0.241412 0.0793279 0.418206 0.775367 0.35275
3 │ 0.19913 0.0633375 0.767805 0.280096 0.721995 0.917259
4 │ 0.708132 0.230088 0.702677 0.947402 0.928979 0.66101
5 │ 0.0267573 0.0122425 0.549734 0.331788 0.32658 0.00476749
julia> df2 .= ifelse.(df .> 0.5, missing, df)
ERROR: MethodError: Cannot `convert` an object of type Missing to an object of type Float64
julia> df2[!, :] .= ifelse.(df .> 0.5, missing, df)
5×6 DataFrame
Row │ x1 x2 x3 x4 x5 x6
│ Float64? Float64? Float64? Float64? Float64? Float64?
─────┼─────────────────────────────────────────────────────────────────────────────────────────────────────
1 │ 0.364225 missing 0.240867 missing 0.331573 missing
2 │ 0.225226 0.241412 0.0793279 0.418206 missing 0.35275
3 │ 0.19913 0.0633375 missing 0.280096 missing missing
4 │ missing 0.230088 missing missing missing missing
5 │ 0.0267573 0.0122425 missing 0.331788 0.32658 0.00476749
Note that df2 originally does not allow missing values in any of the columns. Therefore
df2 .= ifelse.(df .> 0.5, missing, df) fails. However, replacing df2 .= by df2[!, :] .=
works, because the ! selector explicitly requests overwriting the original columns
with new ones, possibly changing their types.
Conclusions
I hope you found these examples useful and they will help you to work with DataFrames.jl more
easily and confidently.
As a final comment let me explain why df2 .= ifelse.(df .> 0.5, missing, df) is fully in-place
(and does not replace the columns with proper element types like df2[!, :] .=) as this is a common question.
There are three reasons for this:
- performance: a fully in-place operation is faster and allocates less;
- safety: in production code we might want to make sure that the type of a column is not changed by mistake;
- design consistency: the operation was designed to work the same way as broadcasting on matrices
(broadcasted assignment used with a matrix does allow to change its element type).