5.1 Macros
You might have noticed that all DataFramesMeta.jl commands start with an “at” @ symbol. These commands have a special category in the Julia language: they are called macros.
NOTE: Macros are a way to perform metaprogramming in Julia. We won’t cover macro’s basics here. However, if you want to find out more, please check the Julia documentation section on metaprogramming and Julius Krumbiegel’s blog post on Julia macros.
DataFramesMeta.jl macros behave similar to DataFrames.jl functions:
- they both take a
DataFrameas a first positional argument - they have in-place mutating functions (as discussed in Section 4.10.1: the bang
!functions)
Nevertheless, there are some differences between using DataFrames.jl functions versus DataFramesMeta.jl macros.
First, using parentheses in the macro commands are optional, and it can be replaced by spaces instead. For example:
@select(df, :col)is the same as:
@select df :colWe will be using the space syntax in this chapter.
Second, macros parse static commands by default. If you want dynamic parsing you’ll need to add $ to your syntax. This happens because macros treat the input as a static string. For example:
@select df :colwill always work because our intended selected column command with the argument :col won’t change in runtime (the time that Julia is executing code). It will always mean the same operation no matter the context.
Now suppose that you want to use one of the column selectors presented in Section 4.4. Here, the expression inside the @select macro needs to be parsed dynamically. In other words, it is not static and the operation will change with context. For example:
@select df Not(:col)Here the columns that we want to select will depend on the actual columns inside df. This means that Julia cannot treat the command as something that won’t change depending on the context. Hence, it needs to be parsed dynamically. In DataFramesMeta.jl, this is solved by wrapping parts of the command that needs to be parsed dynamically with $(). The above command needs to be changed to:
@select df $(Not(:col))This tells DataFramesMeta.jl to treat the Not(:col) part of the macro as dynamic. It will parse this expression and replace it by all of the columns except :col.
Third, most of DataFramesMeta.jl macros have two different versions: a non-vectorized, and a vectorized form. The non-vectorized form is the default form and treats arguments as whole columns, i.e., they operate on arrays whereas the vectorized form has an r prefix (as in rows) and vectorizes all operators and functions calls. This is the same behavior as adding the dot operator . into the desired operation. Similar to the ByRow function from DataFrames.jl that we saw in Section 4.3.2.
These 3 operations are equivalent:
# DataFrames.jl
transform(df, :col => ByRow(exp))# DataFramesMeta.jl non-vectorized (default)
@transform df exp.(:col)# DataFramesMeta.jl vectorized with r prefix
@rtransform df exp(:col)NOTE: For most of the
DataFramesMeta.jlmacros we have four variants:
@macro: non-vectorized@rmacro: vectorized@macro!: non-vectorized in-place@rmacro!: vectorized in-place
