5.2 Column Selection

Let’s revisit our example all_grades() data defined in Chapter 4:

df = all_grades()

The DataFramesMeta.jl @select macro is similar to the select DataFrames.jl function. It performs column selection:

@select df :name

We can add as many columns as we want to @select:

@select df :name :grade

To use the column selectors (Section 4.4), you need to wrap them inside $():

@select df $(Not(:grade))

The DataFramesMeta.jl syntax, for some users, is easier and more intuitive than the DataFrames.jl’s minilanguage source => transformation => target. The minilanguage is replaced by target = transformation(source).

Suppose that you want to represent the grades as a number between 0 and 100:

@select df :grade_100 = :grade .* 10

Of course, the .* can be omitted by using the vectorized form of the macro, @rselect:

@rselect df :grade_100 = :grade * 10

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CC BY-NC-SA 4.0 Jose Storopoli, Rik Huijzer, Lazaro Alonso