Notice only grouping columns and columns passed through an aggregating calculation (such as

`max()`

) are passed through (the column`z`

is not in the result). Now because`y`

is a function of`x`

no substantial aggregation is going on, we call this situation a “pseudo aggregation” and we have taught this before. This is also why we made the seemingly strange choice of keeping the variable name`y`

(instead of picking a new name such as`max_y`

), we expect the`y`

values coming out to be the same as the one coming in- just with changes of length. Pseudo aggregation (using the projection`y[[1]]`

) was also used in the solutions of the column indexing problem.Our

`wrapr`

package now supplies a special case pseudo-aggregator (or in a mathematical sense: projection):`psagg()`

. It works as follows.

In this post, John calls the act of grouping functional dependencies (where we can determine the value of y based on the value of x, for any number of columns in y or x) pseudo-aggregation.

Kevin Feasel

2018-11-01

Data Science, R

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