tapply() Function in R

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tapply() apply different functions on factor sliced rows, or category statistic.

Firstly, loading a data set.

> library(MASS)> head(painters)              Composition Drawing Colour Expression SchoolDa Udine               10       8     16          3      ADa Vinci               15      16      4         14      ADel Piombo              8      13     16          7      ADel Sarto              12      16      9          8      AFr. Penni               0      15      8          0      AGuilio Romano          15      16      4         14      A

We will use the data "painters" in this library. We could compute mean value of a column

> mean(painters$Composition[painters$School == "C"])[1] 13.16667

If we want to compute mean value for every different School, we could use tapply()

> tapply(painters$Composition, painters$School, mean)       A        B        C        D        E        F        G        H 10.40000 12.16667 13.16667  9.10000 13.57143  7.25000 13.85714 14.00000

Max value for every different School

> tapply(painters$Composition, painters$School, max) A  B  C  D  E  F  G  H 17 15 16 15 18  9 18 16

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