Reverse Instructional Design --- The Apply family in R

Jul 23, 2014 • Scott Ritchie

Question 1:

Suppose you have the following data.frame:

> df
 #    a          b  c group
 # 1 12  0.7575236 -6     a
 # 2  2 -0.6816639  7     a
 # 3  9  0.5233682 -5     a
 # 4  0 -0.2917147 -6     b
 # 5 -1  0.5255509 NA     b
 # 6  8 -1.7260456  1     b

Question 1a:
Which of the following commands will return maximum value in each of the columns a,* b, and *c?

  1. sapply(df[1:3], max)
  2. apply(df[1:3], 1, max, na.rm=TRUE)
  3. apply(df[,1:3], 2, max, na.rm=TRUE)
  4. apply(df[-c("group")], 2, max, na.rm=TRUE)

Question 1b:
In the space below, write R code that will split the data frame by the group column, and output the maximum value in each column, for each group: