apply() is giving NA values for every column

Question

I\'ve been having this strange problem with apply lately. Consider the following example:

set.seed(42)
df <- data.frame(cars, foo = sample(LE         


        

Answer 1

apply works on a matrix, and a matrix must be of all one type. So df is being transformed into a matrix, and since it contains a character, all the columns are becoming character.

> apply(df, 2, class)
      speed        dist         foo 
"character" "character" "character" 

To get what you want, check out the colwise and numcolwise functions in plyr.

> numcolwise(mean)(df)
  speed  dist
1  15.4 42.98