[r] Apply function to each column in a data frame observing each columns existing data type

The best way to do this is avoid base *apply functions, which coerces the entire data frame to an array, possibly losing information.

If you wanted to apply a function as.numeric to every column, a simple way is using mutate_all from dplyr:

t %>% mutate_all(as.numeric)

Alternatively use colwise from plyr, which will "turn a function that operates on a vector into a function that operates column-wise on a data.frame."

t %>% (colwise(as.numeric))

In the special case of reading in a data table of character vectors and coercing columns into the correct data type, use type.convert or type_convert from readr.


Less interesting answer: we can apply on each column with a for-loop:

for (i in 1:nrow(t)) { t[, i] <- parse_guess(t[, i]) }

I don't know of a good way of doing assignment with *apply while preserving data frame structure.