Losing Class information when I use apply in R

Question

When I pass a row of a data frame to a function using apply, I lose the class information of the elements of that row. They all turn into \'character\'. The following is a sim

Answer 1

apply only really works on matrices (which have the same type for all elements). When you run it on a data.frame, it simply calls as.matrix first.

The easiest way around this is to work on the numeric columns only:

# skips the first column
a <- apply(df[, -1, drop=FALSE],1, dfunc)

# Or in two steps:
m <- as.matrix(df[, -1, drop=FALSE])
a <- apply(m,1, dfunc)

The drop=FALSE is needed to avoid getting a single column vector. -1 means all-but-the first column, you could instead explicitly specify the columns you want, for example df[, c('foo', 'bar')]

UPDATE

If you want your function to access one full data.frame row at a time, there are (at least) two options:

# "loop" over the index and extract a row at a time
sapply(seq_len(nrow(df)), function(i) dfunc(df[i,]))

# Use split to produce a list where each element is a row
sapply(split(df, seq_len(nrow(df))), dfunc)

The first option is probably better for large data frames since it doesn't have to create a huge list structure upfront.