apply() functions

df <- data.frame(a = 1:2, b = 2:3, c = c("a", "z"), d = c("b", "k"))
df

##   a b c d
## 1 1 2 a b
## 2 2 3 z k

sapply(df, is.numeric)

##     a     b     c     d 
##  TRUE  TRUE FALSE FALSE

vapply(df, is.numeric, logical(1))

##     a     b     c     d 
##  TRUE  TRUE FALSE FALSE

try(vapply(df, is.numeric, character(1)))

## Error in vapply(df, is.numeric, character(1)) : 
##   values must be type 'character',
##  but FUN(X[[1]]) result is type 'logical'