Max date in R column with sapply

Question

I am trying to use sapply to get the max date in a column but it is returning a number instead of a date. Any idea how to resolve this? I can\'t seem to figure out why this is

Answer 1

We need to use lapply instead of sapply

lapply(mtcars, max)

as sapply returns a vector because of the simplify=TRUE default argument and vector can hold only a single class. As there are numeric columns, the 'Date' column (which is stored as integer) gets coerced to integer value. However, we can still use sapply if we use simplify = FALSE to return a list.

sapply(mtcars, max, simplify = FALSE)

Regarding the use of apply

apply(mtcars,2,max)

I would not recommend using apply as this will convert to matrix and matrix can hold only a single class. Here, it will be all character output as the 'Date' class got converted to character.

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Regarding the problem mentioned by OP in the comments, when we have output that belong to different class, we could either use list or data.frame (as data.frame is a list) as c i.e. concatenate returns a vector and as mentioned above, it can hold only a single class.

do.call(rbind, lapply(mtcars, function(x) 
          data.frame(Class=class(x), Max=max(x, na.rm=TRUE))))