How to Simplify Results (or Not) with the sapply Function in R
The sapply() function doesn’t always return a vector. In fact, the standard output of sapply is a list, but that list gets simplified to either a matrix or a vector if possible.

If the result of the applied function on every element of the list or vector is a single number, sapply() simplifies the result to a vector.

If the result of the applied function on every element of the list or vector is a vector with exactly the same length, sapply() simplifies the result to a matrix.

In all other cases, sapply() returns a (named) list with the results.
Say you want to know the unique values of every variable in the data frame clients. To get all unique values in a vector, you use the unique() function. You can get the result you want by applying that function to the data frame clients like this:
> sapply(clients, unique) $hours [1] 25 110 125 40 $public [1] TRUE FALSE $type [1] "public" "abroad" "private"
In the variable hours, you find four unique values; in the variable public, only two; and in the variable type, three. Because the lengths of the result differ for every variable, sapply() can’t simplify the result, so it returns a named list.