Previously we showed you how to write a loop in R. Very often it’s a better idea to use the apply function in R than to write a loop.
So what is the apply function in R?
Well, apply is really a family of functions that have varying uses. Here are the available R apply functions: apply, lapply, sapply, vapply, mapply, rapply and tapply.
Apply is the head of the family. Apply operates on arrays: apply(X, MARGIN, FUN, …). This will return a vector or array or list of values obtained by applying a function to margins of an array.
The Arguments are the following:
X – the array to be used.
MARGIN – a vector giving the subscripts which the function will be applied over. 1 indicates rows, 2 indicates columns, c(1,2) indicates rows and columns.
FUN – the function to be applied: see Details. In the case of functions like +, %*%, etc., the function name must be backquoted or quoted.
… – optional arguments to FUN.
Lets look at an example:
>cars #built-in dataset Speed and Stopping Distances of Cars
I won’t type the entire print but here are a few rows:
speed dist 1 4 2 2 4 10 3 7 4 4 7 22 5 8 16
This goes to row 50.
What if we wanted to get the sum of speed?
>apply(cars,2,sum) speed dist 770 2149
What if you wanted to get the mean?
> apply(cars,2,mean) speed dist 15.40 42.98
These are basic examples of using the apply function in R. Next we will look at the other family members of apply.