Default values for arguments of a function

Doing mathematics with R: Functions

Default values for arguments of a function

You can give some function arguments a default value so that you can omit them in a function call. Consider the following example of a distance function

The Euclidean distance between two points $(x_1,y_1)$ and $(x_2,y_2)$ is given by

$\text{distance}=\sqrt{(x_1-x_2)^2+(y_1-y_2)^2}$ We define the function distance that calculates the distance between two points and in case there is only one point as an argument in a call to this function uses the default value $(0,0)$ for the second point. We use the coordinates of the points as 4 arguments to the function.

The R script

distance <- function(P1.x, P1.y, P2.x=0, P2.y=0) {
  return( sqrt((P1.x-P2.x)^2+(P1.y-P2.y)^2) )
}
cat("distance between (1,2) and (3,4) =", distance(1,2,3,4), "\n")
cat("distance between (1,2) and (3,4) =", distance(1,2,4, P2.x=3), "\n")
cat("distance between (1,2) and (3,0) =", distance(1,2,1), "\n")
cat("distance between (1,2) and (0,0) =", distance(1,2), "\n")

yields the following result:

distance between (1,2) and (3,4) = 2.828427 
distance between (1,2) and (3,4) = 2.828427 
distance between (1,2) and (3,0) = 2 
distance between (1,2) and (0,0) = 2.236068

The second call of the distance function shows that the construction name = default not only allows you to make arguments optional, but it also gives you the option to use arguments with a keyword. The arguments with keywords may be last in the function definition.