Warning Message While Plotting a Piecewise Function in R

Question

I'm trying to plot this function in R

f <- function(a,b,c,x){
 if(a<=x){
   return(c*x)} 
 if(x<=b){
   return(-c*x)}
 return(0)}

but when I try to plot it with:

a<-0
b<-6
c<-3
curve(f(a,b,c,x), xlim=c(0,6), ylim=c(0,3), col='blue', lwd=3, add=FALSE)

I get:

Warning message in if (a <= x) {:
“the condition has length > 1 and only the first element will be used”

Any help please?

Answer 1

Your function f can't operate with vectors for x since the condition (a <= x) isn't straight forward to evaluate for a vector.

Let's take a look at

x <- -6:6
f(a,b,c,x)

Your function f doesn't understand what to do with x while evaluating

if (a <= x) { code }

so the first element of x is always used.

So, what to do?

x   <- -6:6
f_2 <- function(x) { sapply(x, function(x) f(a,b,c,x)) }

curve(f_2, xlim=c(-6,6), ylim=c(0,20), col="blue", lwd=2, add=FALSE)

We define a new function f_2 . sapply takes one element of x and uses it with f(a,b,c,x) than uses the next element etc. This function works with vectors x as input.

Compare it to

curve(f(a,b,c,x), xlim=c(-6,6), ylim=c(0,20), col="blue", lwd=2, add=FALSE)

for better understanding.