I am trying to write a function based on this formula.

(x − x0)^ r  = { (x-x0)^r   if x>x0
                    0       otherwise

what I understood from above is;

y= (x-x0)^r unless x<= x0

so for each element of, identify if greater and x0 if so return y that equals the formula.

tp <- function(x,x0,r) {
  y<-list()
  if (x[i] >x0) {
    for (i in seq_along(x)) {
      y<- append((x[i]-x0)^r)
    } else {
      y <- append(0)
    }
    
  }
  return(y)
}

I have tried doing this but I couldn't make it work. Could anyone advise me if I understood the formula right and if so what is the correct way to coed it.

CodePudding user response：

Use ifelse.

tp <- function(x, x0, r) {
  ifelse(x > x0, (x - x0)^r, 0)
}

x0 <- 2
tp(-2:5, x0, r = 2)
#> [1] 0 0 0 0 0 1 4 9

^{Created on 2022-12-01 with reprex v2.0.2}

CodePudding user response：

An alternative to the if else statement is to use boolean logic, which I like. But both will work.

tp <- function(x, x0, r) {
#  ifelse(x > x0, (x - x0)^r, 0)
  (x > x0)*(x - x0)^r
}

x0 <- 2
tp(-2:5, x0, r = 2)
#> [1] 0 0 0 0 0 1 4 9

CodePudding user response：

Issues with your function:

• append needs a list or vector to which the values are appended, so you need to do y <- append(y, ...);
• Your for expression should be before (outside) the first (x[i] > x0) conditional; and
• Perhaps subjective, but I suggest that y <- numeric(0) makes more sense to me than list. There are certainly times when a list makes sense, but this is not one of them (in my perspective). Again, not wrong, but later processing will likely be more efficient as a vector.

Perhaps something like:

tp <- function(x,x0,r) {
  y <- numeric(0)
  for (i in seq_along(x)) {
    if (x[i] > x0) {
      y <- append(y, (x[i]-x0)^r)
    } else {
      y <- append(y, 0)
    }
  }
  return(y)
}

In R, it is almost always preferred to use vectorized operations instead of for loops. In this case, I think you can easily use the vectorized function that RuiBarradas provided (which is the preferred method for writing this kind of function).