R - Filter a vector using a function

Question

I have a function similar to this one:

isGoodNumber <- function(X)  { if (X==5) return(TRUE) else return(FALSE) }  I have a vector: v<-c(1,2,3,4,5,5,5,5) 

I want to obtain a new vector that contains the elements of v where isGoodNumber(v) == TRUE

How do I do this ?

Tried v [ isGoodNumber(v) == TRUE ] but it doesn't work :-)

Thanks !!

Answer 1

There is a function named "Filter" that will do exactly what you want:

Filter( isGoodNumber, v) #[1] 5 5 5 5 

There would be the option of making a function that was vectorized, either by by the use of the Vectorize function (already illustrated) or writing it with ifelse (also mentioned) and there would be the option of a function that was "Filter"-like

 isGoodNumber3 <- function(X)     { X[ ifelse(X==5, TRUE,FALSE)]    }   isGoodNumber3(v) #[1] 5 5 5 5 

Answer 2

I think here is the easiest method

> v<-c(1,2,3,4,5,5,5,5) > v[v==5] [1] 5 5 5 5