R help - Mar 2013 - A question on function return

Hello again,

Let say I have following user defined function:

fn <- function(x, y) {
		Vec1 <- letters[1:6]
		Vec2 <- 1:5
		return(list(ifelse(x > 0, Vec1, NA), ifelse(y > 0, Vec2, NA)))
	}

Now I have following calculation:
> fn(-3, -3)
[[1]]
[1] NA

[[2]]
[1] NA
> fn(3, -3)
[[1]]
[1] "a"

[[2]]
[1] NA


Here I can not understand why in the second case, I get only the first
element "a" of the corresponding vector 'Vec1'? I want to get
complete
vector(s) as the function return.

Can somebody help me how to achieve that?

Thanks and regards,

Hello,

You get only one value because ifelse is vectorized, and the condition 
has length(x > 0) == 1. (You get as many values as there are in the 
condition.) Try instead


fn2 <- function(x, y) {
	Vec1 <- letters[1:6]
	Vec2 <- 1:5
	list(if(x > 0) Vec1 else NA, if(y > 0) Vec2 else NA)
}

fn2(-3, -3)
fn2(3, -3)


Hope this helps,

Rui Barradas

Em 22-03-2013 18:43, Christofer Bogaso escreveu:
> Hello again,
>
> Let say I have following user defined function:
>
> fn <- function(x, y) {
> 		Vec1 <- letters[1:6]
> 		Vec2 <- 1:5
> 		return(list(ifelse(x > 0, Vec1, NA), ifelse(y > 0, Vec2, NA)))
> 	}
>
> Now I have following calculation:
>
>> fn(-3, -3)
> [[1]]
> [1] NA
>
> [[2]]
> [1] NA
>
>> fn(3, -3)
> [[1]]
> [1] "a"
>
> [[2]]
> [1] NA
>
>
> Here I can not understand why in the second case, I get only the first
> element "a" of the corresponding vector 'Vec1'? I want to
get complete
> vector(s) as the function return.
>
> Can somebody help me how to achieve that?
>
> Thanks and regards,
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

On 2013-03-22 11:43, Christofer Bogaso wrote:
> Hello again,
>
> Let say I have following user defined function:
>
> fn <- function(x, y) {
> 		Vec1 <- letters[1:6]
> 		Vec2 <- 1:5
> 		return(list(ifelse(x > 0, Vec1, NA), ifelse(y > 0, Vec2, NA)))
> 	}
>
> Now I have following calculation:
>
>> fn(-3, -3)
> [[1]]
> [1] NA
>
> [[2]]
> [1] NA
>
>> fn(3, -3)
> [[1]]
> [1] "a"
>
> [[2]]
> [1] NA
>
>
> Here I can not understand why in the second case, I get only the first
> element "a" of the corresponding vector 'Vec1'? I want to
get complete
> vector(s) as the function return.
>
> Can somebody help me how to achieve that?
 From help(ifelse):
   "ifelse returns a value with the same shape as test"

i.e. in your case, the same 'shape' as 'x > 0', a single
value.
You _could_ make ifelse() work with, e.g., ifelse(rep(x, 6) > 0, ....)
but you probably want if() instead.

Peter Ehlers