[R] Puzzled at ifelse()

[Duncan Murdoch]

Ajay Narottam Shah wrote:
> I have a situation where this is fine:
> 
>   > if (length(x)>15) {
>       clever <- rr.ATM(x, maxtrim=7)
>     } else {
>       clever <- rr.ATM(x)
>     }
>   > clever
>   $ATM
>   [1] 1848.929
> 
>   $sigma
>   [1] 1.613415
> 
>   $trim
>   [1] 0
> 
>   $lo
>   [1] 1845.714
> 
>   $hi
>   [1] 1852.143
> 
> But this variant, using ifelse(), breaks:
> 
>   > clever <- ifelse(length(x)>15, rr.ATM(x, maxtrim=7), rr.ATM(x))
>   > clever
>   [[1]]
>   [1] 1848.929
> 
> What am I doing wrong?

if (test) expr1 else expr2

evaluates only one of expr1 or expr2 according to test, and returns that 
result.

ifelse(test, expr1, expr2)

executes all three of test, expr1, and expr2, and puts together a vector 
response where for each element of test that is true (non-zero), the 
corresponding element of one of the expr's is selected.

I'd guess your x is length 1, so your test is length 1, and only the 
first element of the result of rr.ATM is returned (which is not very 
useful).

To get what you want, you should write

clever <- if(length(x) > 15) rr.ATM(x, maxtrim=7) else rr.ATM(x);

Duncan Murdoch