R help - Jul 2005 - Puzzled at ifelse()

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?

-- 
Ajay Shah                                                   Consultant
ajayshah at mayin.org                      Department of Economic Affairs
http://www.mayin.org/ajayshah           Ministry of Finance, New Delhi

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

This is working exactly as documented.  Nothing `breaks'!

What does the help page say?

      'ifelse' returns a value with the same shape as 'test'
which is
      filled with elements selected from either 'yes' or 'no'
depending
      on whether the element of 'test' is 'TRUE' or
'FALSE'.

'test' is of length one and true, so you got the first element of
'yes',
which is a list as the first (and only) element of the answer.

On Tue, 12 Jul 2005, 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?
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595