[R] ifelse question

[Göran Broström]

On 12/12/06, Leeds, Mark (IED) <Mark.Leeds at morganstanley.com> wrote:
> ifelse is vectorized but there is no way you could know what's
> happening with that command
>  because you have rnorm(1) for both conditions. I think you mean to have
> something different in one of them ?
>
> Whewn I run your code in my R session, I get 10 values for y1, so there
> isn't anything wrong except
> That you have the same statement for both cases.

But you got only two (eventually one) distinct values, right? Look at
the code for 'ifelse': yes and no are only called once each, then
recycled to desired length.

I guess you want something like

x <- rnorm(10)
y <- rnorm(10)
z <- rnorm(10)
y1 <- ifelse(x > 0, y, z)

but this whole business is probabilistically the same as

x <- rnorm(10)
y1 <- rnorm(10)

which of course is faster and more transparent.

Göran
>
>
>
>
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of
> jropers at freesurf.fr
> Sent: Tuesday, December 12, 2006 3:13 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] ifelse question
>
> Dear R-helpers,
> How come that in the following code the rnorm() function is evaluated
> only once for each branch of the 'ifelse' ?
>
> x <- rnorm(10)
> y1 <- ifelse(x > 0, rnorm(1) ,  rnorm(1))
>
>
> What is the right way to make it called/evaluated for each row, apart
> from a 'for' loop ?
>
> Thanks,
>
> Jacques.
>
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-- 
Göran Broström