R help - Mar 2011 - inefficient ifelse() ?

dear R experts---

  t <- 1:30
  f <- function(t) { cat("f for", t, "\n"); return(2*t) }
  g <- function(t) { cat("g for", t, "\n"); return(3*t) }
  s <- ifelse( t%%2==0, g(t), f(t))

shows that the ifelse function actually evaluates both f() and g() for
all values first, and presumably then just picks left or right results
based on t%%2.  uggh... wouldn't it make more sense to evaluate only
the relevant parts of each vector and then reassemble them?

/iaw
----
Ivo Welch

Try this:

mapply(function(x, f)f(x), split(t, t %% 2), list(g, f))

On Tue, Mar 1, 2011 at 4:19 PM, ivo welch <ivowel@gmail.com> wrote:
> dear R experts---
>
>  t <- 1:30
>  f <- function(t) { cat("f for", t, "\n");
return(2*t) }
>  g <- function(t) { cat("g for", t, "\n");
return(3*t) }
>  s <- ifelse( t%%2==0, g(t), f(t))
>
> shows that the ifelse function actually evaluates both f() and g() for
> all values first, and presumably then just picks left or right results
> based on t%%2.  uggh... wouldn't it make more sense to evaluate only
> the relevant parts of each vector and then reassemble them?
>
> /iaw
> ----
> Ivo Welch
>
> ______________________________________________
> R-help@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.
>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

	[[alternative HTML version deleted]]

Just use a regular if statement:

X <- if(b == "YY") a else substr(a, 1,3)

Michael

On Wed, Mar 28, 2012 at 11:19 AM, Manta <mantino84 at libero.it>
wrote:
> I have a similar problem. I have a dataset and an element. If the element
is
> equal to "YY", I want to take the first column of the dataset,
otherwise I
> want to take the second column. The following does not work, as it only
> evaluates the first element. Any idea?
>
> a=c("AAAXXX","BBBXXX")
> a=merge(a,c("AAA","BBB"))
> b="YY"
>> ifelse(b=="YY",a,substr(a,1,3))
> [1] "AAAXXX"
>
>
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/inefficient-ifelse-tp3330423p4512579.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> 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 28-03-2012, at 17:19, Manta wrote:
> I have a similar problem. I have a dataset and an element. If the element
is
> equal to "YY", I want to take the first column of the dataset,
otherwise I
> want to take the second column. The following does not work, as it only
> evaluates the first element. Any idea?
> 
yes.
See the description of ifelse.
> a=c("AAAXXX","BBBXXX")
> a=merge(a,c("AAA","BBB"))
> b="YY"
>> ifelse(b=="YY",a,substr(a,1,3))
> [1] "AAAXXX"
If you want the first  column of a when b=="YY" why are you doing a
and not a[,1]?
If you want the second  column of a when !(b=="YY") why are you doing
substr(a,1,3) and not a[,2]?

You are getting a scalar as result because of the way the ifelse function works.

To get what you want you could do:

a[,if(b=="YY") 1 else 2]

or

acolumn <- if( b == "YY" ) 1 else 2
a[,acolumn]

or

a[,2-(b=="YY")]


Berend