[R] (re-titled) help with "if".

[Rolf Turner]

This is an FAQ (although it doesn't seem to be listed in the official  
FAQs.
The ``if'' command compares scalars (only, in effect).

See ?"if"

You need to use the ifelse() function:

folds <- function(x,y) ifelse(x-y>=0,2^(x-y),-2^(y-x))

This if for illustrative purposes only.  The foregoing would be  
better done as:

	folds <- function(x,y) sign(x-y)*2^abs(x-y)

	cheers,

		Rolf Turner

On 4/06/2008, at 1:47 PM, ALAN SMITH wrote:

> Hello R users and developers,
> I am trying to write several functions (fairly new at this) in order
> to avoid using loops on large data frames (because they are slow).  I
> wrote the function below and it seems to get hung on the first part of
> the if statement and then applies that condition to rest of the
> function.  So if (x-y) is greater than 0 the function uses the true
> statement for the calculations.   Could someone please offer some
> advise on how to write these functions a little better or a type
> "apply" that I may use with two (or more) different vectors of data
> required by a single functions.
> #######################  Examples
> #######################################################
> ## example 1 ###
> x<-c(5,6,4,3,5,3,1)
> y<-c(1,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z<-folds(x,y)
> check<-cbind(x,y,z)
> View(check)
>
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be used
>  ### why will it only use the first element and how to I get around
> this ####
>
> ## example 2 making the fist comparison negative ###
> x1<-c(5,6,4,3,5,3,1)
> y1<-c(11,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z1<-folds(x1,y1)
> check2<-cbind(x1,y1,z1)
> View(check2)
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be used
> ###################################################################### 
> ##########
> #### loop I am trying to avoid writing many many times #####
> folds2<-NULL
> xy<-as.data.frame(cbind(x,y))
> for (i in 1:nrow(xy)) {
> diff<-xy$x[i]-xy$y[i]
> folds2[i]<-if(diff>=0) {2^diff} else{-(2^abs(diff))}
> }
> xyz<-cbind(xy,folds2)
> View(xyz)
> #################
>
> Thank you,
> Alan
>
> P.S.  why does ?function not work
>
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> PLEASE do read the posting guide http://www.R-project.org/posting- 
> guide.html
> and provide commented, minimal, self-contained, reproducible code.


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