R help - Dec 2010 - testing with if: what I am doing wrong?

I am running this small program:

x <- factor(c("A","B","A","C"))
y <- c(1,2,3,4)
w <-data.frame(x,y)
if (w$x=="A"){
	w$z=1
}
w
And I obtain:

  x y z
1 A 1 1
2 B 2 1
3 A 3 1
4 C 4 1

And not

  x y z
1 A 1 1
2 B 2 NA
3 A 3 1
4 C 4 NA

Like I should obtain. What am I doing wrong?

Please notice that I get a warning approximately saying - translated from
italian:

In if (w$x == "A") { : the condition length > 1 and only the first
element will be used

Thanks,
Luca

On 18/12/2010 8:34 AM, Luca Meyer wrote:
> I am running this small program:
>
> x<- factor(c("A","B","A","C"))
> y<- c(1,2,3,4)
> w<-data.frame(x,y)
> if (w$x=="A"){
> 	w$z=1
> }
> w
> And I obtain:
>
>    x y z
> 1 A 1 1
> 2 B 2 1
> 3 A 3 1
> 4 C 4 1
>
> And not
>
>    x y z
> 1 A 1 1
> 2 B 2 NA
> 3 A 3 1
> 4 C 4 NA
>
> Like I should obtain. What am I doing wrong?
You're using "if", which only looks at the first element (as the
message
said).  See ?ifelse for a conditional that applies to each vector entry.

Duncan Murdoch
>
> Please notice that I get a warning approximately saying - translated from
italian:
>
> In if (w$x == "A") { : the condition length>  1 and only the
first element will be used
>
> Thanks,
> Luca
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http://www.R-project.org/posting-guide.html
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