[R] For loop with if else statement

[Duncan Murdoch]

On 9/4/2007 9:59 AM, Hans Ole Ørka wrote:
> Hi,
> I try to make a simple for loop with a if else statement (First example - Below) and extend it to a more complex loop (Second example). However, my results
> 
> #First example:
> x=c(1,2)
> t=for(i in 1:length(x)){
> if (x==1){a=x+1}else
> if (x==2){a=x}
> }
> 
> Returned from R:
> Warning messages:
> 1: the condition has length > 1 and only the first element will be used in: if (x == 1) {
> 2: the condition has length > 1 and only the first element will be used in: if (x == 1) {
>> t
> [1] 2 3
> 
> However, the result i had liked to get was t=c(2,2) i.e. using the first function (a=x+1) for x[1] and (a=x) for x[2]. I can remove the Warnings by making: if (x[i]==1) etc. but this do not make the results any better.

x is a vector of length 2.  Using a = x + 1 means that the entire vector 
a will be replaced by the entire vector x.  You need to index each entry 
each time you use it, i.e.

if (x[i] == 1) a[i] <- x[i] + 1
else if (x[i] ==2]) a[i] <- x[i]

or more simply, throw away the loop, and use the ifelse function:

a <- ifelse( x == 1, x + 1,
        ifelse( x == 2,  x,
          NA) )

(where I've used NA for the case where x is neither 1 nor 2.)

Duncan Murdoch

> 
> #Second example:
> x=c(1,2)
> t<-for(i in 1:length(x)){
>         if (x==1){
>                 a=x
>                 b=x-1}else
>         if (x==2){
>                 a=x+1
>                 b=x}
> b<-list(a=a,b=b)
> }
> 
> Returned from R:
> Warning messages:
> 1: the condition has length > 1 and only the first element will be used in: if (x == 1) {
> 2: the condition has length > 1 and only the first element will be used in: if (x == 1) {
>> t
> $a
> [1] 1 2
> 
> $b
> [1] 0 1
> 
> The result i like to get are $a =c(1,3) and $b=c(0,2)
> 
> Probably there are couple of things that I do wrong and I appreciate all help!
> 
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