R help - May 2014 - Loop Issue

Hi everybody.

Consider the following exampling code: 

x=numeric()
  for(i in 1:10){
    u=runif(1,-1,2)
    x[i]=log(u)
  }
This code, in each interation, generates a random value in the (-1,2) interval
and then calculates the log of the value.
When the generated value is less than 0 the log produces a NaN, which gives a
warning.

What I want is to make it start over when a warning is produced, in order to
repeat it until a positive value is generated and therefore the log is
calculated. Logically, would be like: "if there's a warning here, go
back at the beginning and start over", without changing the iteration.

Could someone help me with some directions?

Thanks a lot,

Ricardo 		 	   		  
	[[alternative HTML version deleted]]

Hi Ricardo
Assuming you have a good reason to use such approach (what are you
trying to do ultimately?), you can just increment your counter when
you get a good value, i.e.:

x <- numeric()
n <- 0
while (n < 10) {
    u <- log(runif(1, -1, 2))
    if (is.finite(u)) {
        n <- n+1
        x[n] <- u
    }
}
x

On 23 May 2014 12:11, Ricardo Rocha <ricardorocha23 at hotmail.com>
wrote:
> Hi everybody.
>
> Consider the following exampling code:
>
> x=numeric()
>   for(i in 1:10){
>     u=runif(1,-1,2)
>     x[i]=log(u)
>   }
> This code, in each interation, generates a random value in the (-1,2)
interval and then calculates the log of the value.
> When the generated value is less than 0 the log produces a NaN, which gives
a warning.
>
> What I want is to make it start over when a warning is produced, in order
to repeat it until a positive value is generated and therefore the log is
calculated. Logically, would be like: "if there's a warning here, go
back at the beginning and start over", without changing the iteration.
>
> Could someone help me with some directions?
>
> Thanks a lot,
>
> Ricardo
>         [[alternative HTML version deleted]]
>
> ______________________________________________
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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.


-- 
Yvan Richard

      DRAGONFLY Science

Physical address: Level 5, 158 Victoria St, Te Aro, Wellington
Postal address: PO Box 27535, Wellington 6141
New Zealand
Ph: 04.385.9285
web page

On Thu, 22 May 2014 09:11:43 PM Ricardo Rocha wrote:
> Hi everybody.
> 
> Consider the following exampling code:
> 
> x=numeric()
>   for(i in 1:10){
>     u=runif(1,-1,2)
>     x[i]=log(u)
>   }
> This code, in each interation, generates a random value in the (-1,2)
> interval and then calculates the log of the value. When the generated 
value
> is less than 0 the log produces a NaN, which gives a warning.
> 
> What I want is to make it start over when a warning is produced, in 
order to
> repeat it until a positive value is generated and therefore the log is
> calculated. Logically, would be like: "if there's a warning here,
go back
> at the beginning and start over", without changing the iteration.
> 
> Could someone help me with some directions?
> 
Hi Ricardo,
Perhaps what you want is something like this:

i<-1
while(I < 11) {
 u<-runif(1,-1,2)
 if(u >= 0) {
  x[i]<-log(u)
  i<-i+1
 }
}

Jim