[R] Conditionally incrementing a loop counter: Take 2

[Peter Dalgaard]

Mike Jones wrote:
> My apologies for not including a working example.
>
> Here it is:
>
> for (i in 1:10){
>    cat("initial i = ",i,"\n")
>    x <- runif(1)
>    if (x > 0.7){
>       i <- i-1
>    }   
>    cat("second i = ",i,"\n")
> }
>
> When I ran this i got what follows, so there were four cases where I
> wanted the i not to increment.
>
> initial i =  1 
> second i =  1 
> initial i =  2 
> second i =  1 
> initial i =  3 
> second i =  3 
> initial i =  4 
> second i =  3 
> initial i =  5 
> second i =  4 
> initial i =  6 
> second i =  6 
> initial i =  7 
> second i =  7 
> initial i =  8 
> second i =  7 
> initial i =  9 
> second i =  9 
> initial i =  10 
> second i =  10 
>
>   
Is this the kind of effect you want?

 > x <- runif(10)
 > cbind(x, 1:10, cumsum(x < .7))
                x    
 [1,] 0.384165631  1 1
 [2,] 0.392715845  2 2
 [3,] 0.895936431  3 2
 [4,] 0.910242185  4 2
 [5,] 0.689987301  5 3
 [6,] 0.237071326  6 4
 [7,] 0.225032680  7 5
 [8,] 0.001856286  8 6
 [9,] 0.392034868  9 7
[10,] 0.655076045 10 8

If you insist on using a loop, you need to separate the loop control 
from the manipulation of i, as in (e.g.)

i <- 0
for (j in 1:10){
   i <- i + 1
   cat("initial i = ",i,"\n")
   x <- runif(1)
   if (x > 0.7){
      i <- i-1
   }   
   cat("second i = ",i,"\n")
}


>>  -----Original Message-----
>> From: 	Mike Jones  
>> Sent:	Thursday, December 27, 2007 4:35 PM
>> To:	'r-help at lists.R-project.org'
>> Subject:	Conditionally incrementing a loop counter
>>
>> Hi,
>> I am trying a for loop from 1 to 10 by 1. However, if a condition does
>> not get met, I want to "throw away" that iteration. So if my loop is
>> for (i in 1:10)
>> and i is say, 4 and the condition is not met then I don't want i to go
>> up to 5.  Is there a way to do that? I can't seem to manually adjust i
>> because from what I understand, R creates 10 long vector and uses that
>> to "loops thru" and I'm not sure how to get at the index of that
>> vector. Any suggestions? Thanks in advance.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Mike Jones
>> Westat
>> 1650 Research Blvd. RE401
>> Rockville, MD 20850
>> Ph: 240.314.2312
>>
>>     
>
> 	[[alternative HTML version deleted]]
>
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>   


-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
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