How to subtract the counter i in for loop?

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Use a while loop instead of a for loop. I don't think what you have coded makes any sense, but fighting the for loop over control of the indexing variable is a recipe for failure.
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Hi list,
I am writing a for loop that looks like this:
samples<-rep(NA,10)
x <- rep(c(111, 225), 5)
for(i in 1:10){
   If(x[i]<200){
        samples[i] <- x[i]
    }else{
    i=i-1
   }
}

The problem is that the returning vector still contains NA,  I think
the i
in "else" is not getting subtracted.  How should I get it to work?

Thanks,
Mike

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Hi list,
I am writing a for loop that looks like this:
samples<-rep(NA,10)
x <- rep(c(111, 225), 5)
for(i in 1:10){
   If(x[i]<200){
        samples[i] <- x[i]
    }else{
    i=i-1

If you expected the else clause to assign something to the samples vector, you are mistaken.

   }
}

The problem is that the returning vector still contains NA,  I think the i
in "else" is not getting subtracted.  How should I get it to work? 

You could start by telling us what you wanted to happen. You can change the index of a for loop inside the body, but it will not "back up the process" since at the end of the loop the next "i" will not depend on what you changed it to inside the loop.

David Winsemius, MD
Alameda, CA, USA

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David,
Your results is,

samples

[1] 111  NA 111  NA 111  NA 111  NA 111  NA
It still has NA's in it. I want it look like this,

samples

[1] 111  111  111  111 111

You don't need the for loop at all.

samples <-  x[x<200]

or

samples <-  x[which(x<200)]

Your for loop should look something like this

k <- 1
for(i in 1:10){
  if(x[i]<200){
       samples[k] <- x[i]
       k <- k+1
   }
}
na.omit(samples)

Berend

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thanks, Berend.  Both of your code works great.  Is there a function that can do it?  

Something like this:
x <- matrix(NA, nrow=15, ncol=2)
for(i in 1:15){
   x[i,] <- sample(c(NA, 20, 77), 2, prob=c(0.2, 0.3, 0.4))
}

x

      [,1] [,2]

 [1,]   NA   77

 [2,]   77   NA

 [3,]   NA   77

 [4,]   77   20

 [5,]   77   20

 [6,]   77   20

 [7,]   20   NA

 [8,]   77   20

 [9,]   77   NA

[10,]   77   NA

[11,]   77   20

[12,]   20   77

[13,]   NA   77

[14,]   77   20

[15,]   77   20

I want to have a column of 15 samples without NA's.  Is there an R function like ifelse()?

It's not clear what you exactly want.
A matrix with 15 rows and some columns?
How do you want to remove the NA's in each column?
Then why don't you leave out the NA in the sample?

Berend