changing loop index within loop

Question

I am relatively new to R. I am iterating over a vector in R by using for() loop. However, based on a certain condition, I need to skip some values in the vector. The first thought that comes to mind is to change the loop index within the loop. I have tried that but somehow its not changing it. There must be some what to achieve this in R.

Thanks in advance. Sami

Answer 1

You can change the loop index within a for loop, but it will not affect the execution of the loop; see the Details section of ?"for":

 The ‘seq’ in a ‘for’ loop is evaluated at the start of the loop;
 changing it subsequently does not affect the loop. If ‘seq’ has
 length zero the body of the loop is skipped. Otherwise the
 variable ‘var’ is assigned in turn the value of each element of
 ‘seq’. You can assign to ‘var’ within the body of the loop, but
 this will not affect the next iteration. When the loop terminates,
 ‘var’ remains as a variable containing its latest value.

Use a while loop instead and index it manually:

i <- 1
while(i < 100) {
  # do stuff
  if(condition) {
    i <- i+3
  } else {
    i <- i+1
  }
}

Answer 2

Look at

?"next"

The next command will skip the rest of the current iteration of the loop and begin the next one. That may accomplish what you want.