R: How to increment the incrementing variable within a for loop?

问题

I'm trying to manually increment the i variable when a condition is met.

for(i in 1:x){
   if(condition){
      i <- i + 2
      }
   }

When debugging, the (i<-i+2) line is definitely being run, but i still only increments by 1, instead of 3. (+2 from the line and an additional +1 from the auto increment)

How can I increment while I'm within the loop?

回答1:

So essentially you want to skip a few loop iterations based on a condition. It's a design choice that's rightfully frowned upon, but if you must, you need next. The following code skips the third, fifth and seventh iteration:

for(i in 1:10){
  if(i %in% c(3,5,7)){
    next
  }
  print(i)
}

Say you need to increment with 3 based on a certain condition, then you can use a temporary variable that helps you skip a number of steps. Note that this does go through every iteration, it just breaks out of the iteration in time:

skip <- 0 # the temporary variable helping us keeping track of the iterations
for(i in 1:10){
  if(i == 5){  # the condition that causes the skip
    skip <- 3
  }
  if(skip > 0){ # the control that skips as long as necessary
    skip <- skip - 1
    next
  }
  print(i)
}

回答2:

When you run the loop, the value of the variable i is stored in tmp*. This means that whenever we reach the top the loop, i resets. For example

for(i in 1:2){
  message(i)
  i <- 10
  message(i)
}
#1
#10
#2
#10

To get what you want, you could have something like:

k =1 
for(i in 1:10){
   if(condition) k <- k + 2
}

回答3:

Once the sequence is created, you pretty much lose a lot of control over looping. In cases like this, I change it into a while loop and then do the conditional incrementing/decrementing at the end of the loop.

回答4:

I'm agree with joris-meys, it's "frowned upon". But... A more simple approach is:

for(i in (0:3)*2+1){
  cat(i," ")
}

for(i in (1:4)){
  cat(i," ")
}

回答5:

for(i in seq(0, 10, 2) ){
  print(i)
}

you can do this..

来源：https://stackoverflow.com/questions/39556911/r-how-to-increment-the-incrementing-variable-within-a-for-loop

标签

loops

increment