Repeat function does not stop in R

Question

I am interested in creating a function that would provide a length of a Colatz sequence. To do that I am using a repeat function to go through the sequence. However, it seems that the repeat function does not stop calculating values.

What is a Collatz sequence

Collatz sequence takes a number (call it n) and checks whether it is even or odd. If it is even, then the number is divided by 2 (n<-n/2). If it is odd, then the number is divided by 3n+1 (n<-(3n+1)). It ends when the number equals to 1 ( when n=1).

Initially, I thought that the repeat function does not work simply because of the floatation/integer issue. So I've added isTRUE(all.equal(...)) as advised here. That did not solve a problem.

I suppose that the error is in my logic then. However, I cannot find it.

Code

colatzSeq<-function(n){
  tempVar<-n
  count<-1
  repeat{
    if(isTRUE(all.equal(tempVar%%2,0))){
      tempVar<-tempVar/2
    }else{
      if(isTRUE(all.equal(tempVar,1))){
        tempVar<-1
      }else{
      tempVar<-tempVar/(3*tempVar+1)
      }
    }
    count<-count+1

    if(isTRUE(all.equal(tempVar,1))){
      print(count)
      break
    }
  }
}

The code supposed to return a number of items in a sequence. For example, colatzSeq(20) should return 8 (meaning it took 7 times to change the number 20 to get to 1 according to the Collatz sequence).

As mentioned above, there is no error. Just infinite division.

Thanks!

The number never equals 1. The sequence as defined is 20, 10, 5, 0.3125, ... You can check by adding `message(tempVar); Sys.sleep(0.5)` in the loop. — Roland, Oct 16 '19 at 12:57
@Roland Hmm... I see now. Also, realised that instead of `n/(3n+1)` it should be `3n+1`. Thanks for spotting that! — Joe, Oct 16 '19 at 13:01

score 1 · Accepted Answer · answered Oct 16 '19 at 13:30

Fixing the definition in the question as per https://en.wikipedia.org/wiki/Collatz_conjecture and improving the code we have:

collatzSeq<-function(n) {
  N <- n
  while(n != 1) {
    n <- if (n %% 2 == 0) n/2 else 3*n + 1
    N <- c(N, n)
  }
  N
}

collatzSeq(20)
## [1] 20 10  5 16  8  4  2  1

score 0 · Answer 2 · answered Oct 16 '19 at 13:05

0

Turns out that I have misread the definition. Odd numbers get replaced by 3n+1 not n/(3n+1). The rest works fine.

answered Oct 16 '19 at 13:05

Joe

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Repeat function does not stop in R

2 Answers2