Checking for prime number in R, why this option using 'break' doesn't work-CodePudding

starting to get familiar with R. can't get why this simple chunk doesn't work

want to do it particularly using 'break'

prime.check <- function(num) {
  for (del in 2:(num-1)) {
    if (num %% del == 0) {
      break
    }
    print(paste(num, 'is not prime'))
  }
}

CodePudding user response：

Your function is going to print every time in the loop, which is not really what is desired: you should be finishing the loop and then determining what to print. I suggest assuming it is a prime and then if you break, then know that it is not a prime.

See

prime.check <- function(num) {
  isprime <- TRUE
  for (del in 2:(num-1)) {
    if (num %% del == 0) {
      isprime <- FALSE
      break
    }
  }
  isnot <- if (isprime) "is" else "is not"
  print(paste(num, isnot, "prime"))
  invisible(isprime)
}

prime.check(7)
# [1] "7 is prime"
prime.check(6)
# [1] "6 is not prime"

FYI, the invisible(isprime) allows you to use this function elsewhere, such as if (prime.check(8)) { do_something(); }, whereas with the print alone you couldn't do that. Completely ancillary from your original request.

CodePudding user response：

The condition inside the for loop checks if num is divisible by del without a reminder, e.g. it is not prime. In addition, use return instead of break to finalize a function.

prime.check <- function(num) {
  if(num == 2) return("prime")
  for (del in 2:(num-1)) {
    if (num %% del == 0) {
      return("not prime")
    }
  }
  return("prime")
}

prime.check(2)
#> [1] "prime"
prime.check(3)
#> [1] "prime"
prime.check(4)
#> [1] "not prime"
prime.check(9)
#> [1] "not prime"
prime.check(10)
#> [1] "not prime"

CodePudding user response：

Here is another option with no loop:

prime.check <- function(x){
  if(sum(x %% 1:x  == 0) == 2) return(paste(x, 'is prime'))
  return(paste(x, 'is not prime'))
}

prime.check(2)
#> [1] "2 is prime"
prime.check(3)
#> [1] "3 is prime"
prime.check(4)
#> [1] "4 is not prime"
prime.check(9)
#> [1] "9 is not prime"
prime.check(10)
#> [1] "10 is not prime"

CodePudding user response：

You can do this without a loop:

prime.check <- function(num) {
    if (num==2) return("2 is prime")
    if (all(num %% seq(2, num/2) > 0)) {
        return(paste(num, "is prime"))
        } else return(paste(num, "is not prime"))
}