Twin prime numbers in R programming till 1000

Quote:

Twin prime numbers in R programming till 1000

Not a solution to your question. But hope this solution is still interesting.

As programmer, you are free to use your brain.
You are looking for twin primes numbers.
- 2 is the only prime even number. So there is no twin primes even numbers.

n=2
n=3
N=1000
while(n<N){
  if((isPrime(n) == TRUE) && isPrime(n+2) == TRUE){
    cat("(",n,",",n+2,")","\n")}
  n=n+1
  n=n+2
   }

This makes your code twice as fast as original.

- In IsPrime, you can double the speed by testing 2 and then loop on odd divisors.

- If you study prime detection a little, you see the an integer is prime if there is no divisor other than 1 and itself.
This imply that an integer is not a prime as soon as you found a divisor.
This means that a single modulo with 2 tells you that 1000 is not a prime, your code needs 1000 modulos.

isPrime <- function(a){
  count = 0
  for(i in 1:a+1){
    if(a%%i==0){
      count=count+1
      return (FALSE)
    }
    if(count == 2){
      return TRUE
    }
  }
  return (TRUE)
}

- With a little more study, you will see that there is no need to test divisors beyond the square root of an integer to know if it is a prime or not. For a prime around 1000, you only need to test until 31.

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

private int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.

If you don't know how to use a debugger in R, start here: R Debug – Essential Principles and Functions that you can’t miss![^]

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!