Problem Statement

Sometimes, one may need   (or want)   a loop which its   iterator   (the index variable)   is modified within the loop body   in addition to the normal incrementation by the   (do)   loop structure index.

Demonstrate the best way to accomplish this.

Write a loop which:

Extra credit:   because of the primes get rather large, use commas within the displayed primes to ease comprehension.

Show all output here.

Not all programming languages allow the modification of a loop's index.   If that is the case, then use whatever method that is appropriate or idiomatic for that language.   Please add a note if the loop's index isn't modifiable.

Let's start with the solution:

import "./fmt" for Fmt

var isPrime = Fn.new { |n|
    if (n < 2 || !n.isInteger) return false
    if (n%2 == 0) return n == 2
    if (n%3 == 0) return n == 3
    var d = 5
    while (d*d <= n) {
        if (n%d == 0) return false
        d = d + 2
        if (n%d == 0) return false
        d = d + 4
    }
    return true
}

var count = 0
var i = 42
while (count < 42) {
    if (isPrime.call(i)) {
        count = count + 1
        System.print("%(Fmt.d(2, count)): %(Fmt.dc(18, i))")
        i = 2 * i - 1        
    }
    i = i + 1
}