# Exercise 3: finding prime factors

Exercise 3: finding prime factors

What is the largest prime factor of the number 600851475143?

// What is the largest prime factor of the number 600851475143? target = 600851475143 factors = [] residue = target residue_is_prime = false // NOTE: ``#`` is not a comment character! // While the residue is not prime, keep extracting factors and adding to list while (! residue_is_prime) { // A factor could be from 2 to half the residue, try until one is found // NOTE: don't get integer division automatically in groovy, so have to // explicitly cast // NOTE: originally used ``each``, but ``break`` only works inside loops println ("Factoring \${residue} ...") half = (long) (residue / 2) found_factor = false for (i=2; i <= half; i++) { // If it's a factor, stop looking, add to list & reduce residue if (residue % i == 0) { residue = (long) (residue / i) println ("Reduce to \${i} & \${residue}") // NOTE: this is how you append to a list factors << i found_factor = true break } } // If you haven't found a factor, then the residue must be prime // NOTE: groovy doesn't have `not`, `and` or `or` as syntactic sugar if (! found_factor) { factors << residue residue_is_prime = true } } println ("Factors are \${factors}.") println ("Max factor is \${factors.max()}.") # => 6857