I need to create a flowchart, mainly using do-while loops, that asks for two numbers A and B, verifies that A is smaller than B, and then displays every pair of twin primes between them. Twin primes are two prime numbers that differ by exactly 2. For example, if A = 1 and B = 5, the result should be 3 and 5. I understand that I need to validate the input first, but I am unsure how to structure the prime-number checking and the loop that finds the twin primes.
3 Answers
Break the task into two smaller problems: first create a flowchart that determines whether one number is prime, then use that test while scanning the range. For each number n from A through B - 2, check whether n is prime and whether n + 2 is prime. If both tests are true, output the pair n and n + 2. A do-while loop can handle the repeated divisor checks when determining whether a number is prime.
The overall flow could be: read A and B, repeat the input step while A is greater than or equal to B, set n to A, and then repeat while n + 2 is less than or equal to B. For each n, call your prime-checking logic for n and n + 2. Print the pair when both are prime, then increase n and continue. If no pair is found, you can display a message saying that the range contains no twin primes.
A simple prime test is to try dividing the candidate by every integer from 2 up to the candidate minus 1. If the remainder is zero for any of those divisors, it is not prime. If the loop finishes without finding a divisor, it is prime. Then the main loop can increase n one at a time and test n and n + 2. You can optimize later by checking divisors only up to the square root, but the basic version is easier to turn into a flowchart.
That makes sense. I was trying to solve the whole problem at once, so I should build and test the prime-checking part first.