# Homework Help: How to tell Fortran to get rid of composite numbers?

1. Apr 30, 2013

### Sir Arun

Problem Statement: Do the sieve of Eratosthenes from 2 to 100 and find all the primes

So I'm trying to do the sieve of Eratosthenes in fortran 90 (I'm using Plato IDE)

My efforts to solve this/ method to use:

Obviously, I want the program to do a LOOP starting with every whole number from 2 onwards, and then get rid of all multiples of that number before the next LOOP starts with the next number and so on.

But how do I tell fortran that it should take a number and then strike all multiples of it, essentially getting rid of all composite numbers?

Relevant equation: THATs the one I am searching for, which is why I need your help.

thanks for any help,
Sir Arun

Last edited: Apr 30, 2013
2. Apr 30, 2013

### Staff: Mentor

You need some way to hold information ("prime"/"not prime") about 100 numbers in such a way that you can easily access any random element of them. Which data type would you choose?

3. Apr 30, 2013

### Staff: Mentor

You seem to think that there is some magic formula or equation that will be of use here. That's not true.

What you really need is an algorithm, a sequence of steps to be performed.

For a given number between 2 and 100, how would you manually determine that it is either composite or prime?