Csci 1301, Lab 5
2008.06.20
Primes.java
1
This lab gets us working with nested loops. You will have two lab periods to
complete this one. You will be writing a program to decide which numbers within a user-
given range are prime and which aren’t. Here’s a sample run:
> Enter min: -5
> min must be positive.
> Enter min: 3
> Enter max: 2
> max must be >= min.
> Enter max: 10
>
> min = 3, max = 10
> 3 is prime.
> 5 is prime.
> 7 is prime.
> Checking complete.
Recall that a prime number is any integer greater than 1 that is only evenly
divisible by 1 and itself. That is, a prime number has a remainder when divided by any
other number. Also note that 1 is not prime.
You should use a while loop to make sure that the minimum value entered is
positive. You should also use a while loop to make certain that max is greater than or
equal to min. This is pretty easy. (See ExamAverager.java on page 194.)
How to figure out if an integer iis prime? Try dividing by every number greater
than 1 and less i. If any of them leave a remainder of 0, then i is not prime. How are you
going to test all of these numbers? A loop obviously. I would suggest keeping a boolean
variable that tells you whether you’ve found a divisor or not. If you get through all of the
possible divisors and don’t find one, then the number is prime and you should print a line
saying this. Otherwise, it is compound.
(In fact, you only need to check those numbers that are less than the square root of
i. So if a number x*x is greater than i and you haven’t found a number less than x that
divides i evenly, i is prime. If that just confused you, feel free to ignore it and try every
number between 1 and i.)
So you’ll need one loop to test to see if a single number i is prime. I’d recommend
writing the loop to do that first. But we want to try all possible integers in the user-given
range. So, just wrap that first loop inside another loop that will try every possible prime
number. Easy as pie.
