Question

I want to write a really simple prime app. But i am having some problems with the code below it should generate primes from 0 to 99. But instead it just shows 99.

One problem is that only the last number is displayed. But the other one is that the prime “check” doesn’t work. How can I fix those problems.

for (i=0; i<100; i++) {
    for (n=2; n<i; n++) {
        if (i%n == 0) break;
        else primetext.text = [NSString stringWithFormat:@"%i, ", i];
    }
}

Answer 1

There were a number of problems, I’ll try to explain them

• The problem with prime checking was that you had the if-else statement inside of the for loop that iterated through the numbers to check. So if your number wasn’t divisible by ANY of the numbers you check it would be registered as prime. It would also be registered n times, where n is the number of times it was found to not be divisible.
• All even numbers are not prime (aside from 2), so you can do i+=2 to double your speed
• You only need to check up to the square root of a number to see if it’s prime.
• 1 isn’t a prime so you don’t need to include it (especially because it can make your logic more complex).
• You were reassigning the string instead of adding a component to it
• You were using %i instead of %d. This can sometimes cause weird errors.

Here is your method revised with these points in mind.

int maxNum = 100;
primetext.text = @"2 ";             //Start knowing that 2 is the lowest prime (avoid evaluating evens)
for (i=3; i<=maxNum; i+=2)          //Start at 3 and add 2 (avoid 1 and 2 as well as even numbers) 
{
    bool isPrime = YES;             //Assume that i is prime
    for (n=2; n<sqrt(i); n++)       //Divide by every number up to square root
    {
        if (i%n == 0)               //If evenly divisible by n, not a prime
        {
            isPrime = NO;
            break;                  //Don't need to check any more factors, so break
        }
    }
    if (isPrime)                    //If isPrime was never set to NO, i is prime
    {
        NSString *temp = [NSString stringWithFormat:@"%d ", i];
        primetext.text = [primetext.text stringByAppendingString:temp];
    }
}

Hope this helps.