calculate the number of prime numbers between two inputs

[xcindiix]

ive done the main parts... but i dont get how to calculate the number of prime numbers in between. for instance if the input is 8 and 2, the prime numbers will be 7,5,3,2. i want to output "There are __ prime numbers between"

I believe increment should be used, but where do i put it ?

#include <iostream>
#include <cmath>

using namespace std;

bool isprime(unsigned int x)
{
	unsigned int i = 2;
	unsigned int count = 0;

	while(i<x)
	{
		if(x%i == 0)
			return false;
		else
		{
			i = i+1;
		}
	}

	cout << i << " prime" << endl;

	return 0;
}

int main()
{
	unsigned int m, n;
	int i;
	int count = 0;

	cout << "Please input two positive integers m and n" << endl;
	cin >> m >> n;


	if(m < n)					// m = 2, n = 8
	{
		for(i=m; i<n+1; i++)
		{			
			if(isprime(i))
			{
				count++;
			}
		}
	}

	cout << "There are " << count << " prime numbers between them." << endl;

	system("pause");
	return 0;
}

[Smac89]

line 40

should be if (isprime(i))? If your isprime function works, your program will be stuck at an infinite loop at that point

Counter should be on line 42

[xcindiix]

Thanks for the reply.
I changed it, but the results are the same.

i would get 7, 5, 3, 2 is prime
but "There are 0 prime numbers between them"

:(

[kinley]

Try this

#include <iostream>

using namespace std;

bool isprime(int number)
{
    int numberOfFactors = 0;

    for(int divisor=2; divisor<=number/2; divisor++)
    {
            if(number%divisor == 0)
                     numberOfFactors++;
    }

    if(numberOfFactors == 0)
        return false;
    else
        return true;
}

int main()
{
	unsigned int m, n;
	int i;
	int count = 0;

	cout << "Please input two positive integers m and n" << endl;
	cin >> m >> n;

	if(m > n)					// m = 7, n = 3
	{
		for(i=m; i>n-1; i--)
		{
			if(!isprime(i))
                                count++;
		}

		cout << "There are " << count << " prime numbers between them." << endl;
	}

}

[xcindiix]

Thank You Sooo much!!

I have one more question... what does it mean by "use an array to store the prime numbers you have found?"

[Smac89]

Do you know what an array is?

[xcindiix]

Yes. its a series of elements of the same type placed in contiguous memory locations which can be individually referenced by adding an index to a unique identifier via the use of []?

How would i store the prime numbers found? what does the requirement mean? does it mean to save the prime numbers ? but why would the program want to do that if the input will be changed everytime it runs?

[tntxtnt]

you can save time by re-use the primes saved in the array.

Example:

- User asks for primes in range [2,8]
loop range [2,8] (7 iters)
save {2,3,5,7}
answer: 4

- User asks for primes in range [4,20]
loop range [9,20] (12 iters instead of [4-20] which is 15 iters --> save 3 iters)
answer: 6
save {2,3,5,7,11,13,17,19}

- User asks for primes in range [3,16]
loop range: none 0 iter --> faster than 14 iters
answer: 5
save {2,3,5,7,11,13,17,19}

- User asks for primes in range [1, 10 000 000]
loop range: [21, 100 000] ~ 10mil iters, not saving much time here...
answer: N
save {2,3,5,7,11,13,17,19,...,999983}

- User asks for primes in range [1, 9 000 000]
loop range: none 0 iter --> super fast, saved 9mil iters!
answer: n
save {2,3,5,7,11,13,17,19,...,999983}

[xcindiix]

Thanks tntxtnt. Any clues on how to store them in arrays ?

[kinley]

http://www.cplusplus.com/doc/tutorial/arrays/