[RFernandez]

Here is my code

#include <iostream>
#include <vector>
#include <iomanip>

using namespace std;

void Eratos(int &length, vector< int > &);
ostream &operator<< (ostream &, const vector <int>&);
bool isPrime(int);
int main()
{
	int range;
	vector< int > v;
	cout << "Enter the upper range of number to search for prime numbers\n";
	cin >> range;
	if (range > 300 || range < 2)
	{
		cout << "Invalid range. Your range is not between 2 and 300\n";
	}
	else
	{
		Eratos(range, v);
	}
	return 0;
}


void Eratos(int &length, vector< int > &v)
{

	for (int i = 2; i < length; ++i)
	{
		v.push_back(i);
	}
	for (int i = 2; i < v.size(); ++i)
	{
		if (isPrime(i) == false)
		{
			v.erase(v.begin() + i);
		}
	}

	cout << "Prime numbers are:\n";
	cout << v;
}

ostream &operator <<(ostream& print, const vector<int> &v)
{
	int count = 1;
	for (unsigned int i = 0; i < v.size(); ++i)
	{
		print << setw(5) << v[i];
		if (count % 10 == 0)
		{
			cout << '\n';
		}
		count++;
	}
	print << '\n';
	return print;
}
bool isPrime(int i)
{
	for (int j = 2; j < i; ++j)
	{
		if (i % j == 0)
		{
			return false;
		}
	}
	return true;
}

The problem is my code takes away some prime but leaves other primes in and i cant seem to figure out why

like my output is
2 3 4 5 7 8 10 11 13 15
17 18 20 21 23 25 27 28 30 31
33 35 37 38 40 42 44 46 48 49
51 52 54 56 58 60 62 63 65 67
69 70 72 73 75 77 79 80 82 84
86 88 90 91 93 95 97 99

But the output should be
2 3 5 7 11 13 17 19 23 29
31 37 41 43 47 53 59 61 67 71
73 79 83 89 97

[Salem_c]

I guess you need to understand what erase() actually does to the content and length of a vector.
http://www.cplusplus...r/vector/erase/

Print out the whole vector, after every erase() call.

You should be doing this as part of your general approach to learning how to do your own debugging.

[CTphpnwb]

Hints:
Prime numbers are prime because they have no factors other than themselves and 1. Any factor that is not prime can be broken into prime factors, so a number is prime if it has no prime factors. And if a number has prime factors, at least one of them is equal to or smaller than its square root.

This post has been edited by CTphpnwb: 10 September 2014 - 01:33 PM

[RFernandez]

im not really getting much from that. I believe the problem is how the code is being read. i think the itsprime is skipping numbers but i cant figure out why

[#define]

Hi, your iterator number i does not correspond with the vector element, for two reasons 1) it starts at 2 and 2) you erase elements.

Eg. when i is 4 you erase begin() + i which is begin() + 4, and is the 5th element.

The 5th element is :-

2,3,4,5,6

6. So you don't delete the number 4 but the number 6.

[RFernandez]

how to i fix that?

*do

[#define]

It might be simpler just to push the primes to the back of the initially empty vector.

Are you creating a sieve?

[RFernandez]

yes its a Sieve of Eratosthenes