Using vector to solve Sieve of Eratosthenes with C++ and recieving unusual error message

Question

Previously today, I asked a question about implementing a Sieve of Eratosthenes with 2D arrays and was told by a few people to use vectors instead. The only problem is I had no idea how to use Vectors in C++.

I have rewritten my program today using a vector instead of a 2D array and it was going quite well until near the end of the program, where I receive the following error:

sieve.h: In function ‘void printPrimes(std::vector*, int)’: sieve.h:42:20: error: no match for ‘operator<<’ in ‘std::cout << *(primes + ((unsigned int)(((unsigned int)i) * 12u)))’

I have never received this kind of error message before so I am unsure of how to fix this.

Here is my revised code:

sieve.h

#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <vector>
using namespace std;

vector<int> calc_primes(int);
void printPrimes(int[]);

vector<int> calc_primes(int max)
{
    vector<int> primes;

    for(int i = 2; i < max; i++)
    {
        primes.push_back(i);
    }

    // for each value in the vector
    for(int i = 0; i < primes.size(); i++)
    {
        //get the value
        int v = primes[i];

        if (v!=0) {
            //remove all multiples of the value
            int x = i+v;
            while(x < primes.size()) {
                primes[x]=0;
                x = x+v;
            }
        }
    }
    return primes;
}

void printPrimes(vector<int>* primes, int size)
{
int primearray[size];
for(int i = 0; i < size; i++)
    {
        cout<<primes[i]<<endl;
    }
}

sieve.cpp

#include "sieve.h"
using namespace std;

int main()
{
    int max;
    cout<<"Please enter the max amount of prime numbers:"<<endl;
    cin>>max;
    vector<int> primes = calc_primes(max);
    printPrimes(primes, max);
    return 0;
}

I don't think you need a header file for this little program.... — StoryTeller - Unslander Monica
You're right. I don't really need one. Just getting into the practice is all. — user2812717
In that case you should practice better technique. Don't put entire implementations in header files. Use header files to decalre functions only. — StoryTeller - Unslander Monica

Vaughn Cato Vaughn Cato · Accepted Answer · 2013-09-25T02:08:20

Your loop looks like this:

for(int i = 0; i < size; i++)
{
    cout<<primes[i]<<endl;
}

but since primes is a pointer to a vector, this is treating primes as if it was an array of vectors, and by using primes[i] you are accessing the ith vector in the array of vectors, whereas you meant to access the ith element of the single vector that is passed to the function.

To fix it, just change this:

void printPrimes(vector<int>* primes, int size)

to this

void printPrimes(const vector<int> & primes, int size)

Now you are saying that primes is a reference to a vector, and primes[i] is then the ith element of that vector.

Using vector to solve Sieve of Eratosthenes with C++ and recieving unusual error message

2 Answers