How to display output in rows of five numbers?

user_17236062 0
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I'm new to programming and I have to display all the prime numbers that are the product of this code in rows of five. After too many hours of trying to find something online, this is what I came up with. This way, not even the prime numbers are being displayed in the end; only 1s all the way. I'd be happy to know what I did wrong or what I could change.

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

int main() {
    int n { 0 };
    cout << "Please enter an initial value n<2000 in order for the calculation to begin: " << endl;
    cin >> n;

    vector<bool> cygnus(n + 1);               
    for (int m = 0; m <= n; m++) {
        cygnus[m]=true;
    }

    for (int j = 2; j < n; j++) {
        if (cygnus[j] == true) {
            for (int i = j + 1; i <= n; i++) {
                if (i % j == 0) {
                    cygnus[i] = false;
                }
            }
        }
    }
    
    int s = 0;
    for (auto value : cygnus) {
        if (value == true && s > 0) {
            for (int counter = s; counter++; ) {
                if (counter % 5 == 0) {
                    cout << setw(3) << s << "  \n ";
                }
                
                if (counter % 5 != 0) {
                    cout << setw(3) << s << "  ";
                }
            }
        }

        s++;
    }

    cout << endl;
    return 0;
}
primes counter vector loops c++
1 Answers

You are seriously over-complicating your output logic. Just have a counter variable declared (and initialized to zero) outside the for loop that does the output and then, every time you print a number, increment it. When that reaches the value of 5, print a newline and reset it to zero.

A couple of other points:

  1. The STL containers (like std::vector) use the size_t type (not int) for their sizes and indexes. In the code below, I have changed all your int variables to this type; fortunately, that won't affect your algorithm.

  2. Note that 1 is not a prime number.

Here's a re-worked version of your code:

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

int main()
{
    size_t n{ 0 };
    cout << "Please enter an initial value n<2000 in order for the calculation to begin: " << endl;
    cin >> n;

    vector<bool>cygnus(n + 1);
    for (size_t m = 0; m <= n; m++) {
        cygnus[m] = true;
    }

    for (size_t j = 2; j < n; j++) {
        if (cygnus[j] == true) {
            for (size_t i = j + 1; i <= n; i++) {
                if (i % j == 0) {
                    cygnus[i] = false;
                }
            }
        }
    }
    size_t s = 0;
    size_t counter = 0;
    for (auto value : cygnus) {
        if (value == true && s > 1) { // Note that 1 is NOT a prime number
            cout << setw(3) << s << "  ";
            if (++counter == 5) {
                cout << "\n ";
                counter = 0;
            }
        }
        s++;
    }
    if (counter != 0) cout << "\n "; // Add newline for any partial last line.
    cout << endl;
    return 0;
}
user_108710730
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