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Printing prime numbers from 1 through 100

Asked 2011-03-05T00:57:20.877
17

This c++ code prints out the following prime numbers: 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97.

But I don't think that's the way my book wants it to be written. It mentions something about square root of a number. So I did try changing my 2nd loop to for (int j=2; j<sqrt(i); j++) but it did not give me the result I needed.

How would I need to change this code to the way my book wants it to be?

int main() 
{
    for (int i = 2; i < 100; i++) 
        for (int j = 2; j < i; j++)
        {
            if (i % j == 0) 
                break;
            else if (i == j + 1)
                cout << i << " ";
        }   
    return 0;
}

A prime integer number is one that has exactly two different divisors, namely 1 and the number itself. Write, run, and test a C++ program that finds and prints all the prime numbers less than 100. (Hint: 1 is a prime number. For each number from 2 to 100, find Remainder = Number % n, where n ranges from 2 to sqrt(number). If n is greater than sqrt(number), the number is not equally divisible by n. Why? If any Remainder equals 0, the number is no a prime number.)

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2 Answers

2

I check if a number is prime or not with the following code( of course using sqrt ):

bool IsPrime(const unsigned int x)
{
  const unsigned int TOP
  = static_cast<int>(
      std::sqrt( static_cast<double>( x ) )
    ) + 1;

  for ( int i=2; i != TOP; ++i )
  {
    if (x % i == 0) return false;
  }
  return true;
}

I use this method to determine the primes:

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

#include <cmath>

void initialize( unsigned int *, const unsigned int );
void show_list( const unsigned int *, const unsigned int );
void criba( unsigned int *, const unsigned int );
void setItem ( unsigned int *, const unsigned int, const unsigned int );

bool IsPrime(const unsigned int x)
{
  const unsigned int TOP
  = static_cast<int>(
      std::sqrt( static_cast<double>( x ) )
    ) + 1;

  for ( int i=2; i != TOP; ++i )
  {
    if (x % i == 0) return false;
  }
  return true;
}

int main()
{

    unsigned int *l;
    unsigned int n;

    cout << "Ingrese tope de criba" << endl;
    cin >> n;

    l = new unsigned int[n];

    initialize( l, n );

    cout << "Esta es la lista" << endl;
    show_list( l, n );

    criba( l, n );  

    cout << "Estos son los primos" << endl;
    show_list( l, n );
}

void initialize( unsigned int *l, const unsigned int n)
{
    for( int i = 0; i < n - 1; i++ )
        *( l + i ) = i + 2;
}

void show_list( const unsigned int *l, const unsigned int n)
{
    for( int i = 0; i < n - 1; i++ )
    {
        if( *( l + i ) != 0)
            cout << l[i] << " - ";
    }
    cout << endl;
}

void setItem( unsigned int *l, const unsigned int n, const unsigned int p)
{
    unsigned int i = 2;
    while( p * i <= n)
    {
        *( l + (i * p - 2) ) = 0;
        i++;
    }
}

void criba( unsigned int *l, const unsigned int n)
{
    for( int i = 0;  i * i <
answered 2011-03-05T01:06:15.407
0
#include<iostream>
using namespace std;
void main()
{
        int num,i,j,prime;
    cout<<"Enter the upper limit :";
    cin>>num;
    cout<<"Prime numbers till "<<num<<" are :2, ";

    for(i=3;i<=num;i++)
    {
        prime=1;
        for(j=2;j<i;j++)
        {
            if(i%j==0)
            {
                prime=0;
                break;
            }
        }
        if(prime==1)
            cout<<i<<", ";
    }
}
answered 2012-03-11T17:00:21.867

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