Can't assign a value to a variable of type integer

Question

I am trying to find the prime numbers from 1 to 100. The problem I encounter is that when the program assigns the value 2 to the variable j of type integer. The value of variable j does not change. Does anyone know why this happens?

Create a program to find all the prime numbers between 1 and 100.

One way to do this is to write a function that will check if a number is prime (ie, see if the number can be divided by a prime number smaller than itself) using a vector of primes in order (so that if the vector is called primes, primes[0]==2, primes[1]==3, primes[2]==5, etc.). Then write a loop that goes from 1 to 100, checks each number to see if it is a prime, and stores each prime found in a vector. Write another loop that lists the primes you found. You might check your result by comparing your vector of prime numbers with primes. Consider 2 the first prime.

#include <iostream>
#include <string>
#include <vector>

int main()
{
    std::vector<int> primes = { 2 };
    for (int i = 2; i <= 100; ++i) 
        for (int j = 2; j < i; ++j) 
            if (i % j == 0)
                break;
            else
                primes.push_back(j);
    for (int i = 0; i < primes.size(); ++i)
        std::cout << primes[i] << ' ';
}

Answer 1

There are some mistakes in your initial implementation:

• You are adding elements too soon in the primes vector
• You don't have a clear definition of whatever you consider a prime
• You are trying to put everything in a single function

Some more cleaned up code would look a bit similar to this:

#include <iostream>
#include <string>
#include <vector>

namespace {
    bool isPrime(const std::vector<int> &previousPrimes, int possiblePrime) {
        for (auto prevPrime : previousPrimes)
            if (possiblePrime % prevPrime == 0)
               return false;

        return true;
    }
}

int main()
{
    auto primes = std::vector<int>({2});
    for (int i = 3 /*2 is already a prime*/; i <= 100; ++i)
        if (isPrime(primes, i))
           primes.push_back(i);

    for (auto prime : primes)
        std::cout << prime << ' ';
    std::cout << std::endl;
}

However, as this question looks like a homework assignment, don't blindly copy this and try to understand all the concepts used in here first.

Answer 2

#include <vector>
#include <iostream>
#include <string>

int main()
{
  std::vector<int> primes;
  primes.push_back(2);
  for(int i=3; i < 100; i++)
  {
      bool prime=true;
      for(int j=0;j<primes.size() && primes[j]*primes[j] <= i;j++)
      {
          if(i % primes[j] == 0)
          {
             prime=false;
             break;
          }
      }
      if(prime) 
      {
         primes.push_back(i);
         std::cout << i << " ";
      }
  }

   return 0;
}

this will print the fallowing :

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

We keep track of all of our previously calculated primes. If a number is divisible by a non-prime number, there is also some prime <= that divisor which it is also divisble by. This reduces computation by a factor of primes_in_range/total_range.

Answer 3

For starters there is written in the assignment that you have to write a function that checks whether a number is prime using a vector of prime numbers.

This loop

   for (int j = 2; j < i; ++j) 
        if (i % j == 0)
            break;
        else
            primes.push_back(j);

does not make sense because it does not use the vector to check whether i is divisible by an element of the vector. Also a new prime number should be added to the vector after the loop provided that I is divisible by neither element of the vector.

Also take into account that there is written in the assignment

Then write a loop that goes from 1 to 100 , checks each number to see if it is a prime

Thus only my code shown below satisfies the assignment.:)

The program can look the following way. Instead of type int I used type unsigned int because you are checking only non-negative values.

#include <iostream>
#include <vector>

bool is_prime( const std::vector<unsigned int> &primes, unsigned int x )
{
    std::vector<unsigned int>::size_type i = 0;

    while ( i < primes.size() && x % primes[i] != 0 ) ++i;

    return x != 1 && i == primes.size();        
}


int main() 
{
    const unsigned int N = 100;
    std::vector<unsigned int> primes;

    for ( unsigned int i = 1; i <= N; ++i )
    {

        if ( is_prime( primes, i ) ) primes.push_back( i );
    }

    for ( unsigned int x : primes ) std::cout << x << ' ';
    std::cout << std::endl;

    return 0;
}

The program output is

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 

Answer 4

//Thank you for all your answers, but I just figured out why. The codes are following:

#include <iostream>
#include <string>
#include <vector>

int main()
{
    std::vector<int> primes = { 2 };
    for (int i = 2; i <= 100; ++i) 
        for (int j = 2; j < i; ++j) {
            if (i % j == 0)
                break;
            if (j == i - 1)
                primes.push_back(i);
        }
    for (int x: primes)
        std::cout << x << ' ';
    return 0;
}