C++ Program to Print Prime Numbers

In this article, you will learn and get code to print prime numbers using C++ program in following ways:

Print Prime Numbers between 1 to 100
Print Prime Numbers in a Given Range by user at run-time

Before creating these programs on printing of prime numbers, let's first understand about it.

What is a Prime Number ?

If a number can't be divisibly by any number except 1 and the number itself, then that number is called as prime number. For example, 2, 3, 5, 7, 13, 17 19 etc.

Print Prime Numbers between 1 to 100

This program prints all prime numbers between 1 to 100 using for loop. The question is, write a program in C++ to print prime numbers from 1 to 100. Here is its answer:

// C++ Print Prime Numbers between 1 to 100
// ----codescracker.com----

#include<iostream>
using namespace std;
int main()
{
    int i, chk=0, j;
    cout<<"Prime Numbers Between 1 to 100 are:\n";
    for(i=1; i<=100; i++)
    {
        for(j=2; j<i; j++)
        {
           if(i%j==0)
           {
               chk++;
               break;
           }
        }
        if(chk==0 && i!=1)
            cout<<i<<endl;
        chk = 0;
    }
    cout<<endl;
    return 0;
}

This program was build and run under Code::Blocks IDE. Here is its sample output:

The dry run of above program goes like:

Initial value, chk=0
Now 1 gets initialized to i, and the condition i<=100 or 1<=100 evaluates to be true, therefore program flow goes inside the loop
Inside the loop, there is another for loop, so 2 gets initialized to j and the condition j<i or 2<1 evaluates to be false, therefore program flow does not goes inside this loop's body
Now the condition of if gets evaluated. There are two conditions, and between these two conditions, we've used && operator. So that, program flow only execute the statement of if's body until its both conditions evalautes to be true
But this time, its first condition that is chk==0 or 0==0 evalautes to be true, but its second condition, that is i!=1 or 1!=1 evalautes to be false, therefore program flow does not goes inside its body
Now 0 gets again initialized to chk and program flow goes to the updatation part of outer for loop and increments the value of i. So i=2
The condition i<=100 or 2<=100 again evaluates to be true, therefore program flow again goes inside the loop
2 gets initialized to j and the condition j<i or 2<2 evaluates to be false
Now the condition of if, that is, chk==0 or 0==0 evaluates to be true, and its second condition, i!=1 or 2!=1 also evaluates to be true. Since its both condition evalautes to be true, therefore program flow goes inside its body and the value of i gets printed as prime number, that is 2
Now 0 gets initialized to ch and program flow goes to updatation part of for loop and increments the value of i. So i=3
The condition i<=100 or 3<=100 again evaluates to be true, therefore program flow again goes inside the loop
This process continues, until the condition evalautes to be false
In this way, prime number gets printed one by one

Print Prime Numbers in a Given Range

To print all prime numbers between a particular range (entered by user) in C++ programming, do divisibility test (as done in previous program) using for loop, from 2 to one less than that number (i.e., n-1). If the number is divided to any number from 2 to one less than that number, then the number will not be prime. Otherwise, print it as a prime number as shown here in the following program.

// C++ Print Prime Numbers in a Given Range
// ----codescracker.com----

#include<iostream>
using namespace std;
int main()
{
    int st, en, i, j, chk=0;
    cout<<"Enter the Range\n";
    cout<<"Enter the Starting Number: ";
    cin>>st;
    cout<<"Enter the Ending Number: ";
    cin>>en;
    cout<<"\nPrime Numbers between "<<st<<" and "<<en<<" are:\n";
    for(i=st; i<=en; i++)
    {
        for(j=2; j<i; j++)
        {
           if(i%j==0)
           {
               chk++;
               break;
           }
        }
        if(chk==0 && i!=1)
            cout<<i<<endl;
        chk = 0;
    }
    cout<<endl;
    return 0;
}

Here is the initial output of this program's sample run:

Now supply the two numbers as starting number and ending number say 10 and 50 to print all prime numbers between 10 and 50 as shown in the snapshot given below:

Same Program in Other Languages

C++ Online Test

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