C++ Program to Print Prime Numbers

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

Print prime numbers between 1 and 100
At program runtime, print prime numbers between a given range specified by the user

Before creating these programs for the printing of prime numbers, let's first understand them.

What is a prime number?

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

Print prime numbers between 1 and 100

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

#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 built and runs under the Code::Blocks IDE. Here is its sample output:

The dry run of the above program goes like this:

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 go inside this loop's body.
Now the condition of the if gets evaluated. There are two conditions, and between these two conditions, we've used the && operator. As a result, program flow only execute the if's body statement until both conditions are true.
But this time, its first condition, which is chk==0 or 0==0, evaluates to be true, but its second condition, which is i!=1 or 1!=1, evaluates to be false, therefore program flow does not go inside its body.
Now 0 gets initialized to chk, and the program flow goes to the update part of the outer for loop, which 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 both conditions evaluates to be true, program flow goes inside its body, and the value of i gets printed as a prime number, which is 2.
Now 0 gets initialized to ch, and program flow goes to the update part of the 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 is evaluated as false.
In this way, prime number gets printed one by one.

C++ Print Prime Numbers in a Given Range

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

#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 enter the two numbers, say 10 and 50, as the starting and ending numbers to print all prime numbers between 10 and 50, as shown in the screenshot below:

The same program in different languages

C++ Quiz

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