In this tutorial, we’re going create a **C++ program for Prime Number** that will be able check if the given number is Prime or not. To understand the program or code you have to understand what are prime/composite numbers are and how to identify them.

Complete definition, a prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself. A number that is not a prime is called composite, except 1 because 1 is considered neither a composite nor prime.

5 is a prime because only 1 and 5 can divide it, whereas 6 is a composite because it has the divisors 2 and 3 in addition to 1 and 6.

**A prime number is a number that has NO positive divisors other than 1 and itself.**

**Example of Numbers that are Prime** (between 1-100) : 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 and more (see the image below).

## How C++ program will Check Prime numbers?

Suppose, the number we want to test is N then this program will divide the number N with all the numbers between 1 to N because if the number N is divisible by a number other then 1 or N then it’s not a prime.

Note: Actually we don’t really need to divide N up to N-1 to check if it is a prime or not because logically if there exist any number that can divide N then it can be find before N/2 division.

Suppose N is 35 then the program will divide 35 with 2, 3, 4, 5… up to 16 and check whether it is divisible by any of the number other than 1 and 37 or not. Here 35 is divisible by 5 and 7 therefore the program can easily tell that 35 is not a prime number.

## C++ Program For Prime Number

Now we had learned about prime numbers and we will apply this knowledge into our C++ program for prime number.

[codesyntax lang=”cpp” tab_width=”5″ title=”c++ program to check for prime numbers”]

// C++ program to check whether the given number is Prime or Not! #include<iostream.h> #include<conio.h> void main() { int num, count=0; clrscr(); cout<<"Enter a number :"; cin>>num; for(int i=2; i<num; i++) { if(num%i==0) { count++; break; } } if(count==0) cout<<"Prime number"; else cout<<"Not a Prime number"; getch(); }

[/codesyntax]

If you know what prime numbers are and knows a little programming then there is no need to explain this program but I am still going to explain a little about the program.

STEP1: The program is asking for a number, num.

STEP2: A loop starts from 2 to num-1 and divides num with these numbers,

If division is successful (when remainder is 0) then count is increased by 1 and loop ends (using break keywords).

STEP3: When the loop ends the value of count is checked for 0 (zero). 0 means no successful division and proves the number is a prime number.

**I hope the above C++ Program for Prime Number was easy. Are you facing any problem?**

## This article has 5 comments

November 3, 2013

Excuse me, I try your programme. All the prime number is correct. Only num 4. It is include in the prime number.

November 11, 2013

hmm…?? I don’t think so…

January 8, 2014

Aizat Azami is correct.

Num 4 is included in prime.

use num instead of num/2 to solve this.

January 11, 2014

Yup you are right dude and Aizat too. I noticed it a few days after I commented before. Thanks for the interaction everyone.

February 19, 2014

Number 4 is not a prime.. I tried it.. Program code is correct.. Thanks sir