Checking whether the given number is prime, composite or zero in C++

Prime and composite numbers

Prime numbers are those numbers which are divisible by 1 and itself only. A prime number doesn't have any pattern but can be determined by the division process.
      E.g. 2,3,5,7,...etc. (i.e. 7 is divisible by 1 and itself only, so it is a prime number.)

Composite numbers are those numbers which are not prime numbers. Those numbers which are divisible by any numbers other than 1 and itself are called composite numbers.
      E.g. 4,6,9...etc.(i.e. 4 is divisible by 1,2 and 4, so it is a composite number.)

Zero and one (0,1) are neither prime nor composite numbers. 

Write a program in C++ to check whether the given number is prime, composite or none.

Firstly the number entered by the user is stored in the integer type variable. A for loop is executed from 1 to the input number. A counter is set and increases if any number within the loop divides it. Finally, the prime numbers will have two(2) as their counter value. Composite numbers will have a number greater than two(2) as their counter value. For zero the for loop won't be executed and the statement in if condition will be executed.

Program 

#include<iostream.h>

#include<conio.h>

void main()

{

int num,i,b=0;

clrscr();

cout<<"Enter a number=";

cin>>num;

for(i=1;i<=num;i++)

{

if ((num%i)==0)

b++;

} 
if ( num==0)                                         

              cout<<"\nThe number is neither prime nor composite";

else if(b==2)

cout<<"\nThe number is prime";

else if(b>2)

cout<<"\nThe number is composite";
else
              cout<<"\nInvalid input";

getch();

}

Output

CHECKING WHETHER A NUMBER IS PRIME OR NOT

Explanation

In the above program, we entered a number 7 and it was stored in num. Then the for loop was executed from 1 to 7, 7 was divided by all numbers within the loop. It was divisible by only two number (1 and 7), so the value of counter b increased twice (i.e. b=2). This condition is true for prime numbers, so 7 is a prime number. 

Fact about C++

In C++ we can use endl instead of \n. [endl=end of line]

Syntax:  cout<<"Thank"<<endl<<"You"; 

Thank will be printed and You will be printed in the next line.
Output screen
Thank
You