C++ Program To Find Prime Numbers

What is a PRIME NUMBER?

" A Natural number greater than 1 which has only two divisor 1 and itself is called prime number ".

For Example:  

5 is prime, because it has only two divisors 1 and itself.

1. #include<iostream.h>
2. #include<conio.h>
3.         void main()
4.         {
5.          //clrscr();
6.          int number,count=0;
7. cout<<"ENTER NUMBER TO CHECK IT IS PRIME OR NOT ";
8.           cin>>number;
9.            for(int a=1;a<=number;a++)
10.                {
11.                 if(number%a==0)
12.                    {
13.                   count++;
14.                    }
15.                }
16.        if(count==2)
17.          {
18.           cout<<" PRIME NUMBER \n";
19.          }
20.        else
21.          {
22.           cout<<" NOT A PRIME NUMBER \n";
23.          }
24.        //getch();
25.        }

• UNDERSTANDING PRIME NUMBERS PROGRAM LOGIC
•      To make logic first think about PRIME NUMBER definition that a number which divides by 1 and itself.
•    So we have make a condition like that in which user will enter a number and our program will check it by dividing it from 1 up to itself.
•    To check that how much times it has divided to numbers from 1 to itself we take a variable and increment it each times when a number is divided.
•    In C++ CODING we take a FOR LOOP which will start from 1 up to number that has entered to check whether it is PRIME NUMBER or not with in FOR LOOP we set an IF condition and placed counter (count++) variable in its body so whenever a number from 1 to number which has entered to check divides than IF condition becomes true and counter variable will be incremented.
•    When FOR LOOP is completed we check from IF condition that if counter variables value is equal to 2 than number is PRIME else NUMBER IS NOT PRIME.
•    Because if number divided two times by 1 and itself counter variable will incremented two times if more than two times counter variable will have value greater than 2. 

Program to find perfect number

What is a perfect number?

"Perfect number is a positive number which sum of all positive divisors excluding that number." 

For example 6 is Perfect Number since divisor of 6 are 1, 2 and 3. Sum of its divisor is

1 + 2+ 3 =6

and  28 is also a Perfect Number

 since 1+ 2 + 4 + 7 + 14= 28

Other perfect numbers: 496, 8128

CODE FOR PERFECT NUMBER IN C++

1. #include<iostream.h
2. #include<conio.h>
3. void main()                 //Start of main
4. {
5.   clrscr();
6.    int i=1, u=1, sum=0;
7.    while(i<=500)
8.  {                                  // start of first loop.
9.    while(u<=500)
10.    {                               //start of second loop.
11.      if(u<i)
12.      {
13.       if(i%u==0 )
14.       sum=sum+u;
15.      }                          //End of if statement
16.      u++;
17.    }                           //End of second loop
18.    if(sum==i)
19.    {
20.     cout<<i<<" is a perfect number."<<"\n";
21.    }
22.    i++;
23.    u=1;  sum=0;
24.  }                             //End of First loop
25.    getch();
26.  }

 Sample output

6 is a perfect number.

28 is a perfect number.

496 is a perfect number.

Note:

If you want to calculate perfect number within your desire limit simply take a variable and replaced 500 with it.