# C++ Exercises: Find Perfect numbers and number of Perfect numbers between 1 to 1000

## C++ Numbers: Exercise-5 with Solution

Write a program in C++ to find Perfect numbers and number of Perfect numbers between 1 to 1000.

**Pictorial Presentation:**

**Sample Solution**:

**C++ Code :**

```
# include <iostream>
# include <string>
using namespace std;
int main()
{
int i=1, u=1, sum=0,ctr=0;
cout << "\n\n Find the Perfect numbers and number of Perfect numbers between 1 to 1000:\n";
cout << "------------------------------------------------------------------------------\n";
cout << "The Perfect numbers are : \n";
while(i<=1000)
{
while(u<=1000)
{
if(u<i)
{
if(i%u==0 )
sum=sum+u;
}
u++;
}
if(sum==i)
{
ctr++;
cout<<i<<" is a Perfect number."<<"\n";
}
i++;
u=1; sum=0;
}
cout<<"Number of Perfect numbers between 1 to 1000 is: "<<ctr<<"\n";
}
```

Sample Output:

Find the Perfect numbers and number of Perfect numbers between 1 to 1000: ------------------------------------------------------------------------------ The Perfect numbers are : 6 is a perfect number. 28 is a perfect number. 496 is a perfect number. Number of perfect numbers between 1 to 1000 is: 3

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## C++ Programming: Tips of the Day

**How to use the PI constant in C++?**

On some (especially older) platforms (see the comments below) you might need to

#define _USE_MATH_DEFINES and then include the necessary header file:#include<math.h>

and the value of pi can be accessed via:

M_PI

In math.h (2014) it is defined as:

# define M_PI 3.14159265358979323846 /* pi */

but check your math.h for more. An extract from the "old" math.h (in 2009):

/* Define _USE_MATH_DEFINES before including math.h to expose these macro * definitions for common math constants. These are placed under an #ifdef * since these commonly-defined names are not part of the C/C++ standards. */

However:

- on newer platforms (at least on my 64 bit Ubuntu 14.04) I do not need to define the _USE_MATH_DEFINES
- On (recent) Linux platforms there are long double values too provided as a GNU Extension:

# define M_PIl 3.14159265358979323846

Ref: https://bit.ly/3G4BgzQ

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