﻿ C++ Exercises: Print the first 20 numbers of the Pell series - w3resource

# C++ Exercises: Print the first 20 numbers of the Pell series

## C++ Numbers: Exercise-39 with Solution

Write a program in C++ to print the first 20 numbers of the Pell series.

Sample Solution:

C++ Code :

``````#include <iostream>
using namespace std;
int main()
{
int n,a=1,b=0,c;
cout << "\n\n Find the first 20 numbers of the Pell series: \n";
cout << " --------------------------------------------------\n";
cout<<" The first 20 numbers of Pell series are: "<<endl;
c=0;
cout<<" "<<c<<" ";
for(n=1; n<20; n++)
{
c= a + 2*b;
cout<<c<<" ";
a = b;
b = c;
}
cout<<endl;
}
``````

Sample Output:

```Find the first 20 numbers of the Pell series:
--------------------------------------------------
The first 20 numbers of Pell series are:
0 1 2 5 12 29 70 169 408 985 2378 5741 13860 33461 80782 195025 470832
1136689 2744210 6625109
```

Flowchart:

C++ Code Editor:

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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:

1. on newer platforms (at least on my 64 bit Ubuntu 14.04) I do not need to define the _USE_MATH_DEFINES
2. 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