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# Combinatorial - A class to generate combinations of numbers

, 8 Oct 2008 CPOL
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A combinatorial class and example code.

## Introduction

This is a class to generate combinations of numbers. The generated sets of numbers can be utilized as indexes in your applications. The combinations are with repetition (the same index number can be found in more than one combination), but the combinations are unique.

## Background

The reason I wrote this is that I couldn't find a class that could generate combinations of numbers. That was a year ago. So, I wrote a class to do it. And, here I want to share it.

The code to perform the permutations is not easy to follow, but the code creating the combinations is very easy: it receives the permutation numbers, and if they are found in the current set of combinations, it discards them; otherwise, it adds the combinations to the combinations vector.

Simple and silly, but it works.

## Using the code

Just create an application and pass these as parameters: `size`, `len`, and `combinations`, where:

• `size`: size of the set of numbers to combine
• `len`: length of the resulting sets of combinations
• `combinations`: how many sets we want to generate

The resulting sets of combinations can be used as indexes in your application.

Example:

```#include <span class="code-string">"combinatorial.h"</span>
#include <span class="code-keyword"><iterator> // for the copy on ostream</span>

int main(int argc, char* argv[])
{
ComboVector cv;
if (argc != 4)
return -1;

int size = atoi(argv[1]);
int len = atoi(argv[2]);
int combin = atoi(argv[3]);

if (len >= size)
{
cerr << " len cannot be >= size, exiting";
return -2;
}

Comb::Combinations(size,len,combin,cv);
copy(cv.begin(), cv.end(), ostream_iterator<string>(cout, "\n" ) );

return 0;
}```

## Points of interest

An interesting point (beyond the combinatorial) is the printing of the values of the vector using an algorithm from iterators (copy).

## History

• 8 Oct 2008 - Version 1.

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