## C, , union_find.c

``````/**
* @file union_find.c
* @brief [Union
* find](https://en.wikipedia.org/wiki/Disjoint-set_data_structure) algorithm.
*/
#include <stdio.h>
#include <stdlib.h>
#define MAX_SIZE 1000 /**< maximum number of elements in the set */

/**
* @brief Find index of or value in an array
*
* @param [in,out] p array to search and update
* @param x value to search
* @return value at the index `x`
*/
int find(int *p, int x)
{
if (x >= MAX_SIZE)
{
fprintf(stderr, "Out-of bounds value\n");
exit(EXIT_FAILURE);
}

if (p[x] == x)
{
return x;
}
else
{
p[x] = find(p, p[x]);
return p[x];
}
}

/**
* @brief Function to join
* @param [in,out] p array to join in
* @param x value or index to join to
* @param y value or index to join from
*/
void join(int *p, int x, int y) { p[find(p, x)] = find(p, y); }

/** Main function */
int main()
{
int union_set[MAX_SIZE];

// Have all array indexes that you need to use reference themselves
for (int i = 0; i < 10; i++)
{
union_set[i] = i;
}
// p = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

join(union_set, 3, 5);
printf("The array is now: ");
for (int i = 0; i < 10; i++)
{
printf("%d ", union_set[i]);
}
printf("\n");
// Now 3 and 5 are groupped together, that is find(3) = find(5)
// p = {0, 1, 2, 5, 4, 5, 6, 7, 8, 9}

join(union_set, 3, 8);
printf("The array is now: ");
for (int i = 0; i < 10; i++)
{
printf("%d ", union_set[i]);
}
printf("\n");

// Now 3, 5 and are groupped together, find(3) = find(5) = find(8)
// p = {0, 1, 2, 5, 4, 8, 6, 7, 8, 9}
join(union_set, 0, 5);
if (find(union_set, 0) == find(union_set, 3))
{
printf("0 and 3 are groupped together\n");
}
printf("The array is now: ");
for (int i = 0; i < 10; i++)
{
printf("%d ", union_set[i]);
}
printf("\n");

return 0;
}
``````