What is a union?

A union, is a collection of variables of different types, just like a structure. However, with unions, you can only store information in one field at any one time.
You can picture a union as like a chunk of memory that is used to store variables of different types. Once a new value is assigned to a field, the existing data is wiped over with the new data.

Declaring a union

Declaring a union is exactly the same as declaring a struct, except you use the union keyword:

union human { 

  char *name; 

  int age; 

  float height; 

}; /* don't forget this semi colon!!!! */ 

Once again, you can use a typedef statement to simply the declaration of union variables.

The size of an union is the size of its largest field.

#include <stdio.h>

typedef struct robot1 ROBOT1;

typedef union  robot2 ROBOT2;

struct robot1 {

 int ammo;

 int energy;

};

union robot2 {

 int ammo;

 int energy;

};

int main() {

  ROBOT1 red = {10,200};

/* ROBOT2 blue   = {15,100};  DOESN'T WORK WITH UNIONS  */ 

  ROBOT2 blue;

  blue.ammo = 15;

  blue.energy = 100;

  printf("The red robot has %d ammo ", red.ammo);

  printf("and %d units of energy.\n\n", red.energy);

  printf("The blue robot has %d ammo ", blue.ammo);

  printf("and %d units of energy\n.", blue.energy);

  return 0;

}

Output:
The red robot has 10 ammo and 200 units of energy.

The blue robot has 100 ammo and 100 units of energy.

#include <stdio.h>

typedef union robot ROBOT;

union robot { 

  char *name; 

  int energy; 

};

int main() {

  int i;

  ROBOT robots[3];

  robots[0].name = "Lunar Lee";

  robots[0].energy = 50;

  robots[1].name = "Planetary Pete";

  robots[1].energy = 20;

  robots[2].name = "Martian Matt";

  robots[2].energy = 30;

  for(i=0 ; i<3 ; i++) {

    /*printf("Robot %d is called %s ", i, robots[i].name);*/

    printf("and has %d units of energy.\n", robots[i].energy);

  }

  return 0;

}

Output:
and has 50 units of energy.
and has 20 units of energy.
and has 30 units of energy.