// Demonstrates sharing memory with union types 
// Compile: gcc union1.c
// Execute: a.out
//
#include <stdio.h>
int main(){

    // First we look at format specifiers in printf:
    // printf stands for formatted print
    
    // Declare some variables
    int myInt = 3;
    float myFlt = 3.0;

    // Declare variable myUnion of union type
    // unions allow type checking rules to be violated
    // myUnion can hold an int, or a float, or an array of 4 chars
    union aUnionType{
        int i;
        float f;
        char c[4];
    } myUnion;

    // Print decimal representation of myInt
    // Format specifier %d means print the decimal representation of myInt
    int i = 3;
    printf("an int as d = %d (should be 3)\n", i);    // 3
    printf("an int as f = %f (should be 3)\n\n", i);    // 3
    printf("myInt as a f = %f\n\n", myInt);    // ???

    printf("sizeof int = %d \n", sizeof(int));    // 3
    printf("sizeof float = %d \n\n", sizeof(float));    // 3

    printf("myInt as d = %d (should be 3)\n", myInt);    // 3

    // Now a float
    myFlt = 3.0;

    // Format specifier %f means print the float representation of myFlt
    printf("myFlt as f = %f (should be 3.0)\n\n", myFlt);    // 3.0

    // Can we print myFlt as an int?
    // Yes, but we don't get 3
    printf("myFlt as a d = %d\n", myFlt);    // ???
    printf("myInt as a f = %f\n\n", myInt);    // ???


    myUnion.i = 3;
    printf("myUnion.i as d = %d (should be 3)\n", myUnion.i);    // 3
    printf("myUnion.i as f = %f (should be ?)\n", myUnion.i);    

    printf("myUnion.f as d = %d (should be 3)\n", myUnion.f);    // 3
    printf("myUnion.f as f = %f (should be ?)\n", myUnion.f);    

    // myFlt = myUnion.i;      // Type error!
    
    myFlt = myUnion.f;         // Type check okay

    // myUnion.i and myUnion.f are the SAME memory location
    // So myFlt contains the int 3!
   
    printf("%f\n", myFlt);    // ???, but not 3

    printf("%d\n", myFlt);    // This prints 3


    // Let's use the myUnion.c to examine the BYTES of myUnion.i
    
    myUnion.i = 1048;                     

    // %x means print the hexadecimal representation of 1048
    // 1048 decimal = 00 00 04 18 hexadecimal
    printf("%x\n", myUnion.i);    // 418

    // Print each byte of 1048, as a decimal
    printf("%d ", myUnion.c[0]);   // 0
    printf("%d ", myUnion.c[1]);   // 0
    printf("%d ", myUnion.c[2]);   // 4
    printf("%d\n", myUnion.c[3]);  // 24
    // 18 hexadecimal = 24 decimal
    
}