Thread: Having a problem with unions...

This is a slightly revamped version of a program some you guys helped me with last night. It turns out that it didn't meet the requirements, but this should if I can get it to work:

Code:

#include <stdio.h>
typedef  enum {now=10, next, last} grade;
union {int age; float exactage;} age;

main()
{
  struct {char first_name[20]; char last_name[20];} info;
  int grade = now;
  union age my;
  my.exactage=15.731506849;

  strcpy(info.first_name, "name");
  strcpy(info.last_name, "name");
 
 printf("This program is by %s %s. His exact age is %f, his approximate age is %d. He is currently is grade %d.", info.first_name, info.last_name, my.exactage, my.age, now);
}

error: "aggregate 'union age my' has incomplete type and cannot be initialized"

I've always had problems with unions, so odds are I've made at least one huge mistake. Any and all help would be appreciated.

You're having more than one problem.

Firstly, union declarations are of the form;

Code:

union optional_type_name
{
    /*  contained variables
}
optional_variable_name_1, optional_variable_name_2;   /* etc */

which means that, in your case, your union declaration should be;

Code:

union age {int age; double exactage;};

so that later declarations of your variable "my" will work.

The second problem is that unions can't be used to convert a double to an int in the way you're expecting. In particular, the line;

Code:

    my.exactage = 15.731506849;

does not implicitly cause my.age to have a value of 15. The actual value of my.age could be anything: it depends on the actual layout, in memory, of int and double types.

To convert a double to an int, with rounding down, a correct approach is to use mathematical operations without a union in sight. For example;

Code:

#include <math.h>
#include <stdio.h>

int main()
{
    double exactage = 15.731506849;
    int age = (int)(floor(exactage));
    printf("Exact age is &#37;f and his number of birthdays is %d\n", exactage, age);
}

Originally Posted by anon

Not even a struct. Just keep exact-age, which you can always display as an integer if you want to.

By the way, are ages even mathematically rounded or just truncated? I don't add one to my age half a year before my birthday...

I walked into a bar when I was 20 years and 9 months old, and tried that line of reasoning: "My age rounds up to 21, see?"

Didn't fly.