Difference Between Structure and Union in C Language

Structure and Union in C Programming

Do you know C language can store elements of the same as well as different data types? Arrays store the same data type elements while structure and union store different data type elements. Structure and Union are user-defined data types in C. Both store elements of different data types of varying sizes.

In this C tutorial, we'll study the difference between structure and union with examples in C programming.

Read More: Top 50 Mostly Asked C Interview Questions and Answers

What is a Structure in C?

The structure is a user-defined data type declared with a "struct" keyword. It combines different datatypes into a single datatype under one name. All the elements of a structure are stored at contiguous memory locations. The total memory size of a structure is the sum of the sizes of all its elements. You can access the structure elements using the dot (.) operator.

Syntax of Structure in C

struct structure_name
   {
       type member1;
       type member2;
       type member3;
   };structure_variables;

The structure variables are the structure objects for accessing the structure members.

What is a Union in C?

Union is a user-defined data type declared with a "union" keyword. It is a collection of elements of different data types. All the elements are stored at the same memory location. Only one member can hold a value at a time in a union. The size of the memory depends on the largest member of the union.

Syntax of Union in C

union union_name {
       type member1;
       type member2;
       type member3;
};union_variables;

Difference Between Structure and Union in C Programming

Difference Between Structure and Union with Example

#include <stdio.h>
#include <string.h>

// Define a structure
struct ExampleStruct {
    int intVar;
    float floatVar;
    char str[20];
};

// Define a union
union ExampleUnion {
    int intVar;
    float floatVar;
    char str[20];
};

int main() {
    // Declare and initialize a structure
    struct ExampleStruct s;
    s.intVar = 42;
    s.floatVar = 3.14;
    strcpy(s.str, "Hello, World!");

    // Declare and initialize a union
    union ExampleUnion u;
    u.intVar = 42;

    u.floatVar = 3.14;
    strcpy(u.str, "Hello, World!");

    // Print structure values
    printf("Structure values:\n");
    printf("intVar: %d\n", s.intVar);
    printf("floatVar: %.2f\n", s.floatVar);
    printf("str: %s\n", s.str);

    // Print union values
    printf("\nUnion values (after setting str):\n");
    printf("intVar: %d\n", u.intVar);   
    printf("floatVar: %.2f\n", u.floatVar);
    printf("str: %s\n", u.str);

    return 0;
}   

Output

Structure values:
intVar: 42
floatVar: 3.14
str: Hello, World!

Union values (after setting str):
intVar: 1819043144
floatVar: 1143139122437582505939828736.00
str: Hello, World!  

In the union declared above, after setting the string value, the intVar and floatVar contain garbage values or may be undefined because they share the same memory location with str.

Related Differences:

• Differences Between Stack and Queue Data Structures
• Differences Between Arrays and Linked Lists

Summary