Difference Between Structure and Union in C
Structures in C is a user-defined data type available in C that allows to combining of data items of different kinds. Structures are used to represent a record.
Defining a structure: To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than or equal to one member. The format of the struct statement is as follows:
struct [structure name] { member definition; member definition; ... member definition; }; (OR) struct [structure name] { member definition; member definition; ... member definition; }structure variable declaration;
Union in C is a special data type available in C that allows storing different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple purposes.
Defining a Union: To define a union, you must use the union statement in the same way as you did while defining a structure. The union statement defines a new data type with more than one member for your program. The format of the union statement is as follows:
union [union name] { member definition; member definition; ... member definition; }; (OR) union [union name] { member definition; member definition; ... member definition; }union variable declaration;
Similarities Between Structure and Union
- Both are user-defined data types used to store data of different types as a single unit.
- Their members can be objects of any type, including other structures and unions or arrays. A member can also consist of a bit field.
- Both structures and unions support only assignment = and sizeof operators. The two structures or unions in the assignment must have the same members and member types.
- A structure or a union can be passed by value to functions and returned by value by functions. The argument must have the same type as the function parameter. A structure or union is passed by value just like a scalar variable as a corresponding parameter.
- ‘.’ operator or selection operator, which has one of the highest precedences, is used for accessing member variables inside both the user-defined datatypes.
Differences between Structure and Union
Differences between Structure and Union are as shown below in tabular format as shown below as follows:
Example
C
// C program to illustrate differences // between structure and Union #include <stdio.h> #include <string.h> // declaring structure struct struct_example { int integer; float decimal; char name[20]; }; // declaring union union union_example { int integer; float decimal; char name[20]; }; void main() { // creating variable for structure // and initializing values difference // six struct struct_example s = { 18, 38, "w3wiki" }; // creating variable for union // and initializing values union union_example u = { 18, 38, "w3wiki" }; printf ( "structure data:\n integer: %d\n" "decimal: %.2f\n name: %s\n" , s.integer, s.decimal, s.name); printf ( "\nunion data:\n integer: %d\n" "decimal: %.2f\n name: %s\n" , u.integer, u.decimal, u.name); // difference two and three printf ( "\nsizeof structure : %d\n" , sizeof (s)); printf ( "sizeof union : %d\n" , sizeof (u)); // difference five printf ( "\n Accessing all members at a time:" ); s.integer = 183; s.decimal = 90; strcpy (s.name, "w3wiki" ); printf ( "structure data:\n integer: %d\n " "decimal: %.2f\n name: %s\n" , s.integer, s.decimal, s.name); u.integer = 183; u.decimal = 90; strcpy (u.name, "w3wiki" ); printf ( "\nunion data:\n integer: %d\n " "decimal: %.2f\n name: %s\n" , u.integer, u.decimal, u.name); printf ( "\n Accessing one member at time:" ); printf ( "\nstructure data:" ); s.integer = 240; printf ( "\ninteger: %d" , s.integer); s.decimal = 120; printf ( "\ndecimal: %f" , s.decimal); strcpy (s.name, "C programming" ); printf ( "\nname: %s\n" , s.name); printf ( "\n union data:" ); u.integer = 240; printf ( "\ninteger: %d" , u.integer); u.decimal = 120; printf ( "\ndecimal: %f" , u.decimal); strcpy (u.name, "C programming" ); printf ( "\nname: %s\n" , u.name); // difference four printf ( "\nAltering a member value:\n" ); s.integer = 1218; printf ( "structure data:\n integer: %d\n " " decimal: %.2f\n name: %s\n" , s.integer, s.decimal, s.name); u.integer = 1218; printf ( "union data:\n integer: %d\n" " decimal: %.2f\n name: %s\n" , u.integer, u.decimal, u.name); } |
structure data: integer: 18 decimal: 38.00 name: w3wiki union data: integer: 18 decimal: 0.00 name: sizeof structure : 28 sizeof union : 20 Accessing all members at a time:structure data: integer: 183 decimal: 90.00 name: w3wiki union data: integer: 1801807207 decimal: 277322871721159507258114048.00 name: w3wiki Accessing one member at time: structure data: integer: 240 decimal: 120.000000 name: C programming union data: integer: 240 decimal: 120.000000 name: C programming Altering a member value: structure data: integer: 1218 decimal: 120.00 name: C programming union data: integer: 1218 decimal: 0.00 name: ?
Time Complexity: O(1)
Auxiliary Space: O(1)
Note: Structures are better than unions since memory is shared in a union which results in a bit of ambiguity. But technically speaking, unions are better in that they help save a lot of memory, resulting in the overall advantage over structures in the long run.
Quiz on structures and Union.