A structure is a user-defined data type available in C that allows to combining 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 one member. The format of the struct statement is as follows:
struct [structure name]
{
member definition;
member definition;
...
member definition;
};
A union 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;
};
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 is used for accessing members.
Differences
// 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]; }; // declaraing 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,"geeksforgeeks"}; // creating variable for union // and initializing values union union_example u={18,38,"geeksforgeeks"}; printf("structure data:
integer: %d
" "decimal: %.2f
name: %s
", s.integer, s.decimal, s.name); printf("
union data:
integeer: %d
" "decimal: %.2f
name: %s
", u.integer, u.decimal, u.name); // difference two and three printf("
sizeof structure : %d
", sizeof(s)); printf("sizeof union : %d
", sizeof(u)); // difference five printf("
Accessing all members at a time:"); s.integer = 183; s.decimal = 90; strcpy(s.name, "geeksforgeeks"); printf("structure data:
integer: %d
" "decimal: %.2f
name: %s
", s.integer, s.decimal, s.name); u.integer = 183; u.decimal = 90; strcpy(u.name, "geeksforgeeks"); printf("
union data:
integeer: %d
" "decimal: %.2f
name: %s
", u.integer, u.decimal, u.name); printf("
Accessing one member at time:"); printf("
structure data:"); s.integer = 240; printf("
integer: %d", s.integer); s.decimal = 120; printf("
decimal: %f", s.decimal); strcpy(s.name, "C programming"); printf("
name: %s
", s.name); printf("
union data:"); u.integer = 240; printf("
integer: %d", u.integer); u.decimal = 120; printf("
decimal: %f", u.decimal); strcpy(u.name, "C programming"); printf("
name: %s
", u.name); //difference four printf("
Altering a member value:
"); s.integer = 1218; printf("structure data:
integer: %d
" " decimal: %.2f
name: %s
", s.integer, s.decimal, s.name); u.integer = 1218; printf("union data:
integer: %d
" " decimal: %.2f
name: %s
", u.integer, u.decimal, u.name); } |
Output:
structure data: integer: 18 decimal: 38.00 name: geeksforgeeks union data: integeer: 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: geeksforgeeks union data: integeer: 1801807207 decimal: 277322871721159510000000000.00 name: geeksforgeeks Accessing one member at a 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: ?
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