C Structure
Arrays allow to define type of variables that can hold several data items of the same kind.
Similarly structure is another user defined data type available in C that allows to combine data items of different kinds.
Structures are used to represent a record. Suppose you want to keep track of your books in a library.
You might want to track the following attributes about each book.
- Title
- Author
- Subject
- Book ID
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 tag] {
member definition;
member definition;
...
member definition;
} [one or more structure variables];
The structure tag is optional and each member definition is a normal variable definition,
such as int i; or float f; or any other valid variable definition. At the end of the structure's
definition, before the final semicolon, you can specify one or more structure variables but it is optional.
Here is the way you would declare the Book structure -
struct Books {
char title[50];
char author[50];
char subject[100];
int book_id;
} book;
Accessing Structure Members
To access any member of a structure, we use the member access operator (.). The member access operator is coded as
a period between the structure variable name and the structure member that we wish to access.
You would use the keyword struct to define variables of structure type. The following example shows how to use a
structure in a program-
There are two types of operators used for accessing members of a structure.
- . - Member operator
- -> - Structure pointer operator (will be discussed in the next tutorial)
#include <stdio.h>>
#include <string.h>
struct Books {
char title[50];
char author[50];
char subject[100];
int book_id;
};
int main( ) {
struct Books Book1; /* Declare Book1 of type Book */
struct Books Book2; /* Declare Book2 of type Book */
/* book 1 specification */
strcpy( Book1.title, "C Programming");
strcpy( Book1.author, "Nuha Ali");
strcpy( Book1.subject, "C Programming Tutorial");
Book1.book_id = 6495407;
/* book 2 specification */
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Zara Ali");
strcpy( Book2.subject, "Telecom Billing Tutorial");
Book2.book_id = 6495700;
/* print Book1 info */
printf( "Book 1 title : %s\n", Book1.title);
printf( "Book 1 author : %s\n", Book1.author);
printf( "Book 1 subject : %s\n", Book1.subject);
printf( "Book 1 book_id : %d\n", Book1.book_id);
/* print Book2 info */
printf( "Book 2 title : %s\n", Book2.title);
printf( "Book 2 author : %s\n", Book2.author);
printf( "Book 2 subject : %s\n", Book2.subject);
printf( "Book 2 book_id : %d\n", Book2.book_id);
return 0;
}
When the above code is compiled and executed, it produces the following result -
Book 1 title : C Programming
Book 1 author : Nuha Ali
Book 1 subject : C Programming Tutorial
Book 1 book_id : 6495407
Book 2 title : Telecom Billing
Book 2 author : Zara Ali
Book 2 subject : Telecom Billing Tutorial
Book 2 book_id : 6495700
Structures as Function Arguments
#include <stdio.h>>
#include <string.h>
struct Books {
char title[50];
char author[50];
char subject[100];
int book_id;
};
/* function declaration */
void printBook( struct Books book );
int main( ) {
struct Books Book1; /* Declare Book1 of type Book */
struct Books Book2; /* Declare Book2 of type Book */
/* book 1 specification */
strcpy( Book1.title, "C Programming");
strcpy( Book1.author, "Nuha Ali");
strcpy( Book1.subject, "C Programming Tutorial");
Book1.book_id = 6495407;
/* book 2 specification */
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Zara Ali");
strcpy( Book2.subject, "Telecom Billing Tutorial");
Book2.book_id = 6495700;
/* print Book1 info */
printBook( Book1 );
/* Print Book2 info */
printBook( Book2 );
return 0;
}
void printBook( struct Books book ) {
printf( "Book title : %s\n", Book.title);
printf( "Book author : %s\n", Book.author);
printf( "Book subject : %s\n", Book.subject);
printf( "Book book_id : %d\n", Book.book_id);
}
When the above code is compiled and executed, it produces the following result -
Book 1 title : C Programming
Book 1 author : Nuha Ali
Book 1 subject : C Programming Tutorial
Book 1 book_id : 6495407
Book 2 title : Telecom Billing
Book 2 author : Zara Ali
Book 2 subject : Telecom Billing Tutorial
Book 2 book_id : 6495700
Pointers to Structures
You can define pointers to structures in the same way as you define pointer to any other variable -
struct Books *struct_pointer;
Now, you can store the address of a structure variable in the above defined pointer variable.
To find the address of a structure variable, place the '&'; operator before the structure's name as follows -
struct_pointer = &Book1;
To access the members of a structure using a pointer to that structure, you must use the → operator as follows -
struct_pointer->title;
Let us re-write the above example using structure pointer
#include <stdio.h>>
#include <string.h>
struct Books {
char title[50];
char author[50];
char subject[100];
int book_id;
};
/* function declaration */
void printBook( struct Books book );
int main( ) {
struct Books Book1; /* Declare Book1 of type Book */
struct Books Book2; /* Declare Book2 of type Book */
/* book 1 specification */
strcpy( Book1.title, "C Programming");
strcpy( Book1.author, "Nuha Ali");
strcpy( Book1.subject, "C Programming Tutorial");
Book1.book_id = 6495407;
/* book 2 specification */
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Zara Ali");
strcpy( Book2.subject, "Telecom Billing Tutorial");
Book2.book_id = 6495700;
/* print Book1 info */
printBook( &Book1 );
/* Print Book2 info */
printBook( &Book2 );
return 0;
}
void printBook( struct Books* book ) {
printf( "Book title : %s\n", Book->title);
printf( "Book author : %s\n", Book->author);
printf( "Book subject : %s\n", Book->subject);
printf( "Book book_id : %d\n", Book->book_id);
}
When the above code is compiled and executed, it produces the following result -
Book 1 title : C Programming
Book 1 author : Nuha Ali
Book 1 subject : C Programming Tutorial
Book 1 book_id : 6495407
Book 2 title : Telecom Billing
Book 2 author : Zara Ali
Book 2 subject : Telecom Billing Tutorial
Book 2 book_id : 6495700
Keyword typedef
We use the typedef keyword to create an alias name for data types. It is commonly used with structures to
simplify the syntax of declaring variables.
For example, let us look at the following code:
struct Books {
char title[50];
char author[50];
char subject[100];
int book_id;
};
int main() {
struct Books d1, d2;
}
We can use typedef to write an equivalent code with a simplified syntax:
typedef struct Books {
char title[50];
char author[50];
char subject[100];
int book_id;
};
int main() {
Books d1, d2;
}
Why structs in C?
Suppose, you want to store information about a person: his/her name, citizenship number, and salary. You can create different variables name, citNo and salary to store this information.
What if you need to store information of more than one person? Now, you need to create different variables for each information per person: name1, citNo1, salary1, name2, citNo2, salary2, etc.
A better approach would be to have a collection of all related information under a single name Person structure and use it for every person.
C Union
A union is a special data type available in C that allows to store 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-purpose
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 tag] {
member definition;
member definition;
...
member definition;
} [one or more union variables];
The union tag is optional and each member definition is a normal variable definition, such as int i; or float f;
or any other valid variable definition. At the end of the union's definition, before the final semicolon, you can
specify one or more union variables but it is optional. Here is the way you would define a union type named Data having
three members i, f, and str -
union Data {
int i;
float f;
char str[20];
} data;
Now, a variable of Data type can store an integer, a floating-point number, or a string of characters.
It means a single variable, i.e., same memory location, can be used to store multiple types of data.
You can use any built-in or user defined data types inside a union based on your requirement.
The memory occupied by a union will be large enough to hold the largest member of the union. For example,
in the above example, Data type will occupy 20 bytes of memory space because this is the maximum space
which can be occupied by a character string. The following example displays the total memory size occupied by the above union -
#include <stdio.h>
#include <string.h>
union Data {
int i;
float f;
char str[20];
};
int main( ) {
union Data data;
printf( "Memory size occupied by data : %d\n", sizeof(data));
return 0;
}
When the above code is compiled and executed, it produces the following result -
Memory size occupied by data : 20
Accessing Union Members
To access any member of a union, we use the member access operator (.). The member access operator
is coded as a period between the union variable name and the union member that we wish to access.
You would use the keyword union to define variables of union type. The following example shows how to use unions in a program -
#include <stdio.h>
#include <string.h>
union Data {
int i;
float f;
char str[20];
};
int main( ) {
union Data data;
data.i = 10;
data.f = 220.5;
strcpy( data.str, "C Programming");
printf( "data.i : %d\n", data.i);
printf( "data.f : %f\n", data.f);
printf( "data.str : %s\n", data.str);
return 0;
}
When the above code is compiled and executed, it produces the following result -
data.i : 1917853763
data.f : 4122360580327794860452759994368.000000
data.str : C Programming
Here, we can see that the values of i and f members of union got corrupted because the final value
assigned to the variable has occupied the memory location and this is the reason that the value of
str member is getting printed very well.
Now let's look into the same example once again where we will use one variable at a time which is the
main purpose of having unions -
#include <stdio.h>>
#include <string.h>
union Data {
int i;
float f;
char str[20];
};
int main( ) {
union Data data;
data.i = 10;
printf( "data.i : %d\n", data.i);
data.f = 220.5;
printf( "data.f : %f\n", data.f);
strcpy( data.str, "C Programming");
printf( "data.str : %s\n", data.str);
return 0;
}
When the above code is compiled and executed, it produces the following result -
data.i : 10
data.f : 220.500000
data.str : C Programming
Here, all the members are getting printed very well because one member is being used at a time.
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