Chapter: Programming and Data Structures - C Programming Advanced Features

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Structures

C arrays allow you to define type of variables that can hold several data items of the same kind but structure is another user defined data type available in C programming, which allows you to combine data items of different kinds.

STRUCTURES

 

C arrays allow you to define type of variables that can hold several data items of the same kind but structure is another user defined data type available in C programming, which allows you 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 for your program. The format of the struct statement is this:

 

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 struct keyword to define variables of structure type. Following is the example to explain usage of structure:

 

#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

 

You can pass a structure as a function argument in very similar way as you pass any other variable or pointer. You would access structure variables in the similar way as you have accessed in the above example:

 

#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 title : C Programming

Book author : Nuha Ali

 

Book subject : C Programming Tutorial

Book book_id : 6495407

 

Book title : Telecom Billing

Book author : Zara Ali

 

Book subject : Telecom Billing Tutorial

Book book_id : 6495700

 

Pointers to Structures

 

You can define pointers to structures in very similar way as you define pointer to any other variable as follows:

 

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 above example using structure pointer, hope this will be easy for you to understand the concept:

 

#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 by passing address of Book1 */ printBook( &Book1 );

 

/* print Book2 info by passing address of Book2 */

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 title : C Programming

Book author : Nuha Ali

Book subject : C Programming Tutorial

 

Book book_id : 6495407

Book title : Telecom Billing

 

Book author : Zara Ali

Book subject : Telecom Billing Tutorial

 

Book book_id : 6495700

 

Bit Fields

 

Bit Fields allow the packing of data in a structure. This is especially useful when memory or data storage is at a premium. Typical examples:

Packing several objects into a machine word. e.g. 1 bit flags can be compacted.

Reading external file formats -- non-standard file formats could be read in. E.g. 9 bit integers.

C allows us do this in a structure definition by putting :bit length after the variable. For example:

 

struct packed_struct { unsigned int f1:1; unsigned int f2:1; unsigned int f3:1; unsigned int f4:1; unsigned int type:4; unsigned int my_int:9;

 

} pack;

 

Here, the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4 bit type and a 9 bit my_int.

 

C automatically packs the above bit fields as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer. If this is not the case then some compilers may allow memory overlap for the fields whilst other would store the next field in the next word.

 


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