CP : Structures

Why Use Structures?

We have seen earlier how ordinary variables can hold one piece of information and how arrays can hold a number of pieces of information of the same data type. These two data types can handle a great variety of situations. But quite often we deal with entities that are a collection of dissimilar data types.

For example, suppose you want to store data about a book. You might want to store its name (a string), its price (a float) and a number of pages in it (an int). If data about say 3 such books is to

be stored, then we can follow two approaches:

(a) Construct individual arrays, one for storing names, another for storing prices and still another for storing a number of pages.

(b) Use a structure variable.

Let us examine these two approaches one by one. For the sake of programming, convenience assumes that the names of books would be single character long. Let us begin with a program that uses arrays.

{

char name[3] ;

float price[3] ;

int pages[3], i ;

printf ( "\nEnter names, prices and no. of pages of 3 books\n" ) ;

for ( i = 0 ; i <= 2 ; i++ )

scanf ( "%c %f %d", &name[i], &price[i], &pages[i] );

printf ( "\nAnd this is what you entered\n" ) ;

for ( i = 0 ; i <= 2 ; i++ )

printf ( "%c %f %d\n", name[i], price[i], pages[i] );

}

And here is the sample run...
Enter names, prices and no. of pages of 3 books
A 100.00 354
C 256.50 682
F 233.70 512
And this is what you entered
A 100.000000 354
C 256.500000 682
F 233.700000 512

A structure contains a number of data types grouped together. These data types may or may not be of the same type. The following example illustrates the use of this data type.

main( )
{
struct book
{
char name ;
float price ;
int pages ;
} ;
struct book b1, b2, b3 ;
printf ( "\nEnter names, prices & no. of pages of 3 books\n" ) ;
scanf ( "%c %f %d", &b1.name, &b1.price, &b1.pages ) ;
scanf ( "%c %f %d", &b2.name, &b2.price, &b2.pages ) ;
scanf ( "%c %f %d", &b3.name, &b3.price, &b3.pages ) ;
printf ( "\nAnd this is what you entered" ) ;
printf ( "\n%c %f %d", b1.name, b1.price, b1.pages ) ;
printf ( "\n%c %f %d", b2.name, b2.price, b2.pages ) ;
printf ( "\n%c %f %d", b3.name, b3.price, b3.pages ) ;
}


And here is the output...
Enter names, prices and no. of pages of 3 books
A 100.00 354
C 256.50 682
F 233.70 512

And this is what you entered
A 100.000000 354
C 256.500000 682
F 233.700000 512

Declaring a Structure

struct <structure name>
{
structure element 1 ;
structure element 2 ;
structure element 3 ;
......

......
} ;

Once the new structure data type has been defined one or more variables can be declared to be of that type. For example, the variables b1, b2, b3 can be declared to be of the type struct book, as, struct book b1, b2, b3 ;

Note the following points while declaring a structure type:
(a) The closing brace in the structure type declaration must be followed by a semicolon.
(b) It is important to understand that a structure type declaration does not tell the compiler to reserve any space in memory. All a structure declaration does is, it defines the ‘form’ of the structure.
(c) Usually, the structure type declaration appears at the top of the source code file, before any variables or functions are defined. In very large programs they are usually put in a separate header file, and the file is included (using the preprocessor directive #include) in whichever program we want to use this structure type.