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C - typedef

The C programming language provides a keyword called typedef, which you can use to give a type, a new name. Following is an example to define a term BYTE for one-byte numbers −
typedef unsigned char BYTE;
After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example..
BYTE  b1, b2;
By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows −
typedef unsigned char byte;
You can use typedef to give a name to your user defined data types as well. For example, you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows −
 
#include 
#include

typedef struct Books {
char title[50];
char author[50];
char subject[100];
int book_id;
} Book;

int main( ) {

Book book;

strcpy
( book.title, "C Programming");
strcpy
( book.author, "Nuha Ali");
strcpy
( book.subject, "C Programming Tutorial");
book
.book_id = 6495407;

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);

return 0;
}
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

typedef vs #define

#define is a C-directive which is also used to define the aliases for various data types similar to typedef but with the following differences −
  • typedef is limited to giving symbolic names to types only where as #define can be used to define alias for values as well, q., you can define 1 as ONE etc.
  • typedef interpretation is performed by the compiler whereas #define statements are processed by the pre-processor.
The following example shows how to use #define in a program −
 
#include 

#define TRUE 1
#define FALSE 0

int main( ) {
printf
( "Value of TRUE : %d\n", TRUE);
printf
( "Value of FALSE : %d\n", FALSE);

return 0;
}
When the above code is compiled and executed, it produces the following result −
Value of TRUE : 1
Value of FALSE : 0

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