Difference between typedef and define in C

In this topic, we will discuss the difference between typedef and define in C programming language.

typedef:

A typedef is a keyword used in C programming to define a new name for exiting data types. But it cannot provide a new data type to the predefined data type. Instead, it provides a meaning full names to an already existing data type (int, char, float, etc.). It is defined outside the main() function in a program. In other words, a typedef is used to redefine the names of existing data types in C programming.

Syntax

typedef data_type newName
or typedef <existing_name> <alias_name>

In the above syntax, the existing _name defines the predefined data types or variable name. The 'alias _name' or newName defines the new name for exiting data type or variable name in the C program.

Example 1: Consider a program to use the typedef keyword in C.

Type.c

#include <stdio.h>
typedef int Length; // typedef provide the int data type to a new name as Length 
int main()
{
Length num1, num2, sum; // here Length variable is treated as the int data type
printf(" Enter the first number:");
scanf(" %d", & num1);
printf(" Enter the second number:");
scanf(" %d", & num2);
sum = num1 + num2;
printf(" The sum of the two numbers is: %d", sum);
return 0;
}

Output:

Enter the first number: 20
Enter the second number: 10
The sum of the two numbers is: 30

Example 2: Let's consider another program to use the typedef in unsigned char of C.

program.c

#include <stdio.h>
#include <conio.h>
typedef unsigned char Bit;
int main()
{
Bit B1, B2;
B1 = 'D';
B2 = 'E';
printf( " Demonstrate the use of typedef Keyword ");
printf (" Print a single Byte: %c", B1);
printf (" Print a single Byte: %c", B2);
return 0;
}

Output:

Demonstrate the use of typedef Keyword
Print a single Byte: D
Print a single Byte: E

Use of typedef keyword in Structure

Let's consider a program to use the typedef keyword in structure to provide a new name to the struct and initialize the structure variables, as follow:

Struct.c

#include <stdio.h>
#include <string.h>
 
typedef struct Students {
   int roll_no;
   char name[50];
   char Subject[60];
   char teacher_name[80];
} Stud;
 
int main( ) {

   Stud S1;
   S1.roll_no = 30;
   strcpy( S1.name, "Lockie Ferguson");
   strcpy( S1.Subject, "Mathematics"); 
   strcpy( S1.teacher_name, "Jasmine");
   
 
   printf( "Student Roll No: %d\n", S1.roll_no);
   printf( "Student Name: %s\n", S1.name);
   printf( "Student Subject: %s\n", S1.Subject);
   printf( "Student Teacher Name: %s\n", S1.teacher_name);

   return 0;
}

Output:

Student Roll No: 30
Student name: Lockie Fergusion
Student Subject: Mathematics
Student Teacher Name: Jasmine

In the above program, we use the typedef keyword to define a new name for the struct data type is Students and initialize the variables using it.

define

A #define is the pre-processor used to represents the constant aliases for various data. It is used to define the constant variable for different data types in C. It is defined outside of the main program.

Syntax

In the above syntax, #define is a pre-processor that defines the token as a constant variable name, and the value represents the token value. After initializing the constant variable in the pre-processor, we can use the value using the variable in a program.

Example 1: Consider a program to use the #define pre-processor in C.

Define.c

#include <stdio.h>
#include <conio.h>
#define PI 3.14 // define the constant variable 
void main()
{
printf (" Display the PI Constant value using the #define is: %f", PI);
getch();
}

Output:

Display the PI Constant value using the #define is: 3.14

Example 2: Consider a program to get the area of a circle in C using the #define pre-processor.

Area.c

# include <stdio.h>
#include <conio.h>
#define PI 3.14 // define the constant variable 
void main()
{
int r, area;
printf (" Enter the radius of the circle: ");
scanf ("%d", &r);
area = PI * r * r;
printf (" The area of the circle is: %d", area);
getch();
}

Output:

Enter the radius of the circle : 5
The area of the circle is: 78

In the above program, we use PI as a continuous variable using the #define pre-processor. And when the program is executed, the PI value automatically calls in a program.

Example 3: Consider a program to demonstrate the importance of typedef over the #define for data types.

Pgrm.c

#include <stdio.h>
#include <conio.h>
typedef char* ptr;
#define PTR char
int main()
{
ptr x, y, z;
PTR p, q, r;
printf (" sizeof x : %d\n", sizeof(x) );
printf (" sizeof y : %d\n", sizeof(y) );
printf (" sizeof z : %d\n", sizeof(z) );
printf (" sizeof p : %d\n", sizeof(p) );
printf (" sizeof q : %d\n", sizeof(q) );
printf (" sizeof r : %d\n", sizeof(r) );
return 0;
}

Output:

Sizeof x:8
Sizeof y:8
Sizeof z:8
Sizeof p:8
Sizeof q:1
Sizeof r:1

Difference between the typedef and the #define in C

Following are the differences of the typedef and the #define, as follows:

SN	Typedef	#define
1.	Typedef is a keyword in the C programming language.	#define is a pre-processor and used as macro used in C programming.
2.	It is a keyword used to provide an alternate name to the existing data types only. And that name can be used to initialize the variables in a program.	A #define is used to define an alias for values.
3.	The compiler performs it.	The pre-processor performs it.
4.	It uses a semicolon to terminate the statement.	It does not use a semicolon to terminate the statement.
5.	It defines the actual definition of a new data type.	A #define is used to just copy-paste the define values where it calls or uses.
6.	It follows the scope rule that defines if a new data type is in scope (inside a function), the new type name will only be visible till the scope of the data type.	When the #define pre-processor encounters in a program, it replaces all the occurrence of the variables with the defined values; after that, no scope is followed.
7.	Define outside of the main() function.	Define outside of the main() function.