Tutorial will explain different Storage Classes in C -

This tutorial will provide you brief knowledge regarding pointer in C.

This chapter will cover Function in C Programming -

This Chapter will cover Array in C Programming -

Chapter covers Loop Control Statements in C Programming Language -

This chapter is dedicated to Decision Making in C -

This Chapter will cover all the Data Types in C Programming Languages.

C Programming History : Multiple Choice Questions

How to add two numbers using function pointer ?

#include<stdio.h>
 int sum (int n1,int n2);
 int main()
 {
 int num1 = 10;
 int num2 = 20;
 int result;
 int *(*ptr)(int,int);
 ptr = &sum;
 result = (*ptr)(num1,num2);
 printf("Addition : %d",result);
 return(0);
 }
int sum (int n1,int n2)
{
return(n1 + n2);
}

Explanation :

• Write whole program using normal function call i.e sum(num1,num2);
• After you write whole code, just erase line in which we have called function and replace that line with following line -

Step 1 : Declaring Pointer Variable

int *(*ptr)(int,int);

means -

Declare pointer variable which is capable of storing
address of function which have integer as return type
and which takes 2 arguments.

Step 2 : Initializing function Pointer

ptr = ∑

• Store address of function in Function Pointer

Step 3 : Calling Function

result = (*ptr)(num1,num2);

It will call function sum(num1,num2);
above statement will be elaborated as -

result = (*ptr)(num1,num2);
result = (*&sum)(num1,num2);
result = (*&sum)(num1,num2);
result = (sum)(num1,num2);
result = sum(num1,num2);

In the previous chapter we have learnt about the Lvalue of an expression in this chapter we are going to learn R-value of an expression.

What is R-Value ?

1. R Value stands for Right value of the expression.
2. In any Assignment statement R-Value must be anything which is capable of returning Constant Expression or Constant Value.
3. R Value Can be anything of following -

Diagram Showing Lvalue :

Example of RValue :

#include<stdio.h>
int main()
{
int num;
num = 5;
return(0);
}

In the above example, Constant Value 5 is assigned to the variable will be considered as right Value of the variable.

Important Tips of Using RValue :

Tip 1 : R Value may be Constant or Constant expression

num = 20;      //Constant RValue
num = 20 + 20; //Constant Expression as RValue

1. In the example, we have assigned constant value to the variable.
2. Constant value “20” is considered as RValue.
3. In the next line we have assigned constant expression to the variable. “20 + 20” will be evaluated first and then result of expression will be assigned to an Variable.

Tip 2 : R Value may be MACRO

#define MAX 20
main() {
   int num = MAX;
}

If we have defined a MACRO then we can use MACRO as right value. In the above example we have assigned the 20 to the variable “num”.

In case if we defined the MACRO value of different data type then it may results into compile error.

Tip 3 : R Value may be Variable

#define MAX 20
main() 
{
   int flag = 0;
   int num  = flag;
}

For more information , Visit MSDN library.

What is LValue ?

1. Lvalue stands for left value and Expressions that refer to memory locations are called “l-value” expressions.
2. In any Assignment statement L-Value must be a container ( i.e. which have ability to hold the data)
3. Variable is the only container in C thus L Value must be any Variable.
4. L Value Cannot be Constant,Function or any of the available data type in C

Diagram Showing Lvalue :

Example of LValue :

#include<stdio.h>
int main()
{
int num;
num = 5;
return(0);
}

In the above expression, Constant value 5 is being assigned to a variable ‘num’. Variable ‘num’ is called as storage region’s , ‘num’ can considered as LValue of an expression.

Important Concepts about : LValue of an Expression

Tip 1 : Lvalue cannot be a Constant

int main()
{
int num;
5 = num;  //Error
return(0);
}

Tip 2 : Lvalue cannot be a Constant Variable

int main()
{
const num;
num = 20;  //Error
return(0);
}

Tip 3 : Lvalue cannot be a MACRO

#define MAX 20
int main()
{
MAX = 20;  //Error
return(0);
}

Tip 4 : Lvalue cannot be a Enum Constant

enum {JAN,FEB,MARCH};
int main()
{
JAN = 20;  //Error
return(0);
}

Tip 5 : Lvalue cannot be a Data Type

#define<stdio.h>
#define max 125
struct book{
  char *name;
  int  pages;
};
void main()
{
book = {"C Programming",100};
}

How to solve this error : L-Value Require error ?

Causes of Error :

1. Whenever we are trying to assign value to constant value
2. Whenever we are trying to increment or decrement Constant Expression
3. Inside if statement , if we try to write comparison operator as “= =” instead of ”==”, then we will get this error.

Solution :

Solution to this problem is very simple -

What is the difference between constant to pointer and pointer to constant?