Introduction to Pointers

Introduction to Pointers Pointers are regarded as one of the most difficult topics in C. Although they may appear as little confusing for a beginner, they are most powerful tools and are used frequently with aggregate types, such as arrays and structures. In fact, real power of C lies in the proper use of pointers. Here, we consider only the introductory study of pointers and we shall discuss them in detail in unit-III. Since addresses of variables are not guaranteed to be represented in the same fashion as integers, therefore to store addresses, we need a special type of variable, called a pointer variable. So a pointer variable is another variable that holds the address of the given variable to be accessed. Thus pointers provide a way of accessing a variable without referring to variable directly. To declare a pointer variable, we precede the variable name with an asterisk (∗). The declaration has the form type

∗pt_name;

Where ‘type’ is the pre-defined or user-defined data type and indicates that the pointer will point to the variables of that specific data type.

For example, the statement

int ∗ x;  

declares the variable x as a pointer variable that points to an integer data type. It should be noted that the type ‘int’ refers to the data type of the variable being pointed to by x and not the type of the value of the pointer.

Similarly, the statement

float ∗ y; 

declares y as a pointer to a floating-point variable. 40 Programming in C 8.1.

 Initializing a Pointer : Once a pointer variable has been declared, it can be made to point to a variable by using an assignment statement as x = & pay; which causes x to point to ‘pay’ i.e. x none contains the address of pay. This is known as pointer initialization. A pointer should be initialized before use. We should also ensure that pointer variables always point to the corresponding type of data.

For example,

float a, b;

 int x, ∗y;

 y = & a;

b = ∗ y;

will give wrong results. Further, assigning an absolute address to a pointer variable is prohibited.

Thus

 int ∗ x;

 x = 237; 

is invalid. A pointer variable can be initialized in its declaration itself.

For example

int x, ∗p = & x;

 is valid. It declares x as an integer variable and p as a pointer variable and then initializes p to the address of x. Note carefully that this is an initialization of p, not of ∗p. Further, the target variable x is declared first. 

Thus, the statement 

int ∗p = &x, 

x; is not valid.