Examples Using Pointers in C

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Pointer to a Structure in C

Last updated on July 27, 2020

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We have already learned that a pointer is a variable which points to the address of another variable of any data type like int, char, float etc. Similarly, we can have a pointer to structures, where a pointer variable can point to the address of a structure variable. Here is how we can declare a pointer to a structure variable.

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struct dog { char name [ 10 ]; char breed [ 10 ]; int age ; char color [ 10 ]; }; struct dog spike ; // declaring a pointer to a structure of type struct dog struct dog * ptr_dog

This declares a pointer ptr_dog that can store the address of the variable of type struct dog. We can now assign the address of variable spike to ptr_dog using & operator.

Now ptr_dog points to the structure variable spike.

Accessing members using Pointer #

There are two ways of accessing members of structure using pointer:

1. Using indirection (*) operator and dot (.) operator.
2. Using arrow (->) operator or membership operator.

Let's start with the first one.

Using Indirection (*) Operator and Dot (.) Operator #

At this point ptr_dog points to the structure variable spike, so by dereferencing it we will get the contents of the spike. This means spike and *ptr_dog are functionally equivalent. To access a member of structure write *ptr_dog followed by a dot(.) operator, followed by the name of the member. For example:

(*ptr_dog).name - refers to the name of dog
(*ptr_dog).breed - refers to the breed of dog

and so on.

Parentheses around *ptr_dog are necessary because the precedence of dot(.) operator is greater than that of indirection (*) operator.

Using arrow operator (->) #

The above method of accessing members of the structure using pointers is slightly confusing and less readable, that's why C provides another way to access members using the arrow (->) operator. To access members using arrow (->) operator write pointer variable followed by -> operator, followed by name of the member.

ptr_dog -> name // refers to the name of dog ptr_dog -> breed // refers to the breed of dog

and so on.

Here we don't need parentheses, asterisk (*) and dot (.) operator. This method is much more readable and intuitive.

We can also modify the value of members using pointer notation.

                        strcpy            (            ptr_dog            ->            name            ,            "new_name"            );          

Here we know that the name of the array (ptr_dog->name) is a constant pointer and points to the 0th element of the array. So we can't assign a new string to it using assignment operator (=), that's why strcpy() function is used.

In the above expression precedence of arrow operator (->) is greater than that of prefix decrement operator (--), so first -> operator is applied in the expression then its value is decremented by 1.

The following program demonstrates how we can use a pointer to structure.

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#include <stdio.h> struct dog { char name [ 10 ]; char breed [ 10 ]; int age ; char color [ 10 ]; }; int main () { struct dog my_dog = { "tyke" , "Bulldog" , 5 , "white" }; struct dog * ptr_dog ; ptr_dog = & my_dog ; printf ( "Dog's name: %s \n " , ptr_dog -> name ); printf ( "Dog's breed: %s \n " , ptr_dog -> breed ); printf ( "Dog's age: %d \n " , ptr_dog -> age ); printf ( "Dog's color: %s \n " , ptr_dog -> color ); // changing the name of dog from tyke to jack strcpy ( ptr_dog -> name , "jack" ); // increasing age of dog by 1 year ptr_dog -> age ++ ; printf ( "Dog's new name is: %s \n " , ptr_dog -> name ); printf ( "Dog's age is: %d \n " , ptr_dog -> age ); // signal to operating system program ran fine return 0 ; }

Expected Output:

Dog's name: tyke Dog's breed: Bulldog Dog's age: 5 Dog's color: white After changes Dog's new name is: jack Dog's age is: 6

How it works:

In lines 3-9, we have declared a structure of type dog which has four members namely name, breed, age and color.

In line 13, a variable called my_dog of type struct dog is declared and initialized.

In line 14, a pointer variable ptr_dog of type struct dog is declared.

In line 15, the address of my_dog is assigned to ptr_dog using & operator.

In lines 17-20, the printf() statements prints the details of the dog.

In line 23, a new name is assigned to ptr_dog using the strcpy() function, because we can't assign a string value directly to ptr_dog->name using assignment operator.

In line 26, the value of ptr_dog->age is incremented by 1 using postfix increment operator. Recall that postfix ++ operator and -> have the same precedence and associates from left to right. But since postfix ++ is used in the expression first the value of ptr_dog->age is used in the expression then it's value is incremented by 1.

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Examples Using Pointers in C

Source: https://overiq.com/c-programming-101/pointer-to-a-structure-in-c/

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