undefined | unit 5 structure and pointers

C PPS

UNIT 5STRUCTURE AND POINTERS Q1) What is a structure?A1)A structure can be considered as a template used for defining a collection of variables under a single name. Structures help programmers to group elements of different data types into a single logical unit (Unlike arrays which permit a programmer to group only elements of same data type). Q2)Why do we use structures?A2)• Ordinary variables can hold one piece of information• arrays can hold a number of pieces of information of the same data type.For example, suppose you want to store data about a book. You might want to store its name (a string), its price (a float) and number of pages in it (an int). If data about say 3 such books is to be stored, then we can follow two approaches:

Construct individual arrays, one for storing names, another for storing prices and still another for storing number of pages.

Use a structure variable.

Suppose we want to create a employee database. Then, we can define a structureCalled employee with three elements id, name and salary. The syntax of this structure is asfollows:

struct employee

{ int id;

char name[50];

float salary;

};

Q3) Explain structure variable declaration?A3)We can declare the variable of structure in two ways

Declare the structure inside main function

Declare the structure outside the main function.

1. Declare the structure inside main functionFollowing example show you, how structure variable is declared inside main function

struct employee

{

int id;

char name[50];

float salary;

};

int main()

{

struct employee e1, e2;

return 0;

}

In this example the variable of structure employee is created inside main function that e1 ,e2. 2. Declare the structure outside main functionFollowing example show you, how structure variable is declared outside the main function

struct employee

{

int id;

char name[50];

float salary;

}e1,e2;

Q4) How do we allocate memory for structures?A4)Memory is allocated to the structure only when we create the variable of structure.Consider following example

Q5) Explain structure initialization?A5)1. When we declare a structure, memory is not allocated for un-initialized variable.2. Let us discuss very familiar example of structure student , we can initialize structure variablein different ways –Way 1 : Declare and Initializestruct student{char name[20];int roll;float marks;}std1 = { "Poonam",89,78.3 };In the above code snippet, we have seen that structure is declared and as soon as after declaration wehave initialized the structure variable.std1 = { "Poonam",89,78.3 }This is the code for initializing structure variable in C programmingWay 2 : Declaring and Initializing Multiple Variablesstruct student

{

char name[20];

int roll;

float marks;

}

std1 = {"Poonam" ,67, 78.3};

std2 = {"Vishal",62, 71.3};

In this example, we have declared two structure variables in above code. After declaration of variable we have initialized two variable.std1 = {"Poonam" ,67, 78.3};std2 = {"Vishal",62, 71.3};Way 3 : Initializing Single memberstruct student

{

int mark1;

int mark2;

int mark3;

} sub1={67};

Though there are three members of structure,only one is initialized , Then remaining two membersare initialized with Zero. If there are variables of other data type then their initial values will be – Q6) Explain pointers?A6)When we declare any variable in C language, there are three things associated with that variable. 1. Data type of variable : Data type defines the type of data that variable can hold. Data type tells compiler about the amount of memory allocate to the variable. 2. Address of Variable : Address of variable represent the exact address of memory location which is allocated to variable. 3. Value of variable : It is the value of variable which is store at the address of memory location allocated to variable. Example :int n = 5;In the above example ‘int’ is the data type which tells compiler to allocate 2 bytes of memory to variable ‘n’.Once the variable is declare compiler allocated two bytes of memory to variable ‘n’. Suppose the address of that memory location is 1020. At the address of memory location 1020 the value 5 is store.Memory map for above declaration is as follows. use of &,/and ‘*’ operator Q7) Write a program to swap two variables using pointers?A7)

#include <stdio.h>

// function to swap the two numbers

void swap(int *x,int *y)

{

int t;

t = *x;

*x = *y;

*y = t;

}

int main()

{

int num1,num2;

printf("Enter value of num1: ");

scanf("%d",&num1);

printf("Enter value of num2: ");

scanf("%d",&num2);

//displaying numbers before swapping

printf("Before Swapping: num1 is: %d, num2 is: %d\n",num1,num2);

//calling the user defined function swap()

swap(&num1,&num2);

//displaying numbers after swapping

printf("After Swapping: num1 is: %d, num2 is: %d\n",num1,num2);

return 0;

}

Output:C Program to swap two numbers using pointers

Q8)Write a structure program to get the details of the student?A8)

	/* Created a structure here. The name of the structure is * StudentData. / struct StudentData{ char stu_name; int stu_id; int stu_age; }; int main() { /* student is the variable of structure StudentData/ struct StudentData student; /Assigning the values of each struct member here*/ student.stu_name = "Steve";
	student.stu_id = 1234; student.stu_age = 30; /* Displaying the values of struct members */ printf("Student Name is: %s", student.stu_name); printf("\nStudent Id is: %d", student.stu_id); printf("\nStudent Age is: %d", student.stu_age); return 0; }
Output: Student Name is: Steve Student Id is: 1234
Student Age is: 30

Q9) Write a note on preprocessor directives? A9) Preprocessors are programs that process our source code before compilation. There are a number of steps involved between writing a program and executing a program in C.All of these preprocessor directives begin with a ‘#’ (hash) symbol. The ‘#’ symbol indicates that, whatever statement starts with #, is going to the preprocessor program, and preprocessor program will execute this statement. Examples of some preprocessor directives are: #include, #define, #ifndef etc. Q10) Explain the types of preprocessor directives?A10)

Macros

File Inclusion

Conditional Compilation

Other directives

Let us now learn about each of these directives in detail.

Macros: Macros are a piece of code in a program which is given some name. Whenever this name is encountered by the compiler the compiler replaces the name with the actual piece of code. The ‘#define’ directive is used to define a macro.

File Inclusion: This type of preprocessor directive tells the compiler to include a file in the source code program. There are two types of files which can be included by the user in the program:

Header File or Standard files: These files contains definition of pre-defined functions like printf(), scanf() etc. These files must be included for working with these functions. Different function are declared in different header files. For example standard I/O functions are in ‘iostream’ file whereas functions which perform string operations are in ‘string’ file.
Syntax:

#include< file_name >
where file_name is the name of file to be included. The ‘<‘ and ‘>’ brackets tells the compiler to look for the file in standard directory.
user defined files: When a program becomes very large, it is good practice to divide it into smaller files and include whenever needed. These types of files are user defined files. These files can be included as:
#include"filename"

Conditional Compilation: Conditional Compilation directives are type of directives which helps to compile a specific portion of the program or to skip compilation of some specific part of the program based on some conditions. This can be done with the help of two preprocessing commands ‘ifdef‘ and ‘endif‘.

Other directives: Apart from the above directives there are two more directives which are not commonly used. These are:

#undef Directive: The #undef directive is used to undefine an existing macro. This directive works as:

#undef LIMIT

Using this statement will undefine the existing macro LIMIT. After this statement every “#ifdef LIMIT” statement will evaluate to false.

3. #pragma Directive: This directive is a special purpose directive and is used to turn on or off some features. This type of directives are compiler-specific, i.e., they vary from compiler to compiler. Some of the #pragma directives are discussed below:

#pragma startup and #pragma exit: These directives helps us to specify the functions that are needed to run before program startup( before the control passes to main()) and just before program exit (just before the control returns from main()).

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