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What is Structure and why do we need them in C

 In one of our previous post we dealt with Array.Here in this post we'll be dealing with structure which  I prefer to call 'The advanced version of arrays'.

Structure

Similar to an array, A structure is a collection, however of a dissimilar data types.

Take an example of an office, Where the Manager wants to keep  the records of his/her employees.The record may include employees' name, address, department, pay scale , age and so on.So, why not create a variable with all those features in it. So when we want an entity with multiple features in it we can go with structures.

First Glance of Structure

struct student
{
    int rollNo,intakeYear;
    char name[20],address[20];
};

Above is a structure student with some of the features of the student like:Roll number, intake year, their name and address.

The keyword struct is used to define a structure. Structure definition always ends with a semicolon.

The general form of Structure definition is

struct <structure Name>
{
    structure elements...
};

Since we have defined a structure,we can have a variable of student type. Let's See How!!

Variable Declaration for Structure

For the above structure student we can declare a variable of its type as:


struct student s1;


After the above line of code, the compiler allocates 20 bytes of memory for name , another 20 bytes for address, 4 bytes for roll number and another 4 bytes for intake year.So a total of 48 bytes of memory is allocated for the variable s1 .

However the following declaration of variable is also valid.
struct student
{
    int rollNo,intakeYear;
    char name[20],address[20];
}s1;

We can have as many variables we want.

Initialization of Structure Variable

Structure variable can be initialized when they are declared as:

struct student
{
    int rollNo,intakeYear;
    char name[20],address[20];
};

struct student s1 = {1,2020,"Raju","Kathmandu"};
struct student s2 = {0};


The second variable declaration initializes all its member to zero.

Accessing Structure Elements

We use dot operator(.) to access the structure element.


s1.rollNo;


This refers to the rollNo of the student.

It should be noted that before the dot operator(.) there should always be a structure variable and after the operator there must always be a structure element.

Array of Structure

What if we want to store the data of 100 students?we would require to use 100 different structure variable,which won't be a convenient way. A better approach will be to use an array of  structure.

struct student
{
    int rollNo,intakeYear;
    char name[20],address[20];
}s1[100];

Above line of code declare a variable s1 for 100 student.This provides space in memory for 100 structures of the type struct student.

It should be appreciated what careful thoughts Dennis Ritchie has put into C language.He first defined array as a collection of similar elements then realized dissimilar data types that are often found in real life which cannot be handled using arrays ,so created a new data type called structure.But even using structures programming convenience could not be achieved,because a lot of variables were needed to be handled.Therefore he allowed us to create array of structures:an array of similar data types which themselves are a collection of dissimilar data types.Hats off to the Genius.
------------------Yashavant Kanetkar in his book (Let us C)

Since we have learnt some of the basics of structure,why not implement all of them in a program.

Write a Program to input name,faculty and GPA of 5 students and display the information of those student with faculty Engineering.

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

//structure declration
struct student
{
    char name[20],faculty[20];
    float GPA;
};

int main()
{
struct student s[5]
for(int i = 0 ; i < 5 ; i++)
  {
   printf("Enter name\n");
   scanf(" %[^\n]",s[i].name);

   printf("Enter faculty\n");
   scanf(" %[^\n]",s[i].faculty);

    printf("Enter GPA\n");
    scanf("%f",&s[i].GPA);

    while(s[i].GPA > 4.0)
    {
     printf("GPA cannot be greater than 4.0\n");
     printf("Enter GPA\n");
     scanf("%f",&s[i].GPA);
    }
  }
 for(int i = 0 ; i < 5 ;i++)
  {
   if((strcmp(s[i].faculty,"Engineering")==0))
   printf("Name:%s\n",s1[i].name);
printf("Faculty:%s\n",s1[i].faculty);
printf("GPA:%.2f\n",s1[i].GPA);
   }
return 0;
}

Here we have a structure student
struct student
{
    char name[20],faculty[20];
    float GPA;
};

with structure elements name,faculty and GPA.


struct student s[5]

Here we declare an array of structure (for 5 students). We use  a simple for loop to input the data to our structure variable.


s[i].name

Here we use a dot operator (.) to access the structure element.

Check your Understanding

1.Create a structure book,with structure elements :Book Name , Authors name ,total page , Price .And create array of structure for storing the data for 300 Books and display the information of book with price higher than 1000.
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