The program below, shows how to define a structure, store information into it, and then retrieve information back.

#include <stdio.h>
#include <conio.h>

struct PERSON
{
unsigned char name[100];
unsigned char age;
unsigned char sex;
};

void main(void)
{
struct PERSON user;
printf("Please enter your name : ");
gets(user.name);
printf("\nPlease enter you age : ");
scanf("%d",&user.age);
printf("\nPlease select your sex ( m or f ) : ");
user.sex=getch();
clrscr();
printf("Your details are as follows : \n\n");
printf("Name : %s\n",user.name);
printf("Age : %d\n",user.age);
printf("Sex : %c\n",user.sex);
}

A persons name can be stored in an array of characters, in the example code, a person can't have a name consisting of more than 100 characters. A persons age can be stored in an unsigned char ( 0 - 255 range ). Finally a persons sex ( male or female ) can be stored as a single character ( m or f ). The structure template defines all of this.

The template is given a name which can consist of the same characters used with variable and function naming. The only reason it is in upper case is because of the style I prefer to use, with structure templates always being made up from capital letters, it doesn't have to be the case though.

After there is a template defined, the template can be used to create a variable from it. struct PERSON user; This is the same as declaring a variable called user, but instead of using one of the basic C data types, the PERSON structure template is used ( it has to have struct in front of it though ).

Now a variable has been defined called user to hold a persons details, it can be used. The rest of the program shows how by using the dot notation. So for example, to store the age 73, you would used a line user.age=73;

The program above first, gets the information from user input, and then displays the information.

There is another way to use structures, one which I personally prefer, because it doesn't require the struct keyword to be used when defining a variable for the template.

#include <stdio.h>
#include <conio.h>

typedef struct PERSON
{
unsigned char name[100];
unsigned char age;
unsigned char sex;
}PERSON;

void main(void)
{
PERSON user;
printf("Please enter your name : ");
gets(user.name);
printf("\nPlease enter you age : ");
scanf("%d",&user.age);
printf("\nPlease select your sex ( m or f ) : ");
user.sex=getch();
clrscr();
printf("Your details are as follows : \n\n");
printf("Name : %s\n",user.name);
printf("Age : %d\n",user.age);
printf("Sex : %c\n",user.sex);
}

Both methods work in the same way, but with this latter method you can declare variables PERSON user; in the same way as declaring other variables. The typedef keyword can be placed at the start of the structure definiton, to effectively create a data type. Also the useable data type name has to be placed at the end of the structure template, it doesn't have to be the same as the top name though. You could for example have :

typedef struct PERSONTEMPLATE
{
unsigned char name[100];
unsigned char age;
unsigned char sex;
}PERSON;

The part that is used as the data type name reference is the name given at the end of the structure template though ( PERSON ). There is a reason for two names having to be given, which will be covered when a page on dynamic memory allocation is done.

The examples above are okay if you just want to store one person, but it is easy to extend this by using an array to store more. The example below is the same as the previous examples, but this time can store 100 people. Only the first persons details are entered and displayed though in the code ( index 0 )

#include <stdio.h>
#include <conio.h>

typedef struct PERSON
{
unsigned char name[100];
unsigned char age;
unsigned char sex;
}PERSON;

void main(void)
{
PERSON user[100];
printf("Please enter your name : ");
gets(user[0].name);
printf("\nPlease enter you age : ");
scanf("%d",&user[0].age);
printf("\nPlease select your sex ( m or f ) : ");
user[0].sex=getch();
clrscr();
printf("Your details are as follows : \n\n");
printf("Name : %s\n",user[0].name);
printf("Age : %d\n",user[0].age);
printf("Sex : %c\n",user[0].sex);
}

You can even have structures in structures to create even more complex data types :

typedef struct SALARY
{
float annualearnings;
float taxrate;
float nirate;
}SALARY;

typedef struct PERSON
{
unsigned char name[100];
unsigned char age;
unsigned char sex;
SALARY sal;
}PERSON;

The person structure above is extended to include basic salary details as well. Using the example where an array of 100 people were created called user, the following line could be used to set the annualearnings to £20,000 for the 10th person in the array ( index 9 ).

user[9].sal.annualearnings=20000;

All structures that are referenced have to be defined higher in the code than where they are used, which is why the SALARY structure is defined first, because the PERSON structure references it.

Structures when used with arrays let a program easily manipulate large amounts of data held in a single variable, which is where they are the most useful. Combined with a knowledge of file handling ( next ) and finally pointers structures become a crucial element for most programs.