Structs

Structures

• We already know that arrays are many variables of the same type grouped together under the same name.

• Structures are like arrays except that they allow many variables of different types grouped together under the same name.

• For example you can create a structure called person which is made up of a string for the name and an integer for the age.

Person struct

• Here is how you would create that person structure in C:

• struct person{   char name[30];

   int age;};

Creating a variable of struct type

• The above is just a declaration of a type.

• You must still create a variable of that type to be able to use it.

• Here is how you create a variable called p of the type person:

Program

• #include<stdio.h> struct person{  char name[30];   int age;}; main(){   struct person p;}

Accessing the elements of a structure

• To access the string or integer of the structure you must use a dot between the structure name and the variable name.

• #include<stdio.h> struct person{ char name[30];   int age;}; main(){   struct person p;   p.name = "John Smith";   p.age = 25;   printf("%s",p.name);   printf("%d",p.age);

}

Struct format

• The format for defining a structure is

• struct Tag { Members };

Where Tag is the name of the entire type of structure and Members are the variables within the struct.

e.g. struct example {

int x; };

example is the Tag, and x is a member.

• To actually create a single structure the syntax is

• struct Tag name_of_single_structure;

• struct example an_example;

• an_example is an instance of struct example

• To access a variable of the structure it goes

• name_of_single_structure.name_of_variable;• E.g.

an_example.x = 33;

Putting it all together

//declare the structstruct example {

int x; }; Create variables of type struct examplestruct example an_example; //Treating it like a normal variable type//// Access its members an_example.x = 33;

Another example

• Here is an example program:

struct database { int id_number; int age; float salary; }; main() { database employee; //There is now an employee variable that has modifiable

variables inside it. employee.age = 22; employee.id_number = 1; employee.salary = 12000.21; }

Employee Record

ID_number Age Salary

1 22 12000.10

• Type definitions• You can give your own name to a variable using a type definition. Here is an example of how to create a type definition called intptr for a

pointer to an integer.• #include<stdio.h>

 typedef int *intptr; int main(){   intptr ip;   return 0;}

• Type definitions for a structure• If you don't like to use the word struct when declaring a structure variable then you can create a type definition for the structure. The name

of the type definition of a structure is usually all in uppercase letters.• #include<stdio.h>

 typedef struct person{    char name[30];   int age;} PERSON; int main(){   PERSON p;   p.name = "John Smith";   p.age = 25;   printf("%s",p.name);   printf("%d",p.age);   return 0;}

Exercise

• Create a person struct with fields

• Name, Address, Student no. Grade.

• Create 2 variables of type person

• Assign values to each of their fields.

• Print out who has the highest grade.

#include <stdio.h>

struct person

{char name[40];

char address[80];

char student_no[10];

int grade;

}

main()

{

person p1,p2;

p1.name = “fred”

p1.address = “kevin st”

p1.student_no = “c102445”

p1.grade = 72;

p2.name = “mary”p2.address = “bolton st”p2.student_no = “b10775”p2.grade = 87;if (p1.grade > p2.grade){printf(“fred’s mark %d is greater than mary’s %d”,

p1.grade,p2.grade);}else{printf(“mary’s mark %d is greater than fred’s %d”,

p2.grade,p1.grade);}}

Exercise

• Create a struct which represents a book.

struct book

struct book { chartitle; char author;printf("enter for book %d",i +1); gets(collection[i]. publisher; char isbn;int year;char genre;char illustrator;}

Can have an array of these structs

e.g. catalog is an array of 100 books

struct book catalog[100];

Exercise 2

• Using book struct create a list of 5 books and read in their details

#include <stdio.h>

#include <string.h>

struct book {

char title[40];

char author[40];

char publisher[40];

char isbn[40];

int year;

char genre[40];

char illustrator[40];

}

main()

{struct book collection[5];

int i;

for(i = 0; i < 5,i++)

{printf("enter title for book %d",i +1);

gets(collection[i].title);

printf("enter author for book %d",i +1);

gets(collection[i].author);

printf("enter publisher for book %d",i +1);

gets(collection[i]. publisher);

printf("enter year for book %d",i +1);

scanf(“%d”, collection[i].year);

printf("enter isbn for book %d",i +1);

gets(collection[i]. isbn);

printf("enter genre for book %d",i +1);

gets(collection[i]. genre;

printf("enter illustrator for book %d",i +1);

gets(collection[i]. illustrator);

}

}

Note

• We can also define pointers to structs

• struct book *book1;