1. User Defined Data Types
Objectives: In this chapter you will learn about,
Introduction
Declaring Structure Type & Structure Variables
Referring and initializing structure elements
Passing structures to a function
Using typedef
Example using structure
Enumeration constants
User Defined Data Types - updated

2. Introduction
So far we have only used data types which have been defined by C such as int, double and char.
It is also possible to create our own data types.
A user defined data type is called a structure.
A structure can contain both built-in data types and another structure.
The concept of structure is pretty much the same as arrays except that in an array, all the data is of the same types but in a structure, the data can be of different types.
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3. Definition
A structure is a derived data type that represents a collection of a related data items called components (or members) that are not necessarily of the same data type.
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4. Declaring Structure Type
General syntax:
struct structure_name {
data_type element1;
data_type element2;
. . . };
Example:
struct student {
char name[20];
int studentID;
char major[50];
Also called as structure tag
Components / members
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5. Declaring Structure Variables
After declaring a structure type, we may declare variables that are of that type. A structure variable declaration requires:
The keyword struct
The structure type name
A list of members (variable names) separated by commas
A concluding semicolon
Then, assume that variable of structure type student is my_student. So the declaration should be written as;
struct student my_student;
User Defined Data Types - updated

6. Based on example: struct student
By including this declaration in our program, we are informing the compiler about a new data type which is a user defined data type.
The declaration just makes the compiler aware the existent of new data type but does not take an action yet.
Based on the declaration of
struct student my_student;
causes the compiler to reserve memory space for variable my_student and the components of its structure.
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7. Structure variable
Components
Values
name
major
studentID
Simon
0078 CS
my_student
Conceptual memory structure variable my_student of type student (assuming that the components of variable my_student have already been assigned values)
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8. It is possible to combine the declarations of a structure type and a structure variable by including the name of the variable at the end of the structure type declaration.
struct student {
char name[20];
int studentID;
char major[50]; };
struct student my_student;
struct student {
char name[20];
int studentID;
char major[50];
} my_student; =
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9. Declaring Nested Structure
Members of a structure declaration can be of any type, including another structure variable.
Suppose we have the following structure declaration, which is a member of struct type student:
struct address {
int no;
char street[20];
int zipcode; };
User Defined Data Types - updated

10. We can rewrite the structure student declaration as follow:
This structure type student can be written as;
struct student {
char name[20];
int studentID;
char major[50];
struct address addr;
} ;
User Defined Data Types - updated

11. struct student {
char name[20];
int studentID;
char major[50];
struct address{
int no;
char street[20];
int zipcode; };
} ;
User Defined Data Types - updated

12. Referring and Initializing Structure Elements
A structure contains many elements. Each elements of a structure can be referred to / accessed by using the component selection operator “.” (dot).
Let us use the structure student which we have seen before as an example:
Therefore to refer to the element of a structure, we may write as follow;
my_student.name;
my_student.studentID;
my_student.major;
struct student {
char name[20];
int studentID;
char major[50]; };
struct student my_student;
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13. struct student my_student = {“Ahmad”, 10179, “IT”}
Therefore, we can initialize each elements of a structure individually such as:
struct student my_student;
my_student.studentID = 10179;
Or we can initialize the structure while we are creating an instance of the structure:
struct student my_student = {“Ahmad”, 10179, “IT”}
Notice that it is possible to use the ‘=’ operator on a struct variable. When the ‘=’ sign is used, each elements of the structure at the right hand side is copied into the structure at the left hand side.
User Defined Data Types - updated

14. Example struct birthdate { int month; int day; int year; };
struct birthdate Picasso = {10, 25, 1881};
printf(“Picasso was born : %d/%d/%d\n”, Picasso.day, Picasso.month, Picasso.year);
Output :
Picasso was born : 25/10/1881
User Defined Data Types - updated

15. Passing Structures to a Function
Call by Value:
We can pass the student structure that we have created before to a function called display( ) as follows:
void display (struct student); /* function prototype */
display (student1); /* function call */
void display (struct student s1); /* function header */
where student1 is a variable of type struct student.
In the above function, a copy of the student structure will be created locally for the use of the function. Any changes to the structure inside the function will not affect the actual structure.
User Defined Data Types - updated

16. Example Using Structure: Call by value
#include <stdio.h>
#include <string.h>
struct student{
char name[20];
int id; };
void display(struct student); /* function prototype */
void main(void) {
struct student student1;
strcpy(student1.name, "Ahmad"); /*initialising variable */
student1.id = 12345; /*initialising variable */
display(student1); }
void display(struct student s1) /* make a local copy of the structure */
printf("Name: %s\n", s1.name);
printf("ID: %d\n", s1.id);
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17. Example Using Structure: A Function that return a structure
#include <stdio.h>
#include <string.h>
struct student{
char name[20];
int id; };
struct student read(void); /* function prototype */
void main(void) {
struct student student1;
student1 = read(); /*function call */
printf("Name: %s", student1.name);
printf("\nID: %d\n", student1.id); }
struct student read(void)
struct student s1;
printf("Enter name:");
scanf("%s",s1.name); /* alternative: gets(s1.name); */
printf("Enter ID:");
scanf("%d",&s1.id);
return s1;
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18. where student1 is a variable of type struct student.
Call by reference
It is also possible to use pointers and pass the reference of the structure to the function. This way, any changes inside the function will change the actual structure as well.
To pass a structure variable as a reference, the Read( ) function can be written this way:
void Read(struct student *); /* function prototype */
Read(&student1); /* function call */
void Read(struct student *s1); /* function header */
where student1 is a variable of type struct student.
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19. void Read(struct student *s1)
Take note that when a structure is declared as a pointer, the elements in the structure cannot be referred to using the ‘.’ operator anymore. Instead, they need to be accessed using the ‘->’ operator (indirect component selection operator).
For example:
void Read(struct student *s1) {
s1->studentID = 10179;
scanf(“%s”, s1->name); }
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20. Example Using Structure: Call by reference
#include <stdio.h>
#include <string.h>
struct student{
char name[20];
int id; };
void Read (struct student *); /* function prototype*/
void main(void) {
struct student student1;
Read(&student1); /* function call: passing reference */
printf("Name: %s", student1.name);
printf("\nID: %d\n", student1.id); }
void Read (struct student *s1) /* function header, receive structure as a pointer variable */
printf("Enter name:");
scanf("%s",s1->name); /* you can also use: gets(s1->name) */
printf("Enter ID:");
scanf("%d",&s1->id);
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21. Using typedef in Structure Declarations
The keyword typedef provides a mechanism for creating synonyms (aliases) for previously defined data types.
Here is an example on how to use typedef when declaring a structure:
struct student {
char name[20];
int studentID;
char major[50];
struct address addr;
} ;
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22. typedef struct student StudentData;
By using typedef:
typedef struct student StudentData;
we are now aliasing the structure with a name to be used throughout the program. So instead of writing the word “struct” before declaring a struct variable like the following
struct student my_student;
we can now write:
StudentData my_student;
We could use the alias name when passing the structure to a function:
void display(StudentData s1);
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23. Example : using typedef
#include <stdio.h>
#include <string.h>
struct student{
char name[20];
int id; };
typedef struct student StudentData;
void display(StudentData); /* function prototype */
void main(void) {
StudentData student1;
strcpy(student1.name, "Ahmad");
student1.id = 12345;
display(student1); }
void display(StudentData s1)
printf("Name: %s\n", s1.name);
printf("ID: %d\n", s1.id);
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24. Example: Array of structure
#include <stdio.h>
#define NUM_STUDENTS 10
struct student {
int studentID;
char name[20];
int score;
char grade; };
typedef struct student StudentData;
void Read (StudentData student[]);
void CountGrade (StudentData student[]);
void main ( ) {
StudentData student[NUM_STUDENTS];
Read(student);
CountGrade(student); }
User Defined Data Types - updated

25. User Defined Data Types - updated NI@2006
void Read (StudentData student[]) {
int i;
for (i = 0; i < NUM_STUDENTS; i++) {
printf("Enter the studentID: ");
scanf("%d", &student[i].studentID);
printf("Enter the name: ");
scanf("%s", student[i].name);
printf("Enter the score: ");
scanf("%d", &student[i].score);
printf("\n"); }
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26. User Defined Data Types - updated NI@2006
void CountGrade (StudentData student[]) {
int i;
for (i = 0; i < NUM_STUDENTS; i++) {
if (student[i].score > 90)
student[i].grade = 'A';
else if (student[i].score > 80)
student[i].grade = 'B';
else if (student[i].score > 65)
student[i].grade = 'C';
else if (student[i].score > 50)
student[i].grade = 'D';
else
student[i].grade = 'F';
printf("The grade for %s is %c\n", student[i].name, student[i].grade);
printf("\n"); }
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27. User Defined Data Types - updated NI@2006
/* Sample Output
Enter the studentID:
Enter the name: Salman
Enter the score: 96
Enter the studentID:
Enter the name: Jack
Enter the score: 79 :
The grade for Salman is A
The grade for Jack is C
Press any key to continue */
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28. Example #include <stdio.h> struct car{ char maker[20];
char model[20];
int year; };
void input(struct car*);
void output(char*, char*, int*);
void main() {
struct car firstcar;
input (&firstcar);
output(firstcar.maker, firstcar.model, &firstcar.year);
printf("End of my act!\n"); }
User Defined Data Types - updated

29. User Defined Data Types - updated NI@2006
void input(struct car *sp) {
printf("What is the maker of your car? ");
gets(sp->maker);
printf("What is the model of your car? ");
gets(sp->model);
printf("What year is your car? ");
scanf("%d", &sp->year); }
void output(char *sp1, char*sp2, int*sp3)
printf("Your car is : %s, %s, %d\n", sp1, sp2, *sp3);
printf("Nice car\n");
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30. /* Sample output
What is the maker of your car? Honda
What is the model of your car? Stream
What year is your car? 2003
Your car is : Honda, Stream, 2003
Nice car
End of my act!
Press any key to continue */
User Defined Data Types - updated

31. Enumeration Constants
An enumeration, introduced by the keyword enum, is a set of integer constants represented by identifiers. (to specify one after another)
Example:
enum islamic_months {
muharam, safar, rabiulawal, rabiulakhir,
jamadilawal, jamadilakhir, rejab, syaaban,
ramadhan, syawal, zulkaedah, zulhijjah };
Each of the identifiers actually has a value, starting with 0 (unless specified otherwise). Therefore, we can treat them as integers.
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32. enum islamic_months months;
If we want the enumeration to start with a value other than 0, we can assign the value to the first identifier:
enum islamic_months {
muharam = 1, safar, rabiulawal, rabiulakhir,
jamadilawal, jamadilakhir, rejab, syaaban,
ramadhan, syawal, zulkaedah, zulhijjah };
Same as with the other data types, before an enum can be used, a variable needs to be declared:
enum islamic_months months;
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33. enum islamic_months months;
There are cases where it is appropriate for us to use an enum. This is an example of such a case:
enum islamic_months months;
GetMonth (&months);
switch (months) {
case muharam:
. . .
break;
case safar: }
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34. This is another case where it is appropriate to use an enum:
enum Boolean {FALSE, TRUE};
void main ( ) {
int list[];
Boolean found;
Read(list);
found = Search(list);
if (found == TRUE)
printf(“FOUND!!”);
else
printf(“Cannot find the requested item”); }
User Defined Data Types - updated

35. User Defined Data Types - updated NI@2006
#include <stdio.h>
enum months {JAN = 1, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC};
void main ( ) {
enum months month;
char *monthsName[] = {“January”, “February”, “March”,
“April”, “May”, “June”, “July”, “August”, “September”,
“October”, “November”, “December”};
for (month = JAN; month <= DEC; month++)
printf(“%d %s”, month, monthName[month-1]); }
User Defined Data Types - updated

36. User Defined Data Types - updated NI@2006
Output:
1 January
2 February
3 March
April
May
June
July
August
September
October
November
December
User Defined Data Types - updated

37. SUMMARY In this chapter you have learnt about T.H.E E.N.D
Structure type and variable declarations
How structure members can be accessed
How structure can be initialized
Passing structure to function
Enumeration
T.H.E E.N.D
User Defined Data Types - updated