Structure in C language
Variable and Data Types
int x = 9;
• In programming we need something for holding data, and variable is the way
to do that.
• A data type in a programming language is a set of data with
predefined values. Examples of data types are: integer, float, string,…
• There are two kind of data types in programming languages:
• System-defined data types (also called Primitive data types)
• User-defined data types
System-defined data types
• The data types are defined by languages
• The number of bits are allocated to each data type is fully depend on
the language and compiler
• For example: in C language:
• int: 2 bytes (actual value depends on compiler)
• float: 4 bytes
•…
User-defined data types
• The user-defined data types are defined by the user himself.
• In C we can create structure. In C++/Java, we can create class.
• For example:
struct student{
char[20] name;
int id;
};
struct student x;
Data structure
• Data structure is a particular way of storing and organizing data in a
computer so that it can be used efficiently.
• General data structure: Array, Linked List, Queue, Stack,…
• Data structure is divided into two types:
• Linear data structure
• Non-linear data structure
Type Definition
• Syntax: typedef type Name ;
• Name becomes a name you can use for the type
• Examples: typedef struct {
typedef struct student Student; char[20] name;
Student x; /* x is an struct student */ int id;
Student* PtrToStudent } Student;
typedef char *STRING;
STRING sarray[10];
/* sarray is an array of char *’s, equivalent to declaring:
char *sarray[10]; */
Structured Variables
• Group of related values (but unlike array, where the values are not necessarily of
the same type)
• Each part of structure is a field, with an associated value
• The group is treated as a single unit (a single variable with parts)
VariableName
Field1 Field2 Field3 Field4
Structured Variable
• Declaration • Variable consists of parts
struct {
corresponding to the fields
Type1 FieldName1; • Memory set aside corresponding
Type2 FieldName2; to the total size of the parts
Type3 FieldName3; • Each part is an individual
/* as needed */ variable of the appropriate type
} VarName;
Structure Types
Tag declaration: Type definition:
struct TagName { typedef struct {
Type1 Field1; Type1 Field1;
Type2 Field2; Type2 Field2;
Type3 Field3; Type3 Field3;
/* any more */ /* any more */
}; } TypeName;
Variable declaration: Variable declaration:
struct TagName VarN; TypeName VarN;
Field Selection
• Dot (.) form to refer to field of • Example:
structured var
typedef struct {
• Syntax: VarName.FieldName int month, day, year;
} DATE;
• Each field treated as an
individual variable of that type void main() {
DATE d1;
d1.month = 12;
d1 d1.day = 2;
d1.year = 1970;
month day year
}
12 2 1970
Structure Initialization
• Can initialize structured variable by giving value for each field (in the
order fields are declared)
• Syntax:
STYPE Svar = { FVal1, FVal2, FVal3, … };
• Example:
typedef struct { d1
int month, day, year;
month day year
} DATE;
12 2 1970
DATE d1 = { 12, 2, 1970 };
Structure Assignment
• Can assign value of one structured var to another
• variables must be of same type (same name)
• values are copied one at a time from field to corresponding field
• Example:
d1
typedef struct {
int month, day, year; month day year
} DATE; 12 2 1970
DATE d1 = { 12, 2, 1970 };
DATE d2; d2
month day year
d2 = d1; /* Assignment */
12 2 1970
Structure Comparison
• Should not use == or != to compare structured variables
• compares byte by byte
• structured variable may include unused (garbage) bytes that are not equal
(even if the rest is equal)
• unused bytes are referred to as slack bytes
• to compare two structured variables, should compare each of the fields of the
structure one at a time
Slack Bytes
• Many compilers require vars to start on even numbered (or divisible by 4)
boundaries, unused bytes called slack bytes
• Example:
typedef struct { MyType v1;
char ch; MyType v2;
int num;
} MyType;
v1 v2
ch num ch num
2048
2049
2050
2051
2052
2053
2054
2056
2060
2061
2062
2063
2064
2065
2066
2067
Structure Example
#include <stdio.h>
printf(”Enter:\n”);
typedef struct { printf(" ID#: ");
int id; scanf("%d",&s1.id);
float gpa; printf(" GPA: ");
scanf("%f",&s1.gpa);
char class;
printf(" Class: ");
} Student;
scanf(" %c",&s1.class);
void main() {
Student s1; printf(”S#%d (%c) gpa =
%.3f\n",s1.id,s1.class,s1
s1 .gpa);
id gpa class }
Passing Structures as Parameters
• A field of a structure may be passed as a parameter (of the type of
that field)
• An advantage of structures is that the group of values may be passed
as a single structure parameter (rather than passing individual vars)
• Structures can be used as
• value parameter: fields copied (as in assignment stmt)
• reference parameter: address of structure passed
• return value (resulting structure used in statement) -- not all versions of C
allow structured return value
• best to use type-defined structures
Structure as Value Parameter
#include <stdio.h> void printS(Student s) {
/* Struc. Param. named s
typedef struct { created, fields of arg
int id; (s1) copied to s */
printf("S#%d (%c) gpa =
float gpa; %.3f\n",s.id,s.class,s.gpa);
char class; }
} Student; void main() {
Student s1;
s1
id gpa class s1 = readS();
printS(s1);
}
Structure as Return Value
typedef struct { Student readS() {
int id; Student s; /* local */
float gpa;
printf(”Enter:\n");
char class; printf(" ID#: ");
} Student; scanf("%d",&s.id);
void main() { printf(" GPA: ");
scanf("%f",&s.gpa);
Student s1;
printf(" Class: ");
scanf(" %c",&s.class);
s1 = readS();
printS(s1); s1 return s; /* local as
} id gpa class return val */
}
Structure as Reference Parameter
typedef struct { void readS(Student *s) {
int id; printf(”Enter:\n");
float gpa; printf(" ID#: ");
scanf("%d",&((*s).id));
char class; printf(" GPA: ");
} Student; scanf("%f",&((*s).gpa));
void main() { printf(" Class: ");
scanf(" %c",&((*s).class));
Student s1;
}
/*
readS(&s1); s - address of structure
printS(s1); s1 *s - structure at address
} id gpa class (*s).id - id field of struc
at address */
The Pointer Selection Operator
• Passing a pointer to a structure rather than the
entire structure saves time (need not copy structure)
• Therefore, it is often the case that in functions we
have structure pointers and wish to refer to a field:
(*StrucPtr).Field
• C provides an operator to make this more readable
(the pointer selection operator)
StrucPtr->Field /* equivalent to (*StrucPtr).Field */
• StrucPtr must be a pointer to a structure
• Field must be a name of a field of that type of structure
Pointer Selection Example
typedef struct { void readS(Student *s) {
int id; printf(”Enter:\n");
float gpa; printf(" ID#: ");
scanf("%d",&(s->id));
char class; printf(" GPA: ");
} Student; scanf("%f",&(s->gpa));
void main() { printf(" Class: ");
scanf(" %c",&(s->class));
Student s1;
printf(“Id is %d”,s->id);
readS(&s1); }
printS(s1); s1
} id gpa class
Derived Types as Fields
• The fields of a structure may be any type, including derived types
such as arrays, structures, enumerations, etc.
• An array within a structure is given a field name, to refer to individual
elements of the array we give the field name and then the array ref
([x])
• A structure within a structure is referred to as a nested structure --
there are a couple of ways to declare such structures
Array Within Structure
typedef struct {
s1
int id;
id gpa class
float gpa;
char class;
char name[20];
name
int score[5];
} Student;
012345678 20
Student s1; score
/* With large structure,
more efficient to pass as
pointer */ 0 1 2 3 4
Array Within Structure (cont)
void readS(Student *s) { void printS(Student *s) {
int i; int i;
printf(”Enter:\n"); printf("%s id=#%d (%c)
printf(" Name: "); gpa = %.3f\n",s->name,
scanf("%20s",s->name); s->id,s->class,s->gpa);
printf(" ID#: "); for (i = 0; i < 5; i++)
scanf("%d",&(s->id)); printf("%d ",s->score[i]);
printf(" GPA: "); printf("\n");
scanf("%f",&(s->gpa)); }
printf(" Class: ");
void main() {
scanf(" %c",&(s->class));
Student s1;
printf(" 5 grades: ");
for (i = 0; i < 5; i++)
readS(&s1);
scanf("%d",
printS(&s1);
&(s->score[i]));
}
}
Nested Structure
• Fields of V:
One mechanism, declare nested
V.A /* int field */
structure directly within type
definition: V.D /* structure field */
V.D.B /* char field */
typedef struct {
V.D.C /* float field */
int A;
struct {
char B;
float C; V
} D; /* struc field */ D
A
} MyType; B C
MyType V;
Nested Structure (cont)
Fields of V:
Alternate mechanism (preferred):
V.A /* int field */
typedef struct {
V.D /* structure field */
char B;
V.D.B /* char field */
float C;
V.D.C /* float field */
} MyDType;
typedef struct {
int A;
V
MyDType D;
} MyType; D
A
B C
MyType V;
Initializing Nested Structures
• To initialize a nested structure we give as the value of the structure a
list of values for the substructure:
StrucType V = { Field1Val, Field2Val, Field3Val, … }
where FieldXVal is an item of the form
{ SubField1Val, SubField2Val, SubField3Val, … }
if Field X is a structured field
• Previous example (MyType)
MyType V = { 43, { ‘A’, 3.5 } }; V
D
A
B C
43
A 3.5
Representing Table Data
• For many programs it is appropriate to keep track of a table of
information where we know several things about each entry in the
table:
Name ID Class GPA
Rich 42 F 2.00
Cathy 38 O 3.96
Keith 2 J 3.95
Karen 1 S 4.00
• We would like to keep the values for each entry in the table together
as one unit
Table: Array of Structures
• One mechanism for representing a table of information is to use an array where
each member of the array is a structure representing the info about a line of the
table:
students
id gpa class id gpa class id gpa class
42 2.0 F 38 3.96 O 2 3.95 J
name name name
Ri c h Ca t h y K e i t h
012345678 20 012345678 20 012345678 20
0 1 2
Array of Structures
• Define type corresponding to individual element (structured type)
typedef struct {
/* Fields */
} Student;
• Declare a named array of that type
Student Ss[100];
• Often use an integer variable to keep track of how many of the array
elements are in use:
int numS = 0;
Array of Structures Example
readSFile(Ss,&numS,"stu.dat");
#include <stdio.h> do {
option = select();
#define MAXSTUDENT 100 switch (option) {
case 'I': case 'i':
typedef struct { insS(Ss,&numS); break;
case 'R': case 'r':
int id;
remS(Ss,&numS); break;
float gpa;
case 'P': case 'p':
char class; prntS(Ss,numS); break;
char name[20]; case 'S': case 's':
sort(Ss,numS); break;
} Student;
case 'Q': case 'q': break;
}
void main() {
printf("\n");
Student Ss[MAXSTUDENT]; } while ((option != 'Q') && (option != 'q'));
int numS = 0; prntSFile(Ss,numS,"stu.dat");
int option; }
