Structures
• A structure combines several data items of different
datatypes under one name, unlike an array which holds
several data items of the same datatype
• A structure is commonly used to define a record, e.g., for
a student, using the following attributes:
Surname
First-name(s)
ID number,
Student-number
Date-of-birth
• A structure is a derived datatype created using other
datatypes
• A structure is a user-defined datatype defined by using
the keyword struct
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1) Defining a structure
Before you declare a structure variable, you need to first define
its datatype (define a structure) using struct
• The format of the struct statement is as follows:
struct structure-name // defining a structure, and, thus, creating a datatype
{
member1 declaration;
member2 declaration;
...
};
struct keyword
• Here is an example:
struct Employee
{
char lastName[20];
char firstName[20];
int age;
float salary;
};
structure-tag or
structure-name
Structure elements or members or
fields of structure – normal
variable declarations
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• Employee is the name of a structure that holds the details
of an employee, namely, last-name, first-name, age and
salary, and struct Employee is a datatype
• Variables within a structure are called members of the
structure
• Members of the same structure must have different names,
but two structures may have members with similar names
2) Declaring structure variables
• Declaring structure variables can be done in two ways:
a) Declaring structure variables separately
• Structure variables are declared after the structure is defined,
as shown in the next slide
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struct Employee
{
char lastName[20];
char firstName[20];
int age;
float salary;
}; //defining a structure
/*Separately declaring structure variable, array and
pointer */
struct Employee Worker1, Cleaners[5], *EmployeePtr ;
• First, a structure, struct Employee, is defined
• Separately, a variable, array and pointer, all of type struct
Employee (or simply Employee) are declared:
Worker1 is a variable of type Employee
Cleaners[5] is an array of type Employee
*EmployeePtr is a pointer to type Employee
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b) Declaring structure variables with structure definition
struct Employee
{
char lastName[20];
char firstName[20];
int age;
float salary;
} Worker1, Cleaners[5], *EmployeePtr;
/*Structure variable, array and pointer declared
with structure definition */
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c) Declaring structure variables globally and locally
• A structure can be defined globally, and structure variables
separately declared globally or locally in functions, for
example:
struct Student
{
char surname[20];
char firstname[20];
char gender[6];
int age;
float balance;
};
//defining a structure globally
struct Student s1; //declaring a structure variable globally
int main()
{
struct Student s2; //declaring a structure variable locally in a function
…
}
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• Alternatively, a structure can be defined locally in a function
and structure variables declared locally in the function, for
example:
int main() // main function
{
struct Student
{
char surname[20];
char firstname[20];
char gender[6];
int age;
float balance;
} s1; //declaring variable with structure definition, locally
struct Student s2; //separately declaring structure variable, locally
…
}
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3) Initialising a structure
• A structure can be initialised with data
Example 1
struct Student
{
char surname[20];
char firstname[20];
char gender[6];
int age;
float balance;
} ;
//Initialising a structure
struct Stud Std1 = {“Msibi”, “Ray”, “Male”, 20, 2.50};
•
•
•
•
•
surname contains the string, Msibi
firstname contains the string, Ray
gender contains the string, Male
age contains the integer value, 20
balance contains the floating-point value, 2.50
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Example 2
struct Point
{
int x;
int y;
} Pt1 = {4, 5};
• X contains the value 4
• y contains the value 5
4) Accessing members of a structure
a) Using the dot operator
• Members of a structure can be accessed using the dot (.)
operator
• The operator accesses the members via structure variable names
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i. Using printf()
• Members of a structure can be accessed for displaying using
printf()
• Here is one example:
#include <stdio.h>
#include <conio.h>
int main()
{
struct Student
{
char Name[30];
int age;
float balance;
};
struct Student Std1 = {"Ray Msibi",19,250.60};
printf("Name:\t\t%s\n",Std1.Name);
printf("Age:\t\t%d\n",Std1.age);
printf("Fees balance:\t%.2f\n",Std1.balance);
getch();
return 0;
}
• The output is:
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• Here is an another example:
#include <stdio.h>
#include <conio.h>
int main()
{
struct Student
{
char Name[30];
int age;
float balance;
};
struct Student StdArr[] =
{
{"John Dube",20,500},
{"Jabu Sithole",19,705.70},
{"Mandla Nkosi",21,1004.40}
};
int i;
for(i=0;i<3;i++)
{
printf("Student %d\n",i+1);
printf("Name:\t\t%s\n",StdArr[i].Name);
printf("Age:\t\t%d\n",StdArr[i].age);
printf("Fees balance:\t%.2f\n\n",StdArr[i].balance);
}
getch();
return 0;
}
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• The output is:
ii. Reading input data from user at run-time
• Members of a structure can be accessed to store data read
from the user at run-time (from the standard input)
• fgets() can read a string from the standard input
• getch() can read a char value from the standard input
• scanf() can read int, float and double values from the standard
input
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• Here is an example:
#include <stdio.h>
#include <conio.h>
int main()
{
struct Student
{
char Name[30];
char Gender;
int Age;
float Bal;
};
struct Student Std1;
printf("Enter name and surname: ");
fgets(Std1.Name, sizeof(Std1.Name),stdin);
printf("Enter letter for gender: ");
Std1.Gender = getch(); ///assigning to Gender member
printf("%c\n",Std1.Gender);
printf("Enter age: ");
scanf("%d",&Std1.Age);
printf("Enter balance: ");
scanf("%f",&Std1.Bal);
printf("\nName and surname:\t%s",Std1.Name);
printf("Gender:\t\t\t%c\n",Std1.Gender);
printf("Age:\t\t\t%d\n",Std1.Age);
printf("Fees balance:\t\t%.2f\n",Std1.Bal);
getch();
return 0;
}
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• The output is:
iii. Assigning to members of a structure
• Members of a structure can be accessed to assign data for
storage
• A string can only be assigned when declaring a structure
• Values of other datatypes (int, float, char, double) can be
assigned after a structure is declared
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• Here is an example:
#include <stdio.h>
#include <conio.h>
int main()
{
struct Student
{
char Name[30];
char Gender;
int Age;
float Bal;
};
struct Student Std1 = {"Roy Gumede"};
Std1.Gender = 'M';
Std1.Age = 20;
Std1.Bal = 400;
printf("Name and surname:\t%s\n",Std1.Name);
printf("Gender:\t\t\t%c\n",Std1.Gender);
printf("Age:\t\t\t%d\n",Std1.Age);
printf("Fees balance:\t\t%.2f\n",Std1.Bal);
getch();
return 0;
}
• The output is:
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