PIC10A Lecture 17

Reading

Structures

Sometimes it is useful to have a collection of values of different types and to treat the collection as a single item.
Example

bank account	balance, interest rate
student	name, student ID, gender
rectangle	height, width
car	manufacturer, model, year, license plate, millege

An example of bank account.

#include<iostream> #include<cmath> using namespace std; struct BankAccount { double balance; double interestRate; }; // get data from the user void getData(BankAccount& account); // calculate the interest double interest(BankAccount account, int year); // display the information void display(BankAccount account); int main() { // magic formula cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.precision(2); BankAccount myAccount; // declare a BankAccount getData(myAccount); // get data display(myAccount); // display int year = 2; cout << "The interest of your account is " << interest(myAccount, year) << " after " << year << " years.\n"; return 0; } void getData(BankAccount& account) { cout << "Enter account balance: "; cin >> account.balance; cout << "Enter account interest rate: "; cin >> account.interestRate; } double interest(BankAccount account, int year) { return account.balance*pow(1+account.interestRate,year) - account.balance; } void display(BankAccount account) { cout << "Balance: " << account.balance << endl; cout << "Interest Rate: " << account.interestRate << endl; }

Output

Enter account balance: 5000 Enter account interest rate: 0.05 Balance: 5000.00 Interest Rate: 0.05 The interest of your account is 512.50 after 2 years.

Another example: Rectangle

#include<iostream> using namespace std; struct Rectangle { int height; int width; }; // display information void display(Rectangle rect); // return the area of the rect int area(Rectangle rect); int main() { Rectangle rect = {4,5}; display(rect); cout << "The area of the rectangle is " << area(rect) << endl; return 0; } void display(Rectangle rect) { cout << "height: " << rect.height << " " << "width: " << rect.width << endl; } int area(Rectangle rect) { return rect.height*rect.width; }

Output

height: 4 width: 5 The area of the rectangle is 20

After reading the examples, you should have some basic idea about

• how to define a structure.
• how to declare a variable as structure.
• how to access members of a structure.
• how to use structure as arguments (call-by-reference or call-by-value) in functions

Structure definition

struct structure_tag {
    type1 member_variable_1;
    type2 member_variable_2;
	...
	...
    typen member_varialbe_n;
}
      

struct BankAccount {
    double balance;
    double interestRate;
};

struct Rectangle {
    int height;
    int width;
};

• The keyword struct announces that this is a structure type definition.
• The identifier BankAccount or Rectangle are the names of structure types, which is known as the structure tag.
• The name of the structure tag can be any legal identifier. However, they usually spelled starting with an uppercase letter.
• The variables declared inside the braces { } are called member variables.
• Don't forget the semi-color ; after the brace.
• a structure definition is placed outside any function definition.
• the structure type is then a global definition that is available to all the code follows the structure definition.

declaration and initialization

• Once a structure type definition has been given, the structure can be used just like the predefined types int, char and so forth.

structure_tag variable_name
      

BankAccount myAccount;

• You can initialize a structure at the time that it is declared.
• The initialing values must be given in the order that corresponds to the order of member variables in the structure type definition.
• It is an error if there are more initializer value than struct members.
• If there are fewer initializer values than struct members, the provided values are used to initialize data members, in order. Each data member without an initializer is initialized to a zero value of an appropriate type for the variable.

structure_tag variable_name = {member_value_1, member_value_2,... member_value_N};
      

Rectangle rect = {4,5};

#include<iostream>
using namespace std;


struct Date {
    int month;
    int day;
    int year;
};

// display information
void display(Date date);


int main() {
    Date date = {12,5,1972};
    display(date);
    cout << endl;
    Date date2 = {11,17}; //miss the last value
    display(date2);
    cout << endl;
    return 0;
}

void display(Date date) {
    cout << date.month << "/" << date.day << "/" << date.year;
}

      

12/5/1972
11/17/0

Members

• A structure variable can hold values just like any other variables can.
• A structure value is a collection of smaller values called member values.
• each structure type has a list of member variables
• Example,

  structure	member variables
  BankAccount	balance, interestRate
  Rectangle	height, width
  Date	month,day,year

• use dot operator . to access members.
• you can use the members as ordinary variables, i.e. you can use them in arithmetic, assignments, etc etc

structure_variable_name.member_variable_name
      

account.balance
account.interestRate
rect.height
rect.width

#include<iostream>
using namespace std;

struct Date {
    int month;
    int day;
    int year;
};

// display information
void display(Date date);


int main() {
    Date date;
    date.month = 12;
    date.day = 5;
    date.year = 1972;
    // same as Date date = {12,5,1972};
    cout << "Today is ";
    display(date);
    cout << endl;
    date.day++;
    cout << "Next day is ";
    display(date);
    cout << endl;

    return 0;
}

void display(Date date) {
    cout << date.month << "/" << date.day << "/" << date.year;
}
      

Today is 12/5/1972
Next day is 12/6/1972

Copy

• You can copy a structure to another structure.

#include<iostream>
using namespace std;

struct Date {
    int month;
    int day;
    int year;
};

// display information
void display(Date date);


int main() {
    Date date = {12,5,1972};
    display(date);
    cout << endl;
    Date date2 = date;
    display(date2);
    cout << endl;
    return 0;
}

void display(Date date) {
    cout << date.month << "/" << date.day << "/" << date.year;
}

      

12/5/1972
12/5/1972

Structures as function arguments

• A function can have call-by-value parameters of a structure type or call-by-reference parameters of a structure type, or both. Refer to the BankAccount and Rectangle examples.
• you can't change the member variables of a call-by-value parameter in a function.
• you can change the member variables of a call-by-reference parameter in a function.

#include<iostream>
using namespace std;

struct Date {
    int month;
    int day;
    int year;
};

// display information
void display(Date date);

void nextDay(Date date);


int main() {
    Date date = {12,5,1972};
    display(date);
    cout << endl;
    nextDay(date);
    cout << "After the function call: \n";
    display(date);
    cout << endl;
    return 0;
}

void display(Date date) {
    cout << date.month << "/" << date.day << "/" << date.year;
}

void nextDay(Date date) {
    date.day++;
}
      

12/5/1972
After the function call:
12/5/1972

#include<iostream>
#include<iostream>
using namespace std;

struct Date {
    int month;
    int day;
    int year;
};

// display information
void display(Date date);

void nextDay(Date& date);


int main() {
    Date date = {12,5,1972};
    display(date);
    cout << endl;
    nextDay(date);
    cout << "After the function call: \n";
    display(date);
    cout << endl;
    return 0;
}

void display(Date date) {
    cout << date.month << "/" << date.day << "/" << date.year;
}

void nextDay(Date& date) {
    date.day++;
}
      

12/5/1972
After the function call:
12/6/1972