1 Chapter 3 Expressions and Interactivity

2 Topics 3.1 The cin Object 3.2 Mathematical Expressions 3.3 When You Mix Apples and Oranges: Type Conversion 3.4 Overflow and Underflow 3.5 Type Casting 3.6 Named Constants

3 Topics (continued) 3.7 Multiple Assignment and Combined Assignment 3.8 Formatting Output 3.9 Formatted Input 3.10 More About Member Functions 3.11 More Mathematical Library Functions 3.12 Introduction to File Input and Output

4 3.1 The cin Object Standard input object Like cout, requires iostream file Used to read input from keyboard Often used with cout to display a user prompt first Information retrieved from cin with >> Input information stored in one or more variables

5 The cin Object User input goes from keyboard to keyboard buffer cin converts information to the type that matches the variable: int height; cout << "How tall is the room? "; cin >> height;

6 The cin Object Can be used to input > 1 value: cin >> height >> width; Multiple values from keyboard must be separated by spaces Order is important: first value entered goes to first variable, etc.

7 Example 1 // This program asks the user to enter the length and width of // a rectangle. It calculates the rectangle's area and displays // the value on the screen. #include using namespace std; int main() { int length, width, area; cout << "This program calculates the area of a "; cout << "rectangle.\n"; cout << "Enter the length and width of the rectangle "; cout << "separated by a space.\n"; cin >> length >> width; area = length * width; cout << "The area of the rectangle is " << area << endl; return 0; } This program calculates the area of a rectangle. Enter the length and width of the rectangle separated by a space [Enter] The area of the rectangle is 200

8 Example 2 // This program demonstrates how cin can read multiple values // of different data types. #include using namespace std; int main() { int whole; float fractional; char letter; cout << "Enter an integer, a float, and a character: "; cin >> whole >> fractional >> letter; cout << "Whole: " << whole << endl; cout << "Fractional: " << fractional << endl; cout << "Letter: " << letter << endl; return 0; } Enter an integer, a float, and a character: b [Enter] Whole: 4 Fractional: 5.7 Letter: b Enter an integer, a float, and a character: b [Enter] Whole: Fractional: Letter: 5.74b[Enter]

9 The cin Object Can be used to read in a string Must first declare an array to hold characters in string: char myName[20]; myName is name of array, 20 is the number of characters that can be stored (the size of the array), including the NULL character at the end Can be used with cin to assign a value: cin >> myName;

10 Example // This program demonstrates how cin can read a string into // a character array. #include using namespace std; int main() { char name[21]; cout << "What is your name? "; cin >> name; cout << "Good morning " << name << endl; return 0; }

Mathematical Expressions Can create complex expressions using multiple mathematical operators An expression is a programming statement that has a value. It can be a constant, a variable, or a mathematical combination of constants and variables Can be used in assignment, cout, other statements: result=x; result=4; area = 2 * PI * radius; cout << "border is: " << 2*(l+w);

12 Order of Operations Operator Precedence: In an expression with > 1 operator, evaluate in this order: - (unary negation), * / %, + -, In the expression * 2 – 2, evaluate first evaluate second evaluate third Do first Do last Do next

13 Associativity of Operators - (unary negation) associates right to left *, /, %, +, - associate left to right parentheses ( ) can be used to override the order of operations: * 2 – 2 = 4 (2 + 2) * 2 – 2 = * (2 – 2) = 2 (2 + 2) * (2 – 2) = 0

14 Converting Algebraic Expressions to Programming Statements Multiplication requires an operator: Area=lw is written as Area = l * w; There is no exponentiation operator: Area=s 2 is written as Area = pow(s, 2); Parentheses may be needed to maintain order of operations: is written as m = (y2-y1) /(x2-x1);

15 Library function: pow pow(x, y): raise x to power y cmath header file: #include return a double value pow function x y

16 Example // This program calculates the area of a circle. // The formula for the area of a circle is Pi times // the radius squared. Pi is #include #include // needed for pow function using namespace std; int main() { double area, radius; cout << "This program calculates the area of a circle.\n"; cout << "What is the radius of the circle? "; cin >> radius; area = * pow(radius, 2); cout << "The area is " << area << endl; return 0; }

When You Mix Apples and Oranges: Type Conversion Operations are performed between operands of the same type. If not of the same type, C++ will convert one to be the type of the other This can impact the results of calculations.

18 Hierarchy of Types Highest: Lowest: Ranked by largest number they can hold Exception: unsigned int outranks long if int and long are the same size long double double float unsigned long long unsigned int int

19 Type Coercion Type Coercion: automatic conversion of an operand to another data type Promotion: convert to a higher type Demotion: convert to a lower type

20 Coercion Rules 1) char, short, unsigned short automatically promoted to int Exception: unsigned short is promoted to unsigned int if short and int are in the same size 2) When operating on values of different data types, the lower one is promoted to the type of the higher one. 3) When using the = operator, the type of expression on right will be converted to type of variable on left

21 Example int x, y=4; float z=2.7 x=y*z; cout<<x; y*z=10.8 x=10

Overflow and Underflow Occurs when assigning a value that is too large (overflow) or too small (underflow) to be held in a variable Variable contains value that is ‘wrapped around’ set of possible values

23 Example // This program demonstrates integer overflow and underflow. #include using namespace std; int main() { short testVar = 32767; cout << testVar << endl; testVar = testVar + 1; cout << testVar << endl; testVar = testVar - 1; cout << testVar << endl; return 0; } short ranges -32,768 to +32,

24 Handling Overflow and Underflow Different systems handle the problem differently. They may display a warning / error message stop the program or just continue execution

Explicit Type Conversion Also called type casting Used for manual data type conversion Format static_cast (expression) Example: cout (65); // Displays ‘A’

26 More Type Casting Examples Useful to see int value of a char variable: char ch = 'C'; cout << ch << " is stored as " (ch); Useful for floating point division using ints: float m = static_cast (y2-y1)/(x2-x1);

27 C-Style and Prestandard Type Cast Expressions C-Style cast: data type name in () cout << ch << " is " << (int)ch; Prestandard C++ cast: value in () cout << ch << " is " << int(ch); Both are still supported in C++, although static_cast is preferred

28 Ex. What are the values of the following expressions: int a=5, b=12; double x=3.4, z=9.1; A) b/a B) x*a C) static_cast (b/a) D) static_cast (b)/a E) b/static_cast (a) F) static_cast (b)/static_cast (a) G) b/static_cast (x) H) static_cast (x) * static_cast (z) I) static_cast (x*z) J) static_cast (static_cast (x)*static_cast (z))

29 #include using namespace std; int main() { int integer1, integer2; float float1; integer1 = 19; integer2 = 2; float1 =integer 1/ integer2; cout << float1 <<endl; float1 = static_cast (integer1) / integer2; cout << float1 <<endl; float1 = static_cast (integer1 / integer2); cout <<float1 <<endl; return 0; } Ex. What will the following program display?

Named Constants Named constant (constant variable): variable whose content cannot be changed during program execution Used for representing constant values with descriptive names: const float TAXRATE = ; const int NUMSTATES = 50; Often named in uppercase letters

31 const vs. #define #define C-style of naming constants #define NUMSTATES 50 Interpreted by pre-processor rather than compiler Does not occupy a memory location like a constant variable defined with const Instead, causes a textual substitution to occur. In above example, every occurrence in program of NUMSTATES will be replaced by 50 no ; goes here

32 Example 1 // This program calculates the area of a circle. // The formula for the area of a circle is pi times // the radius squared. Pi is #include #include // needed for pow function using namespace std; int main() { const float PI = ; double area, radius; cout << "This program calculates the area of a circle.\n"; cout << "What is the radius of the circle? "; cin >> radius; area = PI * pow(radius, 2); cout << "The area is " << area << endl; return 0; }

33 Example 2 // This program calculates the area of a circle. // The formula for the area of a circle is Pi times // the radius squared. pi is #include #include // needed for pow function using namespace std; #define PI int main() { double area, radius; cout << "This program calculates the area of a circle.\n"; cout << "What is the radius of the circle? "; cin >> radius; area = PI * pow(radius, 2); cout << "The area is " << area << endl; return 0; }

Multiple Assignment and Combined Assignment The assignment operator ( = ) can be used > 1 time in an expression x = y = z = 5; Associates right to left x = (y = (z = 5)); Done Done Done 3 rd 2 nd 1 st

35 Combined Assignment Applies an arithmetic operation to a variable and assigns the result as the new value of that variable sum = sum + newnum; Note: not an algebraic equation! Operators: += -= *= /= %= Example: sum += amt; is “short” for sum = sum + amt;

36 More Examples x += 5; means x = x + 5; x -= 5; means x = x – 5; x *= 5; means x = x * 5; x /= 5; means x = x / 5; x %= 5; means x = x % 5; The entire right hand side is evaluated before the combined assignment operation is done. x *= a + b; means x = x * (a + b);

37 More Examples Prog 3-13

Formatting Output Can control how output displays for numeric, string data: size position number of digits Requires iomanip header file: #include

39 Stream Manipulators Used to control features of an output field Some affect just the next value displayed: setw(n) : print in a field at least n spaces wide. Use more spaces if field is not wide enough Some affect values until changed again: fixed : use decimal notation for floating-point values setprecision(n) : When used with fixed, print floating-point value using n digits after the decimal. Without fixed, print floating-point value using n significant digits showpoint : always print decimal for floating-point values left: cause output is left-justified right: cause output is right-justified

40 Manipulator Examples const float e = 2.718; float price = 25.0; Displays cout << setw(8) << e << endl; ^^^2.718 cout << setprecision(2); cout << e << endl; 2.7 cout << fixed << e << endl; 2.72 cout << setw(6) <<showpoint<< price; ^25.00

41 Ex. Write cout statements with stream manipulators that perform the following : A) Display the number in a field of nine spaces with two decimal places of precision B) Display the number 7.0 in a field of five spaces with three decimal places of precision. The decimal point and any trailing zeroes should be displayed. C) Display the number 67 left justified in a field of seven spaces

Formatted Input Can format field width for use with cin setw(n) Useful when reading string data to be stored in a character array: char fName[10]; cout << "Enter your name: "; cin >> setw(10) >> fName; cin reads one less character than specified in setw() directive

43 Formatted Input To read an entire line of input, use cin.getline( ) char address[81]; cout << "Enter your address: "; cin.getline(address, 81); cin.getline takes two arguments: Name of array to store string Size of the array

44 Formatted Input To read a single character: Use cin : char ch; cout << "Strike any key to continue"; cin >> ch; Problem: will skip over blanks, tabs, Use cin.get() : cin.get(ch); Will read the next character entered, even whitespace

45 Formatted Input Mixing cin and cin.get() in the same program can cause input errors that are hard to detect char ch; //Define a character variable. int number; //Define an integer variable. cout << “Enter a number:”; cin >> number; //Read an integer cout << “Enter a character: “; cin.get(ch); //Read a character. cout << “Thank You!\n”; 100[Enter]

46 Formatted Input To skip over unneeded characters that are still in the keyboard buffer, use cin.ignore() : cin.ignore(); // skip next char cin.ignore(10, '\n'); // skip the next 10 char. or until a '\n' char ch; //Define a character variable. int number; //Define an integer variable. cout << “Enter a number:”; cin >> number; //Read an integer cin.ignore( ); // Skip the new line character cout << “Enter a character: “; cin.get(ch); //Read a character. cout << “Thank You!\n”;

More About Member Functions Member Function: procedure that is part of an object cout, cin are objects Some member functions of the cin object: getline: cin.getline(address, 81); get: cin.get(ch); ignore: cin.ignore(); cin.ignore(10, '\n');

More Mathematical Library Functions Require cmath header file Take double as input, return a double Commonly used functions: sin Sine cos Cosine tan Tangent sqrt Square root log Natural (e) log abs Absolute value (takes and returns an int)

49 More Mathematical Library Functions These require cstdlib header file rand() : returns a random number ( int ) between 0 and the largest int the compute holds. Yields same sequence of numbers each time program is run. srand(x) : initializes random number generator with unsigned int x

50 Example // This program demonstrates random numbers. #include using namespace std; int main() { unsigned seed; cout << "Enter a seed value: "; cin >> seed; srand(seed); cout << rand() << endl; return 0; } Enter a seed value: 5 [Enter] Enter a seed value: 16 [Enter]

Introduction to File Input and Output Can use files instead of keyboard, monitor screen for program input, output Allows data to be retained between program runs since f iles are stored on secondary storage media, such as disk Steps: Open the file Use the file (read from, write to, or both) Close the file

52 Files: What is Needed Use fstream header file for file access File stream types: ifstream for input from a file ofstream for output to a file fstream for input from or output to a file Define file stream objects: ifstream infile; ofstream outfile;

53 Opening Files Create a link between file name (outside the program) and file stream object (inside the program) Use the open member function: infile.open("inventory.dat"); outfile.open("report.txt"); Filename may include drive, path info. outputFile.open(“a:\\invtry.dat”); Output file will be created if necessary; existing file will be erased first Input file must exist for open to work

54 Using Files Can use output file object and << to send data to a file: outfile << "Inventory report"; Can use input file object and >> to copy data from file to variables: infile >> partNum; infile >> qtyInStock >> qtyOnOrder;

55 Closing Files Use the close member function: infile.close(); outfile.close(); Don’t wait for operating system to close files at program end: may be limit on number of open files may be buffered output data waiting to send to file

56 Example 1 // This program uses the << operator to write information to a file. #include using namespace std; int main() { ofstream outputFile; outputFile.open("demofile.txt"); cout << "Now writing information to the file.\n"; // Write 4 great names to the file outputFile << "Bach\n"; outputFile << "Beethoven\n"; outputFile << "Mozart\n"; outputFile << "Schubert\n"; // Close the file outputFile.close(); cout << "Done.\n"; return 0; } Now writing data to the file. Done. Bach Beethoven Mozart Schubert On Screen In demofile.txt

57 Example 2 // This program uses the >> operator to read information // from a file. #include using namespace std; int main() { ifstream inFile; char name[81]; inFile.open("demofile.txt"); cout << "Reading information from the file.\n\n"; inFile >> name;// Read name 1 from the file cout << name << endl;// Display name 1 inFile >> name;// Read name 2 from the file cout << name << endl;// Display name 2 inFile >> name;// Read name 3 from the file cout << name << endl;// Display name 3 inFile >> name;// Read name 4 from the file cout << name << endl;// Display name 4 inFile.close();// Close the file cout << "\nDone.\n"; return 0; } Reading data from the file. Bach Beethoven Mozart Schubert Done. On Screen

58 Class review The cin Object requires iostream file read input from keyboard Information retrieved from cin with >> Can be used to input > 1 value Order is important Mathematical Expressions Order of the operations: Parentheses overrides the order of operations pow(x,y), include, double Type conversion Operations are performed between operands of the same type. If not of the same type, C++ will convert one to be the type of the other Type coercion, promotion, demotion Lower->higher; “=“: left->right

59 Class review Overflow and Underflow Occurs when assigning a value that is too large (overflow) or too small (underflow) to be held in a variable type casting Used for manual data type conversion static_cast (expression) char ch = 'C'; cout (ch); Multiple Assignment and Combined Assignment Formatting Output #include setw(n), fixed, setprecision(n), showpoint, left, right Formatted Input To read an entire line of input, use cin.getline(a, b) To read a single character: cin, cin.get(a) Member function