Chapter 3: Expressions and Interactivity

1. Chapter 3: Expressions and Interactivity

2. 3.1 The cin Object

3. The cin Object • Standard input object • Like cout, requires iostream file • Used to read input from keyboard • Information retrieved from cin with >> • Input is stored in one or more variables

4. The cin Object • cin converts data to the type that matches the variable: int height; cout << "How tall is the room? "; cin >> height;

5. Displaying a Prompt • A prompt is a message that instructs the user to enter data. • You should always use cout to display a prompt before each cin statement.cout << "How tall is the room? ";cin >> height;

6. The cin Object • Can be used to input more than one 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. 3.2 Mathematical Expressions

8. evaluate third evaluate first evaluate second Order of Operations In an expression with more than one operator, evaluate in this order: - (unary negation), in order, left to right * / %, in order, left to right + -, in order, left to right In the expression 2 + 2 * 2 – 2

9. Order of Operations

10. Associativity of Operators • - (unary negation) associates right to left • *, /, %, +, - associate left to right • parentheses ( ) can be used to override the order of operations: 6 / 2 * 3 = 9 6 / ( 2 * 3 ) = 1

11. Algebraic Expressions • Multiplication requires an operator: Area=lw is written as Area = l * w; • There is no exponentiation operator: Area=s2 is written as Area = pow(s, 2); • Parentheses may be needed to maintain order of operations: is written as m = (y2-y1) /(x2-x1);

12. 3.3 When You Mix Apples with Oranges: Type Conversion

13. When You Mix Apples with 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.

14. Hierarchy of Types Highest: Lowest: Ranked by largest number they can hold long double double float unsigned long long unsigned int int

15. 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

16. Coercion Rules 1) char, short, unsigned short automatically promoted to int 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

17. 3.5 Type Casting

18. Type Casting • Used for manual data type conversion • Useful for floating point division using ints: double m;int a =3, b=2;m = a / b; //1m = static_cast<double>(a)/ static_cast<double>(b); //1.5 • Useful to see int value of a char variable: char ch = 'C'; cout << ch << " is " << static_cast<int>(ch); //C is 67

19. Type Casting in Program 3-9

20. 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

21. 3.6 Multiple Assignment and Combined Assignment

22. Multiple Assignment and Combined Assignment • The = can be used to assign a value to multiple variables: x = y = z = 5;

23. Combined Assignment • The combined assignment operators provide a shorthand for different operations. • The statement sum = sum + 1; is equivalent to sum += 1;

24. Combined Assignment Operators

25. 3.7 Formatting Output

26. Formatting Output • Can control how output displays for numeric, string data: • Requires iomanip header file

27. The setw, left and right Manipulators • setw(x): print in a field at least x spaces wide. Use more spaces if field is not wide enough • left: cause values to be left justified • right: cause values to be right justified

28. The setw Stream Manipulator in a Program #include <iostream> #include <iomanip> using namespace std; int main() { int a=1, b=10, c=100, d=1000; cout<<a<<endl; cout<<b<<endl; cout<<c<<endl; cout<<d<<endl; cout<<setw(4)<<a<<endl; cout<<setw(4)<<b<<endl; cout<<setw(4)<<c<<endl; cout<<setw(4)<<d<<endl; return 0; }

29. The left and right Manipulators //Causes values to be left or right justified #include <iostream> #include <iomanip> using namespace std; int main() { int a=1, b=10, c=100; cout<<setw(4)<<a<<endl; cout<<setw(4)<<b<<endl; cout<<setw(4)<<c<<endl; cout<<left<<setw(4)<<a<<endl; cout<<setw(4)<<b<<endl; cout<<setw(4)<<c<<endl; cout<<right<<setw(4)<<a<<endl; cout<<setw(4)<<b<<endl; cout<<setw(4)<<c<<endl; return 0; }

30. The fixed, setprecision and showpoint Manipulators • fixed: use decimal notation for floating-point values • setprecision(x): when used with fixed, print floating-point value using x digits after the decimal. Without fixed, print floating-point value using x significant digits • showpoint: always print decimal point and trailing zeroes

31. The setprecision and fixed Manipulator in a Program #include <iostream> #include <iomanip> using namespace std; int main() { float a=3.14159; cout<<setprecision(3)<<a<<endl; //3.14, here, 3 represents number of significant digits cout<<setprecision(3)<<fixed<<a<<endl; //3.142, here, 3 represents number of decimal points return 0; }

32. The showpoint Manipulator in a Program //showpoint pads a number with trailing zeros #include <iostream> #include <iomanip> using namespace std; int main() { float a=3.14; cout<<setprecision(5)<<a<<endl; //3.14 cout<<setprecision(5)<<showpoint<<a<<endl; //3.1400 return 0; }

33. Stream Manipulators

34. 3.8 Working with Characters and string Objects

35. Working with Characters and string Objects • Using cin with the >> operator to input strings can cause problems: • It passes over and ignores any leading whitespace characters (spaces, tabs, or line breaks) • To work around this problem, you can use a C++ function named getline.

36. Using getline in Program 3-19

37. Working with Characters and string Objects • To read a single character: • Use cin: char ch; cout << "Strike any key to continue"; cin >> ch; Problem: will skip over blanks, tabs, [enter] • Use cin.get(): cin.get(ch); Will read the next character entered, even whitespace

38. Using cin.get() in a Program #include <iostream> using namespace std; int main() { char ch; cout<<"Press enter to continue: "; //cin>>ch; //skips the enter key ch=cin.get(); // OR cin.get(ch); return 0; }

39. Mixing cin and cin.get() • Mixing cin >> and cin.get() in the same program can cause input errors that are hard to detect • To skip over unneeded characters that are still in the keyboard buffer, use cin.ignore(): cin.ignore(); // skip next char • use cin.ignore(n, c): Skip n characters or until character c is encountered cin.ignore(10, '\n'); // skip the next // 10 char. or until a '\n'

40. Example of an Input Error due to Mixing cin and cin.get() //Problem due to mixing cin>> with cin.get() #include <iostream> using namespace std; int main() { int num = 100; char ch; cout<<"Enter a number: "; cin>>num; cout<<"Enter a character: "; ch=cin.get(); return 0; }

41. Solution to Problemdue to Mixing cin and cin.get() //Solution to the problem due to mixing cin>> with cin.get() – use cin.ignore() #include <iostream> using namespace std; int main() { int num = 100; char ch; cout<<"Enter a number: "; cin>>num; cin.ignore(); cout<<"Enter a character: "; ch=cin.get(); return 0; }

42. stringMember Functions and Operators • To find the length of a string: • To concatenate (join) multiple strings: string state = "Texas"; int size = state.length(); greeting2 = greeting1 + name1; greeting1 = greeting1 + name2; Or using the += combined assignment operator: greeting1 += name2;

43. 3.9 Mathematical Library Functions

44. Mathematical Library Functions • Require cmath header file • Take double as input, return a double • Commonly used functions:

45. Random Function • Requires 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

46. Using Random Function //Program to generate random numbers #include <iostream> #include <cstdlib> using namespace std; int main() { int num; num = rand(); cout<<num<<endl; num = rand(); cout<<num<<endl; num = rand(); cout<<num<<endl; return 0; }

47. Using srand() with Random Function //srand() generates different sequence of random numbers #include <iostream> #include <cstdlib> using namespace std; int main() { int num; srand(5); //initializing the seed to 5 num = rand(); cout<<num<<endl; num = rand(); cout<<num<<endl; num = rand(); cout<<num<<endl; return 0; }

48. Generating Random Numbers within a Range //Program to generate random numbers within a range #include <iostream> #include <cstdlib> using namespace std; int main() { int v1 = rand() % 100; // v1 in the range 0 to 99 int v2 = rand() % 100 + 1; // v2 in the range 1 to 100 int v3 = rand() % 30 + 1985; // v3 in the range 1985-2014 cout<<v1<<endl<<v2<<endl<<v3<<endl; return 0; }

49. 3.11 A Case Study

50. A Case Study • General Crates, Inc. builds custom-designed wooden crates. • Cost: $0.23 per cubic foot • Charge: $0.50 per cubit foot • You have been asked to write a program that calculates the: • Volume (in cubic feet) • Cost • Customer price • Profit of any crate GCI builds