3
Expressions Chapter and Interactivity
3.1
The cin Object
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
The cin Object
cin converts data to the type that matches the variable: int height; cout << "How tall is the room? "; cin >> height;
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;
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.
Reading Strings with cin
Can be used to read in a string Must first declare an array to hold characters in string:
char myName[21];
nyName is name of array, 21 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;
3.2
Mathematical Expressions
Mathematical Expressions
Can create complex expressions using multiple mathematical operators An expression can be a literal, a variable, or a mathematical combination of constants and variables Can be used in assignment, cout, other statements: area = 2 * PI * radius; cout << "border is: " << 2*(l+w);
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
evaluate second evaluate first evaluate third
Order of Operations
Associativity of Operators
- (unary negation) associates right to left *, /, %, +, - associate right to left parentheses ( ) can be used to override the order of operations: 2 + 2 * 2 2 = 4 (2 + 2) * 2 2 = 6 2 + 2 * (2 2) = 2 (2 + 2) * (2 2) = 0
Grouping with Parentheses
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 = y 2 - y1 m of operations: x 2 - x1 is written as
Algebraic Expressions
3.3
When You Mix Apples and Oranges: Type Conversion
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.
Hierarchy of Types
Highest: long double double float unsigned long long unsigned int int Lowest: Ranked by largest number they can hold
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
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
3.4
Overflow and Underflow
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 Different systems may display a warning/error message, stop the program, or continue execution using the incorrect value
3.5
Type Casting
Type Casting
Used for manual data type conversion Useful for floating point division using ints:
double m; m = static_cast<double>(y2-y1) /(x2-x1);
Useful to see int value of a char variable:
char ch = 'C'; cout << ch << " is " << static_cast<int>(ch);
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
3.6
Named Constants
Named Constants
Named constant (constant variable): variable whose content cannot be changed during program execution Used for representing constant values with descriptive names: const double TAX_RATE = 0.0675; const int = NUM_STATES 50; Often named in uppercase letters
Constants and Array Sizes
It is a common practice to use a named constant to indicate the size of an array: const int SIZE = 21; char name[SIZE];
const vs. #define
#define C-style of naming constants: #define NUM_STATES 50
Note no ; at end
Interpreted by pre-processor rather than compiler Does not occupy memory location like const
3.7
Multiple Assignment and Combined Assignment
Multiple Assignment and Combined Assignment
The = can be used to assign a value to multiple variables: x = y = z = 5; Value of = is the value that is assigned Associates right to left: x = (y = (z = 5));
value is 5 value is 5 value is 5
Combined Assignment
Look at the following statement: sum = sum + 1; This adds 1 to the variable sum.
Other Similar Statements
Combined Assignment
The combined assignment operators provide a shorthand for these types of statements. The statement sum = sum + 1; is equivalent to sum += 1;
Combined Assignment Operators
3.8
Formatting Output
Formatting Output
Can control how output displays for numeric, string data:
size position number of digits
Requires iomanip header file
Stream Manipulators
Used to control how an output field is displayed Some affect just the next value displayed:
setw(x): print in a field at least x spaces wide. Use more spaces if field is not wide enough
Stream Manipulators
Some affect values until changed again:
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 for floatingpoint values
Stream Manipulators
3.9
Formatted Input
Formatted Input
Can format field width for use with cin Useful when reading string data to be stored in a character array:
const int SIZE = 10; char firstName[SIZE]; cout << "Enter your name: "; cin >> setw(SIZE) >> firstName;
cin reads one less character than specified with the setw() manipulator
Formatted Input
To read an entire line of input, use cin.getline(): const int SIZE = 81; char address[SIZE]; cout << "Enter your address: "; cin.getline(address, SIZE); cin.getline takes two arguments:
Name of array to store string Size of the array
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, <CR>
Use cin.get():
cin.get(ch);
Formatted Input
Mixing cin >> and cin.get() in the same program can cause input errors that are hard to detect To skip over u...