3.1. Chapter 3: The cin Object. Expressions and Interactivity

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1 Chapter 3: Expressions and Interactivity 3.1 The cin Object Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-1 The cin Object Standard input stream object, normally the keyboard, used to read input from outside the computer into the program Like cout, needs to include iostream file #include <iostream> Information retrieved from cin with stream extraction operator >> Input is stored in one or more variables Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-3 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-4

2 The cin Object Displaying a Prompt cin converts data to the type that matches the variable: int height; double degree; cout << "How tall is the room? "; cin >> height; cout << "What is the temperature? "; cin >> degree; Notice that << and >> point in the direction that the data is flowing Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-5 Reading data from the user is normally a two-step process: Use cout to display a prompt on the screen Use cin to read a value from the keyboard Always use a prompt, which is a message that instructs the user to enter data. cout << "How tall is the room? "; cin >> height; Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-6 The cin Object Can be used to input more than one value: cout << "Enter the height and width: "; cin >> height >> width; Multiple values from keyboard must be separated by spaces Order is important: first value entered goes to first variable, etc. Use the correct data type Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-7 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-8

3 Reading Strings with cin Reading Strings with getline Can be used to read in an input and store it as a string Must first define a string cin is mainly used to read numeric data and can only read one word string getline function can read any string: string name; cout << "What is your name? "; cin >> name; string name; cout << "Enter your full name:\n"; getline(cin, name); need to #include <string> Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-9 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-10 Mathematical Expressions 3.2 Mathematical Expressions Various arithmetic calculations can be performed by creating complex expressions using multiple mathematical operators An expression can be a literal, a variable, or a mathematical combination of literals, variables and mathematical operators Examples: 4 x 4*x + 3*y 2*x*y Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-12

4 Mathematical Expressions Order of Operations Can be used in assignment (=), cout, and other statements Ex. Or: circumference = 2 * radius * PI; perimeter = 2*(l+w); cout << "perimeter is: " << perimeter; cout << "perimeter is: " << 2*(l+w); (No need for the assignment statement beforehand) Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-13 In an expression with more than one operator, evaluate in this order: - (unary negation) (highest) * / % + - (lowest) In the expression * 2 2 evaluate second evaluate first evaluate third Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-14 Order of Operations 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 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-15 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-16

5 Grouping with Parentheses Algebraic Expressions Multiplication requires an operator: Area=lw is written as Area = l * w; Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-17 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-18 Algebraic Expressions Algebraic Expressions Parentheses may be needed to maintain order of operations: y 2 y m = 1 x 2 x 1 3x a = 4 y m = (y2-y1)/(x2-x1); m = y2-y1/x2-x1; //wrong a = (3*x+2)/(4*y-1); There is no exponentiation operator: area=s 2 is written as area = pow(s, 2); or area = s*s; area = s^2; // wrong!!! pow() a C++ library function Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-19 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-20

6 When You Mix Apples and Oranges: Type Conversion 3.3 When You Mix Apples and Oranges: Type Conversion Operations are performed between operands of the same type If operands are not of the same type, C++ will convert one to the type of the other Data types are ranked. Lower types will be converted to higher types This can impact the results of calculations Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-22 Hierarchy of Types Type Coercion Highest: long double double float unsigned long long unsigned int Lowest: int One data type outranks another if it can hold a larger number (or it has larger memory size) Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-23 Type Coercion: automatic conversion of an operand to another data type implicit Type Casting: manual conversion of an operand to another data type (covered in 3.5) explicit Promotion: convert to a higher type Demotion: convert to a lower type Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-24

7 Coercion Rules Coercion Rules 1) char, short, unsigned short are automatically promoted to int if they are used in a math expression 2) When operating on values of different data types, the lower one is promoted to the type of the higher one ex: int years=5; double interestrate=0.05, total; total = years * interestrate; 3) When using the = operator, the type of the expression on right will be converted to the type of the variable on left ex: int x, y=4; double z=2.2, parts; x = y*z; //? 8 parts = 5/2; //? 2.0 parts = 5.0/2; //? 2.5 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-25 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-26 Overflow and Underflow 3.4 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 Similar to mileage recording in an odometer Different systems may display a warning/error message, stop the program, or continue execution using the incorrect value Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-28

8 Overflow and Underflow // This program demonstrate data overflow & underflow #include <iostream> Using namespace std; void main() { short testvar = 32767; // max short = testvar = testvar + 1; cout << testvar << endl; testvar = testvar 1; } cout << testvar << endl; Program Output: Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Type Casting Type Casting Used for manual data type conversion General format: static_cast<datatype>(value) Useful when C++ will not perform the required conversion automatically Useful for floating point division using ints: int x1,x2,y1,y2; 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); Output: C is 67 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-31 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-32

9 Problems with Constants 3.6 Named Constants Problem 1: Meaning not clear e.g. newamount = balance * 0.02 Problem 2: What if the value changes frequently e.g. newamount = balance * 0.03 Both problems can be solved by using named constants Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-34 Named Constants Advantages of Named Constants Named constant (constant variable): variable whose content cannot be changed during program execution (read-only) Used for representing constant values with descriptive names: const double TAX_RATE = ; const int NUM_STATES = 50; Must be initialized when defined Often named in uppercase letters Make programs self-documenting newamount = balance * 0.02; can be changed to newamount = balance * INTEREST_RATE; Widespread changes made easier. Change the initialization value. Then recompile the program const double INTEREST_RATE = 0.3; Useful to define difficult to remember constants const double PI = ; Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-35 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-36

10 3.7 Multiple Assignment and Combined Assignment Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-37 Multiple Assignment and Combined Assignment The = can be used to assign a value to multiple variables: x = y = z = 5; Replace multiple assignment statements with one Associates right to left: x = (y = (z = 5)); Combined Assignment Look at the following statement: sum = sum + 1; This adds 1 to the variable sum. value value value is 5 is 5 is 5 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-39 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-40

11 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; Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-41 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-42 Combined Assignment Operators 3.8 Formatting Output Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-43

12 Formatting Output The way a value is displayed on the screen is called its formatting The cout object has a standard way of formatting variables of each data type The size of a float is maximum 7 spaces including the decimal point Use number of spaces needed for each number Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-45 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-46 Stream Manipulators Stream Manipulators The cout object also provides ways to control the formatting through the use of manipulators Requires to include <iomanip> header file setw(x): specifies the field width for the value immediately following it to be at least x spaces wide. Use more spaces if field is not wide enough value = 15; cout << "[" << setw(5) << value << "]"; Output: [ 15] 3 spaces before 15, right adjusted setw() affects the value only once Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-47 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-48

13 Continued Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-49 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-50 Stream Manipulators Others affect values until changed again: fixed: use decimal notation for floating-point values (not scientific notation) 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 zeros for floating-point values left/right: left / right adjust the output Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-51 Continued Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-52

14 3.11 More Mathematical Library Functions 2 spaces Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-53 More Mathematical Library Functions Besides the pow function, the C++ runtime library provides numerous other functions Requires to include <cmath> header file Most take double as input, return a double Commonly used functions: sin y = sin(x); sine cos y = cos(x); cosine tan y = tan(x); tangent sqrt y = sqrt(x); square root log y = log(x); natural (e) log abs y = abs(x); absolute value (takes and returns an int) Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-55 Copyright 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3-56