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INTRODUCTION TOC++

Chapter 03: Expression & Interactivity

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Topics

3.1 The cinObject

3.2 Mathematical Expressions

3.3 Named Constants

3.4 Multiple and Combined Assignment

3.5 Working with Characters and String

Objects

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3.1 The cinObject

Standard input object

Like cout, requires iostreamfile

Used to read input from keyboard

Often used with coutto display a user prompt

first

Data is retrieved from cinwith >>

Input data is stored in one or more variables

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The cinObject

User input goes from keyboard to the input

buffer, where it is stored as characters

cinconverts the data to the type that matchesthe variable

int height;cout > height;

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The cinObject

Can be used to input multiple valuescin >> height >> width;

Multiple values from keyboard must beseparated by spaces or [Enter]

Must press [Enter] after typing last value

Multiple values need not all be of the same type

Order is important; first value entered is storedin first variable, etc.

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3.2 Mathematical Expressions

An expression can be a constant, a variable,or a combination of constants and variablescombined with operators

Can create complex expressions usingmultiple mathematical operators

Examples of mathematical expressions:

2 height a + b / c

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Using Mathematical Expressions

Can be used in assignment statements, withcout, and in other types of statements

Examples:area =2 * PI * radius;cout

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Order of Operations

In an expression with > 1 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 * 22,

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Do first:

Do last:

Do next:

Evaluate1st

Evaluate2nd

Evaluate3rd

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Associativity of Operators

-(unary negation) associates right to left

* / % + - all associate left to right

parentheses ()can be used to override theorder of operations

2 + 2 * 2 2 = 4

(2 + 2) * 2

2 = 62 + 2 * (2 2) = 2

(2 + 2) * (2 2) = 0

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Algebraic Expressions

Multiplication requires an operator

Area = lw is written asArea = l * w;

There is no exponentiation operator

Area = s2 is written asArea = pow(s, 2);

(note:powrequires the cmathheader file)

Parentheses may be needed to maintain order of

operationsis written as

m = (y2-y1)/(x2-x1);

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12

12

xx

yym

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3.3 Named Constants

Also called constant variables

Variables 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

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constvs. #define

#define C-style of naming constants

#define NUM_STATES 50

Interpreted by pre-processor rather than compilerDoes not occupy a memory location like a constant

variable defined with const

Instead, causes a text substitution to occur. In

above example, every occurrence in program ofNUM_STATESwill be replaced by 50

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no ;

goes here

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3.4 Multiple and Combined

Assignment

The assignment operator (=) can be used more

than 1 time in an expression

x = y = z = 5;Associates right to left

x = (y = (z = 5));

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Done Done Done3rd 2nd 1st

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Combined Assignment

Applies an arithmetic operation to a variable

and assigns the result as the new value of that

variable Operators: += -= *= /= %=

Example:

sum += amt; is short for sum = sum+amt;

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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 right hand side is evaluated before thecombined assignment operation is done.

x *= a + b; means x = x * (a + b);

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3.5 Working with Characters and

String Objects

char: holds a single character

string: holds a sequence of characters

Both can be used in assignment statements

Both can be displayed with coutand

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String Input

Reading in a string object

string str;

cin >> str; // Reads in a string// with no blanks

getline(cin,str);// Reads in a string

// that may contain// blanks

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Character Input

Reading in a character

char ch;

cin >> ch; // Reads in any non-blank char

cin.get(ch);// Reads in anychar

cin.ignore();// Skips over next char in// the input buffer

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String Operators

= Assigns a value to a stringstring words;words = "Tasty ";

+Joins two strings togetherstring s1 = "hot", s2 = "dog";string food = s1 + s2; // food = "hotdog"

+=Concatenates a string onto the end of another onewords += food;// words now = "Tasty hotdog"

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Copyright 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Chapter 3: Expressions and Interactivity

Starting Out with C++

Early Objects

Seventh Edition

by Tony Gaddis, Judy Walters,and Godfrey Muganda