Chapter 3:

Expressions and Interactivity

Outline

• cin object• Mathematical expressions • Type Conversion and Some coding styles

The cin Object

• Standard input object

• Like cout, requires iostream file

• Used to read input from keyboard

How to use cin?

SYNTAX

These examples yield the same result.cin >> length ;cin >> width ;

orcin >> length >> width ;

cin >> Variable >> Variable . . . ;

The cin Object

• Information retrieved from cin with >> (the extraction operator)– operator >> attempts to extract the next item

from the input stream and store its value in the right operand variable

• Input is stored in one or more variables

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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;

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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.

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Outline

• cin object• Mathematical expressions • Type Conversion and Some coding styles

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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

Mathematical Expressions

• Can be used in assignment, cout, other statements:area = 2 * PI * radius;cout << "border is: " << 2*(l+w);

Some C++ operatorsPrecedence Operator Description Higher + Positive - Negative

----------------------------------------------------------- * Multiplication / Division % Modulus

------------------------------------------------------------ + Addition

- Subtraction-------------------------------------------------------------

Lower = Assignment

Example

7 * 10 - 5 % 3 * 4 + 9means (7 * 10) - 5 % 3 * 4 + 9

70 - 5 % 3 * 4 + 970 - (5 % 3) * 4 + 9 70 - 2 * 4 + 9 70 - ( 2 * 4 ) + 9 70 - 8 + 9 ( 70 - 8 ) + 9

62 + 9 71

Parentheses

• Parentheses can be used to change the usual order; expressions inside ( ) are evaluated first

• evaluate (7 * (10 - 5) % 3) * 4 + 9 ( 7 * 5 % 3 ) * 4 + 9

( 35 % 3 ) * 4 + 9 2 * 4 + 9 8 + 9 17

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Algebraic Expressions

• Multiplication requires an operator:Area=lw is written as Area = l * w;

• Parentheses may be needed to maintain order of operations:

is written asm = (y2-y1) /(x2-

x1); 12

12

xx

yym

When calculation average

• Average = a + b + c / 3.0; (Wrong!!!)

• Average = (a + b + c) / 3.0; (Correct!!!)

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Algebraic Expressions

Algebraic Expressions

Algebra and C++ expressions

23x

y y = 3 * x /2;

A = (3*x+2)/(4*a-1);

d = b*b -4*a*c;

14

23

a

xa

acbd 42

Outline

• cin object• Mathematical expressions • Type Conversion and Some coding styles

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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.

Example

• double parts;• parts = 15/6; //parts=2

• double parts;• parts = 15.0/6; //parts=2.5

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Named (Symbolic) Constants

• Named constant (constant variable): variable whose content cannot be changed during program execution

• Often named in uppercase letters

• Used for representing constant values with descriptive names:const double TAX_RATE = 0.0675;const int NUM_STATES = 50;

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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));

valueis 5

valueis 5

valueis 5

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Combined Assignment

• Look at the following statement:

sum = sum + 1;

This adds 1 to the variable sum.

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Other Similar Statements

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Other Similar Statements

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Combined Assignment

• The combined assignment operators provide a shorthand for these types of statements.

• The statementsum = sum + 1;

is equivalent to sum += 1;

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Combined Assignment Operators

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Combined Assignment Operators

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Mathematical Library Functions #include <cmath> Commonly used functions:

where n can be an integer, double, float.

sin(n) Sine

cos(n) Cosine

tan(n) Tangent

sqrt(n) Square root

log(n) Natural (e) log

abs(n)pow(b,n)

Absolute value (takes and returns an int)Exponents

Mathematical Library Functions

In order to user the functions you need to know: name of the function you need what the function does (input/output) input parameter and the data type of parameter data type of returned value

Syntax: value = function_name (argument);

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Mathematical Library Functions

Examples:sqrt_value = sqrt(49); //Result is 7abs_value = abs(-34.5); //Result is 34.5power_value = pow(2.1,3); //Result is 9.261cosine_value = cos(3.1415196/6);

//Result is 0.866

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