Chapter 4 Literals, Variables and Constants

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Chapter 4 Literals, Variables and Constants

Spring 2014

Chapter 4

Literals, Variables and

Constants

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4.1 Literals

A literal is a value that is interpreted as it is written

Any numeric literal starting with 0x specifies that the following is a hexadecimal value

0xFF – this is a hexadecimal value

Any numeric literal starting with 0 is an octal value

0777 – this is an octal value

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4.1 Literals

Character literals - single characters placed between apostrophes (‘)

‘a’

String literals - one or more characters placed between quotes ( “ )

“string”

Usually, treat single character as a character literal

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4.2 Escape Sequences

Escape sequence - exception to the rule that literals are interpreted exactly as they are written

Escape sequences start with a backslash (\) followed by a single character

Two types of escape sequences• Character• Numeric

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4.2.1 Character Escape Sequences

Can be embedded in a string literal or be used as a character literal

Null character - a special character used, among other things, to give a character variable an initial value

C++ is case sensitive, so \T is not a tab

EscapeSequence

Character Representation

\nCarriage return and line feed (new line)

\tTab (eight characters wide)

\" Double quote

\' Single quote

\\ Backslash

\0 Null character

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4.2.1 Character Escape Sequences

cout << "This is on one line\n This is on another\n";cout << "\tHe said, \"Stop!\"";

// OutputThis is on one line This is on another He said, "Stop!"

cout << "This is an apostrophe: ";

cout << '\'';

// OutputThis is an apostrophe: '

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4.3 Variable Declarations

Variable - a placeholder whose contents can change

Everything must be declared before it is used

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4.3 Variable Declarations

A variable declaration has several purposes:• informs operating system how much internal memory

(RAM) the variable will need

• identifies the memory address to use for that variable

• identifies the type of data to be stored in that physical memory location

• indicates what operations (i.e., +, -, /, etc.) can be performed on the data contained within that variable

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4.3 Variable Declarations

Basic declaration syntax<data type> identifier;

Data types discussed in the next section

Identifier - the variable name

int salary; // Notice the semicolon

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4.3 Variable Declarations

Multiple variables can be declared in the same statement

int age, iq, shoe_size;

Variables can be declared anywhere as long as they are declared before used

Usually good idea to have all declarations at beginning of program

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4.3.1 Variable’s Initial Value

When declared, its initial value is unknown

Important to provide an initial value for all variables

Initialization - process of giving a variable a value during its declaration - resulting in the variable always being in a known state

int sum = 0; //literalint Ralphs_age = RETIREMENT_AGE; //constant valueint Randys_age = Ralphs_age - 26; //variable

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4.3.1 Variable’s Initial Value

Can initialize a variable to another variable’s value

<data type> identifier2 = identifier;

Initialization is the process of giving a variable a value during its declaration. As a result, the variable will always be in a known state.

int base_salary = 30000;

int num_dependents, staff_salary = base_salary;

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4.3.1 Variable’s Initial Value

Another form of initialization - uses parentheses instead of the assignment operator

int base_salary( 30000 );

int num_dependents, staff_salary( base_salary );

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4.3.2 Initialization

Always know the state, or value, of all variables

Variables should always be initialized

If you do not initialize, value unknown

Variables, even characters, are usually initialized to 0

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4.3.3 Data Types

A data type:• Specifies how much memory a variable will

take up in memory

• Indicates operations that can be performed on the variable

Primitive data type - data type whose definition is built into the language

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4.3.3 Primitive Data TypesC++

Data TypeDescription of Data

Memory Allocated

Range

char Character 1 byte -128 to 127

int Integer OS Dependent OS Dependent

float Floating point (decimal) 4 bytes3.4E +/- 38 with 7 digits of accuracy

doubleDouble precision floatingpoint

8 bytes1.7E +/- 308 with 15 digits of accuracy

bool Boolean data 1 byte true or false

short (or short int)

Smaller integer 2 bytes –32,768 to 32,767

long Larger integer 4 bytes–2,147,483,648 to2,147,483,647

long double Larger double 8 bytes1.7E +/- 308 with 15 digits of accuracy

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4.3.3 Data Types

Boolean value - either true or false

Size of an integer (int) - dependent upon the operating system• On a 16-bit operation system such as Windows

3.x, an integer is 16 bits, or 2 bytes

• On a 32-bit operation system (Windows XP), an integer is 32 bits, or 4 bytes

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4.3.3 Data Types

Size of an integer (int) - dependent upon the operating system (continued)

• On a 64-bit operation system - some versions of Windows Vista - an integer is 64 bits, or 8 bytes

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4.3.3 Data Types

The amount of memory an integer requires determines the range of values

In a 32-bit operating system - since a bit can have one of two values - there will be 232 different possibilities

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4.3.3 Data Types

Most significant bit is used as a sign bit• Zero meaning the number is positive

• One means its negative

• Therefore, left with 31 bits, or 231 different values

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4.3.3 Data TypesUnsigned prefix for integral data types - the

sign bit is used for data instead of the sign

Integral data type - only holds whole numbers• A char data type is an integral data type • Under the hood a char holds an ASCII number

representing a character

Use smallest data type that will work with the data

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4.3.4 The sizeof Operator

sizeof operator - determines number of bytes required for a specific data type

// Part 1cout << sizeof( char ) << '\n';

// Part 2unsigned short age = 21; cout << sizeof( age ) << '\n';

// Output12

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4.3.5 Numeric Literal Suffixes

Numeric literal suffix - special character used to specify the type of literal

Numeric literal with an F suffix specifies a float, while L specifies a long value

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4.3.5 Numeric Literal Suffixes

Either case will work for suffixes – but use capitals to avoid confusion between lower case l and a numeric 1

float money = 123.45F;// Flt pt (4 bytes) numericfloat avg = 95.5f; // literals are treated as long flag = 0L; // doubles (8 bytes) // Last character is not a one but a lowercase llong salary = 50000l;

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4.3.6 Naming Rules

Variable naming rules:• Only made up of letters, digits and underscores

• abcdefghijklmnopqrstuvwxyz• ABCDEFGHIJKLMNOPQRSTUVWXYZ• 0123456789• _

• Can’t start with a digit (must begin with a letter or underscore)

• name01 is OK, 01name is not

• Can’t be a reserved word (if, else, while, etc.)• Else is OK as well as else1

Variable names should be descriptive, aiding in code readability

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Identifiers

• An identifier is a sequence of characters used to denote one of the following:• Object or variable name• Class, structure, or union name• Enumerated type name• Member of a class, structure, union, or enumeration• Function or class-member function• typedef name• Label name• Macro name• Macro parameter

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Identifiers (cont.)• The following characters are legal as the first character of

an identifier, or any subsequent character:• _ a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M

N O P Q R S T U V W X Y Z

• The following characters are legal as any character in an identifier except the first:• 0 1 2 3 4 5 6 7 8 9

• Cannot be the exact spelling and case as keywords• Identifiers that contain keywords are legal

• Pint

• Use of two underscore characters (__) at the beginning of an identifier, or a single leading underscore followed by capital letter, is reserved for C++ implementation

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4.4 ASCII Characters

ASCII chart - associates characters with a number (page 630 in textbook)

American Standard Code for Information Interchange (ASCII)

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ASCII Table and Description

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4.4 ASCII Characters

Allow for the storage of characters in memory

Some important ASCII values:• 65 = ‘A’ (4116, 010000012)

• 97 = ‘a’ (6116, 011000012)

• 32 = ‘ ’ (space, 2016, 001000002)

• 48 = ‘0’ (character zero, 3016, 001100002)

The number 010 is stored as 0016, 000000002

The number 2010 is stored as 1416, 000101002

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4.4 ASCII Characters

To display characters given an ASCII value use numeric escape sequences

cout << "Hexadecimal ASCII character: " << "\x4E" << endl;cout << "Octal ASCII character: " << "\77" << endl;

cout << "Hexadecimal number: " << 0x4E << endl;cout << "Octal number: " << 077 << endl;

//OutputHexadecimal ASCII character: NOctal ASCII character: ?Hexadecimal number: 78Octal number: 63

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4.5 Constants

Constants - identifiers that have a value that will never change• Aid in code readability and maintainability

• Should have a name that is descriptive of their purpose

const int SPEED_LIMIT = 65;const int RETIREMENT_AGE = 67;const double PI = 3.1416;

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4.6 const versus #define

To declare constants use the #define preprocessor directive

#define SPEED_LIMIT 65 // Notice no = or semicolons#define RETIREMENT_AGE 67#define PI 3.14

Preprocessor searches through the code replacing the identifier with the value associated with it

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4.6 const versus #define

#define statements can cause compilation errors while looking syntactically correct

#define PI = 3.14; // Notice the = and ;

int main(){ int circumference = 0, radius = 5; circumference = 2 * PI * radius; return 0;}

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4.6 const versus #define

Although the statement looks correct, it causes a compilation error

circumference = 2 * PI * radius;

Error becomes clearer if we show what was created by the preprocessor

circumference = 2 * = 3.14; * radius;

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4.6 const versus #define

Use const versus #define because:

• const uses a data type and participates in type checking

• const has scope

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4.7 Bringing It All Together

Useful to picture how variables and constants might be placed in memory

Examine the declarations below:

short int age;char grade = 'A';float gpa(0.0);const float PI = 3.14;

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4.7 Bringing It All Together

They may be placed in memory as shown below:

? ? A 0 0 0 0 3 . 1 4

agegrade

gpa PI

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Remember, pseudocode is a language independent representation of an algorithm

Using data types has a tendency to make the solution to closely tied to C++ (or any other language)

4.8 Variable Declarations in Pseudocode

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4.8 Variable Declarations in Pseudocode

Do not put variable declarations in pseudocode

#includes are not specified in pseudocode and are considered necessary overhead to the algorithm

4.10 C – The Differences• C doesn’t have a Boolean data type (and no true or false)

• Doesn’t allow for the use of parentheses to initialize variables or constants

• In older versions of C, variables must be declared as the first statement in a block of code (after an opening curly brace)

4.10 C – The Differences

Current C standard allows a programmer to use const to create constants

Legacy C programs written must use the #define to create constants