CPS120: Introduction to Computer Science

Variables and Constants

Anatomy of a C++ Program//Simple C++ Program

//

// Purpose: To demonstrate the

// parts of a simple C++ program

#include <iostream.h>

main ( )

{

cout << "This is a program ";

return 0;

}

Comments //

Compiler Directive #

Main Function ( )

Braces { }

Statements ;

Variables

Used to store values in virtually every computer program

Used for “remembering” things during program execution

Variables have names, types and values• Values can change during execution

Languages and Data Types

Strong typing: the requirement that only a value of the proper type can be stored into a variable

A data type is a description of the set of values and the basic set of operations that can be applied to values of the type

Data Types

Integer numbers

Real numbers

Characters

Boolean values

Strings

Declarations

A declaration is a statement that associates an identifier with a variable, an action, or some other entity within the language that can be given a name so that the programmer can refer to that item by name

Declarations in Various Languages

Data Types

You need to first choose an appropriate data type when you use a variable.Values can either be:

whole numbers

decimal numbers

letters (i.e. characters)

whole words (i.e. string values

Choosing a Type

Most computer languages have a select number of different data types

You must select the proper data type for each variable that you use in a program in order to program efficiently

This decreases memory (RAM) usage

This increases the speed of your program

Integers

The range varies depending upon how many bytes are assigned to represent an integer valueSome high-level languages provide several integer types of different sizesOperations that can be applied to integers are the standard arithmetic and relational operators

Data Types - Whole Numbers

In C++, to store whole numbers in a variable, we use a variable of the int data type.

An int variable uses 4 bytes of memory.

An int variable can store a number as low as -2,147,483,648.

An int variable can store a number as high as 2,147,483,647.

Other Data Types

unsigned char, short, unsigned int, long, and unsigned long for whole numbers

Real Numbers

Like the integer data type, the range varies depending on the number of bytes assigned to represent a real numberMany high-level languages have two sizes of real numbersThe operations that can be applied to real numbers are the same as those that can be applied to integer numbers

Data Types - Decimal Numbers

To store decimal numbers in a variable, we use a variable of the double data type

A double variable uses 8 bytes of memoryA double variable can store a number as low as -1.7 x 10308A double variable can store a number as high as 1.7 x 10308A double variable can store a number with up to 15 digits of precision (significant digits)

Other Data Types

float and long double for decimal values

Characters

It takes one byte to represent characters in the ASCII character set

Two bytes to represent characters in the Unicode character set

Our English alphabet is represented in ASCII, which is a subset of Unicode

Characters

Applying arithmetic operations to characters doesn’t make much sense

Comparing characters does make sense, so the relational operators can be applied to characters

The meaning of “less than” and “greater than” when applied to characters is “comes before” and “comes after” in the character set

Data Types - Characters

To store a letter or a single character (such as #, $, *, etc.), we use a variable of the char data type.

A char variable only uses 1 byte of memory.

A char variable can only hold one letter, digit, or character.

Strings

A string is a sequence of characters considered as one data value

For example: “This is a string.”Containing 17 characters: one uppercase letter, 12 lowercase letters, three blanks, and a period

The operations defined on strings vary from language to language

They include concatenation of strings and comparison of strings in terms of lexicographic order

Data Types – Words / Phrases

To store a word or phrase (string value), we use a variable that is a string

Technically string is not a data type

You can think of it as a data type for now

Boolean

The Boolean data type consists of two values: true and falseNot all high-level languages support the Boolean data typeIf a language does not, then you can simulate Boolean values by saying that the Boolean value true is represented by 1 and false is represented by 0

Data Types – True and False

The data type bool is useful to store true and false values

Alternatively, we can simply use an int variable with either a 1 value (to represent true) or a 0 value (to represent false) if necessary

Using Variables in C++

Variables must be declared before they are used in C++. Get into the habit of doing this at the top of your functions

char grade; // a students semester grade

int numStudents; // number of students in our class

double price; // price of item

string userName; // user's name

Remember

A reserved word is a word in a language that has special meaning

Case-sensitive means that uppercase and lowercase letters are not considered the same

Remember, C++ is completely case sensitive

Variable Names in C++

Variable names are technically known as identifiers

Choose your own variable names but you must be careful to use valid ones. Otherwise, the compiler will be confused and errors will result. When choosing your variable names:

do not use keywords that are defined in the programming language (Reserved Words)

do not include spaces or other disallowed characters

do not use more than 31 characters

do begin the identifier with a letter

Conventions for Naming Variables

Use a conventional method of making your variables easy to read at a quick glance. For example:

1. Begin variable identifiers with lowercase letters (eg. score)

if you wish to use more than one word within the identifier, you must capitalize the following words or parts of words (eg. semesterGrade, testScore)

2. Separate successive words with underscore characters ( _ ) (eg. semester_grade, card_value)

3. Hungarian notationBegin with type (eg. iTestScore)

Common Reserved Wordsbreak

case

char

const

default

do

double

else

extern

float

for

if

int

long

return

switch

void

while

Initializing VariablesC++ does not automatically initialize all variables to the value 0 If you do not initialize a variable to a certain value, the variable will have an indeterminate value that can corrupt the logic of your program

You should usually initialize your variables at the same time that you declare them. This is done with a declaration statement that is also an initialization statement

int numberOfPizzas = 3;          double monthlyCarPayment = 685;char letterGrade = 'A';string firstName = "Paul";

Assignment statement

Assignment statement: an action statement (not a declaration) that says to evaluate the expression on the right-hand side of the symbol and store that value into the place named on the left-hand side

Sample Assignment Statements

Constants

Named constant: A location in memory, referenced by an identifier, that contains a data value that cannot be changed

Constants

Sometimes you need to use the same value many times throughout a program. In this case, it is proper to use a constant rather than a variable

Constants allow you to give a name to a value used several times in a program

The value never changes

Use of Constants (Literals)Numeric53.14159-17.29

Characters'a''7''*'

Strings (a sequence of symbols)"I will be an better person "

Naming Constants

Constants are defined in a way that is similar to variables

Select a data type and give the constant a name

Any valid identifier name can be used to name a constant

do start with letter or underscore

don’t use reserved words

Conventions for Naming Constants

Traditionally, all uppercase letters have been used when naming constantsUse the underscore character ( _ ) between consecutive words. This allows other programmers to be able to "pick out" your constants at a quick glance

Examples:const double PI = 3.14159const double PA_SALES_TAX = 0.06const int SPEED_OF_LIGHT = 299792458; // commas can't be used here

Type Compatibilities

You cannot store a value of one type in a variable of a different type – a type mismatch occurs

Promotion occurs automatically

You can typecastSupply the name of the data type you want to use to interpret the variable followed by the variable placed in parenthesis

• C = PI * float (diameter);