CPS120: Introduction to Computer Science

Variables and Constants

Lecture 8 - B

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

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

Data Types - Whole Numbers

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.

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)

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.

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

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

Other Data Types

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

float and long double for decimal values

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

Variable NamesVariable names are technically known as identifiersChoose 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 charactersdo not use more than 31 charactersdo begin the identifier with a letter

Remember, C++ is completely case sensitive

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";

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

Start with letter or underscore

Can’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);