Transcript CPS120: Introduction to Computer Science

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 memory
A double variable can store a number as low as
-1.7 x 10308
A double variable can store a number as high
as 1.7 x 10308
A 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 Names
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
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.
3.
Separate successive words with underscore characters
( _ ) (eg. semester_grade, card_value)
Hungarian notation
Begin with type (eg. iTestScore)
Common Reserved Words
break
case
char
const
default
do
double
else
extern
float
for
if
int
long
return
switch
void
while
Initializing Variables
C++ 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)
Numeric
5
3.14159
-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 constants
Use 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 = 3.14159
const double PA_SALES_TAX = 0.06
const 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 typecast
Supply 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);