Transcript Document

Chapter 2
Variables and Constants
Objectives
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Explain the different integer variable
types used in C++.
Declare, name, and initialize variables.
Use character variables.
Explain the different floating-point types
and use variables of those types.
Describe Boolean variables.
Use constants.
Understanding Variables
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A variable is a data structure whose
contents can change while a program is
running.
In C++ you must select the data type
that best fits the nature of the data to
be stored.
Integer Data Types
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An integer is a whole number.
There are many ways to store integers;
each has its own range and memory
requirements.
The integer data types in C++ are:
char, unsigned char, short, unsigned short,
int, unsigned int, long and unsigned long.
Declaring and Naming Variables
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Indicating to the compiler the name and type
of variable you want to use is called
“declaring” the variable.
You must declare a variable for you can use
it.
A typical variable declaration might look like
the following:
int i;
// declare i as an integer
Initializing Variables
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Initializing a variable means assigning a value
to it.
Variable values are indeterminate when they
are first declared.
You use the = sign to assign a value to a
variable.
A typical variable initialization statement
might look like the following:
i = 2;
// initialize i to 2
Naming Variables
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Names of variables are called identifiers.
Identifiers must start with a letter or an
underscore (_).
You can use letters, numerals, or underscores
in the rest of the identifier.
Use names that make the purpose of the
variable clear.
There can be no spaces in identifiers.
Keywords cannot be used as identifiers.
Characters and the Char Data Type
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Characters are stored as numbers
according to the ASCII codes.
C++ includes a char data type for
storing characters.
Each variable of the char data type can
hold only one character.
A group of characters put together to
form a word or phrase is called a string.
Floating-point data types
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Tasks such as working with money
require floating-point data types.
Floating-point data types can store
fractional numbers.
The floating-point data types in C++
are float, double and long double.
With these types, you can also use
exponential notation: 6.378164e6
Boolean Variables
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A Boolean variable can have two
possible values.
One value usually represents true and
one usually represents false.
Some C++ compilers do not support
the Boolean data type.
Constants
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Constants hold data that does not
change as the program runs.
Constants must be given a data type
and a name.
An example of a statement that
declares a constant:
const double PI = 3.14159;
Summary
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Computers store data in data structures.
Data is stored in variables or constants.
Within one data type, there may be types
with different ranges and memory
requirements.
Variables must be declared before they are
used.
Boolean variables can have one of two
values.