1. CPS120: Introduction to Computer Science
Variables and Constants

2. 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 ;

3. 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

4. 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

5. Data Types Integer numbers Real numbers Characters Boolean values
Strings

6. 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

7. Declarations in Various Languages

8. 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
For example, if you want to store coffee in a container, you would use a Styrofoam cup and not a Dixie cup. Otherwise, you would burn your hands. Likewise, if you want to store chicken soup in a container, you would use a bowl and not a plate. Otherwise, the soup would spill off of the edge of the plate.

9. 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

10. Integers
The range varies depending upon how many bytes are assigned to represent an integer value
Some high-level languages provide several integer types of different sizes
Operations that can be applied to integers are the standard arithmetic and relational operators

11. 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 ,147,483,648.
An int variable can store a number as high as 2,147,483,647.

12. Other Data Types
unsigned char, short, unsigned int, long, and unsigned long for whole numbers

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

14. 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)

15. Other Data Types
float and long double for decimal values

16. 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

17. 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

18. 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.

19. 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

20. 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

21. Boolean The Boolean data type consists of two values: true and false
Not all high-level languages support the Boolean data type
If 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

22. 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

23. 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

24. 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

25. 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
Use meaningful variable names
Rate, RATE and rate are different variable

26. Conventions for Naming Variables
Use a conventional method of making your variables easy to read at a quick glance. For example:
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)
Separate successive words with underscore characters ( _ ) (eg. semester_grade, card_value)
Hungarian notation
Begin with type (eg. iTestScore)

27. Common Reserved Words break case char const default do double else
extern
float
for if
int
long
return
switch
void
while

28. 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";
Garbage values can corrupt the logic of your program and cause errors when you least expect
Note: char values must be surrounded with single quotes and string values must be surrounded with double quotes.
Open up vc_demo1.cpp
// Variable declarations
float StartingMileage = 0.0; // stores mileage at beginning of trip
float EndingMileage = 0.0; // stores mileage at end of trip
float MilesTravelled = 0.0; // stores the total distance
float GallonsOfGas = 0.0; // stores the amount of fuel used
float PriceOfGas = 0.00; // stores the amount paid for gas
float PricePerMile = 0.00; // store the price per mile
float MilesPerGallon = 0.0; // stores the miler per gallon achieved

29. 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
Note this is not a variable declaration. It is a program statement in the body of the program.

30. Sample Assignment Statements
Declares and initializes

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

32. 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
For example, if you need to use the number 6% many times because of the fact that the Michigan tax rate is 6%then it would be wise and efficient to use a constant named TAXRATE that is initialized to the value .06 rather than to type the number .06 itself throughout your program. Using the constant named TAXRATE makes your program easier for others to understand. It also makes the program easy to upgrade down the road. For example, if the government were to change the TAXRATE, then it would be easier to update your program if you had used the constant TAXRATE.

33. Use of Constants (Literals)
Numeric 5
-17.29
Characters
'a'
'7'
'*'
Strings (a sequence of symbols)
"I will be an better person "

34. 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

35. 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 =
const double PA_SALES_TAX = 0.06
const int SPEED_OF_LIGHT = ; // commas can't be used here
Open up vc_demo2.cpp

36. 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);
Promotion:
int answer, I;
float x;
i =3;
x= 0.5;
answer = x * I;