Classes, Objects, and Simple Data Version 1.0

Topics Simple data types Variables and constants Declarations Assignment Input and output Classes and objects Style

Objectives At the completion of this topic, students should be able to: Create proper identifiers in a C# program Describe the difference between an object and simple data Describe the simple data types in the C# language Write C# programs that correctly use declarations use assignment statements use literal data use the Console class format simple floating point data Describe the object model of programming Describe the way that data is stored in the computer

We can imagine computer memory to be something like a set of post office boxes.

We can imagine computer memory to be something like a set of post office boxes. Some boxes hold small things (like integers) Some boxes hold larger things (like real numbers) Some boxes hold really big things (like objects) and each box has an address

When we write a program, we need to reserve space in memory for any data that the program will use. We do this by giving the data a name and telling the computer what kind of data it is. The computer translates this name into an address.

In order to be able to refer to the data stored at some address in the computer, we give the data at that address a name. These names are called identifiers. In the computer’s memory, a piece of data is known only by it’s address. 12 X000054EA – hex address

Identifiers The name that you use to refer to a piece of data or a method in C# is called an identifier. A C# identifier must begin with either a letter or an underscore character i.e. [a-z A-Z _] The remaining characters may be letters, digits, or the underscore character[a-z A-Z 0-9 _]. All other characters are invalid. Identifiers can be of any length. Identifiers are case sensitive.

Some Valid Identifiers x x1 _abc sum data2 oldValue It is common in C# to run words together like this. Just capitalize all words after the first.

Some Invalid Identifiers 123 &change 1_dollar my-data

Good programmers select names that are meaningful and somehow describe the data that they name.

In Lab #2 you have to find the un-watered area of Farmer John’s field. x would be a bad choice to name that variable unWateredArea would be a good choice

Keywords C# has a set of built in keywords that are considered to be part of the language. You cannot use any of these as identifiers in your program. In Visual C# Express Edition, By default these will show up in blue. Examples include bool break char int double class const do …

A C# program manages two general kinds of data Simple Data the most basic forms of data - numbers and characters Objects more complex data – strings and others, usually composed of many pieces of simple data

Every piece of data in C# has a given size and shape and is stored at an address. The values that we store in a memory address must fit the size and shape that we specified for that address.

A Memory Chip

binary digits (bits) Address: Where the data resides in memory Size: How many bits make up the data Shape: How the data is coded

Simple Data Simple data elements all have a data type that defines its size and shape and an identifier that is an alias for its address in memory. The data type defines the possible set of values that a simple data element can have, and the operations that can be performed on the data.

Simple (Sort of) Numeric Data Types TypeStorageMax Value int 32 bits- 2,147,483,647 to 2,147,483,648 double 64 bitsover The C# language defines a number of different kinds of data. In this course we will mainly use the following numeric data types:

Integer Numbers Integers are whole numbers they have no fractional part. Integer operations include addition subtraction multiplication division remainder assignment

Examples of Integers ,905,301

Real Numbers Real numbers have fractional parts Real numbers are often written in scientific format The most common real data type is double Operations on real numbers include addition subtraction division multiplication assignment

Examples of Real Numbers ,905, X 10 -4

Character Data Different standards exist for encoding characters The ASCII standard, finalized in 1968, uses 7 bits for each character. In the ASCII standard, is interpreted as the character ‘A’. The 8 bit ASCII standard was added later to add lower-case characters. 7 bits only allows for the definition of 128 unique characters. Subsequent standards (ISO8859 and ISO10646) define much larger, multi-national character sets. However, both are supersets of ASCII. Character data is defined by the keyword char When interpreted as a character, certain bit patterns represent printable characters and control characters.

The ASCII Code Table 1 st hex digit 2 nd hex digit x41 = ‘A’

Characters * Characters are written, in C# programs, as 'A', 'B', etc. * 'A' is stored as in memory or hex 0X41 or dec 65 * 'B' is stored as in memory or hex 0X42 or dec 66 * 'a' is stored as in memory or hex 0X61 or dec 97 * 'b' is stored as in memory or hex 0X62 or dec 98

Control Characters Control characters are characters that do not print, but cause some action, such as moving to a new line, to occur. In C# we write control characters as a backslash, followed by a character that denotes the action to be taken. ' \b ' backspace ' \t ' tab ' \n ' new-line ' \r ' carriage return

Boolean Data A piece of Boolean data can only have one of two values: true false Boolean data is defined by the keyword bool

Variables and Constants A variable is a name for a memory location (address) that holds some piece of data. The value stored in that location may change during execution of the program; however, the type may not. A constant is a name for a memory location (address) that holds some piece of data, where the value of the data cannot change during execution of the program.

Declarations In C#, all variables and constants must be declared before they are used in a program. C# is what is known as a strongly typed language. This means that we must tell the compiler what the data type is for every variable. The compiler then checks all operations to make sure that they are valid for the given type of data.

Question... Assume that you are able to peek into the memory of your computer, and you see the bit pattern What does this bit pattern mean?

The correct answer is that you don’t know. Unless you know what type of data you are looking at, it is impossible to interpret the bits stored in memory.

Integer Representation In most modern digital computers, integer numbers are stored internally in binary. The number of bits used to store an integer in C# is 32 bits. Example: the integer 5 is

Floating Point Representation Numbers that contain decimal points are stored internally in a very different format. The exact format depends upon the processor used in the computer, but in general it looks like: signexponentMantissa or Coefficient for example, the number 6, ( x 10 4 ) would have sign of 0 an exponent of 4 and a mantissa of The actual binary representation is beyond the scope of this course.

Character Representation Characters are stored internally in a coded format. For example, using the standard ASCII code, the character ‘A’ would be stored as C# actually uses a superset of ASCII called Unicode, that supports a multiple byte character code and The character ‘A’ is stored as

Computer Instructions Locations in memory can hold both data and instructions. A special register, called the program counter points in memory to the next instruction to be executed. The computer fetches the next instruction from memory. The program counter moves to the next instruction. The computer then decodes an executes the instruction it just fetched.

Machine Language We call the instructions stored in computer memory machine language instructions. They are defined by the chip manufacturer. For example, the machine instruction might mean something like take the byte stored in memory location 0024 and put it into the A register.

Summary Integersstraight binary representation Real Numberssplit into sign, exponent and coefficient Characterscoded bytes – Unicode an ASCII superset Instructionscoded bytes – machine language

Declaring a Variable int someNumber; char firstLetter; bool theAnswer; double density = 12.45; int hoursWorked = 14; char key = ‘g’; this statement reserves space in computer memory for an integer. We can then refer to the data in this location using the name “someNumber” which is an alias for the the address in memory of that int value. this statement reserves space in computer memory for a character. The bit pattern for ‘g’ is then stored in that location. We can now refer to the data in this location using the name “key”.

int idata = 500; data type (size & shape) identifier (“address” in memory) value (value in memory)

Declaring a Variable int value1, value2, value3; This statement, termed a comma delimited list, declares three variables, all of which are int’s with an unknown value.

Declaring a Variable int value1= 12, value2= 4, value3= 21; This statement, comma delimited list, declares three variables, all of which are int’s, and initializes them.

Declaring a Constant const int SCALE_VALUE = 14; The keyword const means that this is a constant. You cannot change the value after it is declared and initialized. We normally use all upper case letters when writing the name of a constant.

Assignment The easiest way to change the value of a variable is to use an assignment statement. temperature = 68.4; note that all statements end with a semicolon. the right hand side of the assignment statement may be a literal value, or an expression involving variables, literal values, and operators, or even method calls. the expression on the right side of the operator is evaluated and the resulting value is stored in the storage location allocated to the variable “temperature”

Assignment Compatibility In general, it is invalid to assign a variable of one type to a variable of another. For example if you write int a = 6.52; The compiler will issue the warning Cannot implicitly convert type 'double' to 'int'. … This means you are trying to put a square peg in a round hole. It won’t fit. REMEMBER variables have sizes and shapes.

Assignment Compatibility Note that you can do this assignment. double a = 6; The compiler will force a conversion. Why? Sort of like being able to put a round peg in a square hole

The compiler will allow you to do Widening Conversions double a = 3; because no information will be lost. The compiler will not allow you to do Narrowing Conversions int pi = ; because information is lost.

Un-initialized Data In C#, numbers are not always initialized to a known value. Thus they may not always be what you expect. So …… always initialize data when it is declared.

Initializing Data int numOne = 5, numTwo = 4, numThree = 17; int numOne = 5; int numTwo = 4; int numThree = 17;

Literal Data In the statement sum = a + 5; the value 5 is what is called literal data. It is good programming practice to use constants instead of literal data in your program. const int MAX = 5;. sum = a + MAX; Exceptions are 1, -1 and 0

We use the term “Magic Numbers” to refer to literal data that is written into an expression in your program. double avgTemperature = sumTemperature / 2; This is a magic number You do not want magic numbers in your programs. They make programs hard to maintain. You will lose points if I see magic numbers in your programs.

Objects and Classes Object oriented languages give programmers the ability to model real-world objects.

for example, a car has attributes * it is black * it has a 200 hp engine * it has 2 doors * it was built in 1943 * etc it also has behaviors * when you turn the key it starts * when you press the brake it stops * when you push the horn it beeps * etc object-oriented languages encapsulate the data and the methods that operate on that data into an object.

size color GetSize ( ) GetColor( ) an object’s methods (behaviors) manage specific pieces of data (attributes) inside the object. External Method methods outside of the object cannot see or manipulate the object’s data, which is private. However, they can call public methods inside the object to access the data.

Classes Later on, we will spend much more time talking about objects and classes. For now, just think of a class as a blueprint that the computer uses when creating objects of that class. When we write an object oriented program, much of our time is devoted to designing and writing classes.

Languages that primarily deal with objects are called object-oriented languages.

Some Convenient Classes C# has built in to it some classes that will make our programming tasks much easier. The first of these we will talk about is the Console class.

keyboard buffer Console.In program keyboard buffer output buffer Console.Out program display buffer When a Console program executes, the C# runtime environment automatically creates these two stream objects to help manage Console input and output.

Console.ReadLine( ) The Console class provides the ReadLine( ) method to read data from the standard input stream, Console.In. This method waits for the user to type in some data and press the Enter key. The ReadLine( ) method returns the data that the user typed in as a string object. keyboard buffer Console.In program keyboard buffer

String name; name = Console.ReadLine( ); keyboard buffer Console.In name keyboard buffer John Doe

When dealing with numbers, we have to use the Parse method to convert the string value into the appropriate numerical data type int age = 0; age = int.Parse(Console.ReadLine( ) ); keyboard buffer Console.In age keyboard buffer 25 Parse method

When dealing with numbers, we have to use the Parse method to convert the string value into the appropriate numerical data type double money = 0.0; money = double.Parse(Console.ReadLine( ) ); keyboard buffer Console.In money keyboard buffer Parse method

output buffer Console.Out program Console.WriteLine display buffer The Console class provides the WriteLine( ) method to write To the standard output stream, Console.Out. This method takes a string as it’s parameter. After writing to the display, the cursor is moved to the next line.

output buffer Console.Out name Console.WriteLine display buffer string name = “Joe Coder”; Console.WriteLine( name ); Joe Coder

output buffer Console.Out money Console.WriteLine display buffer Numbers are automatically converted to strings by the WriteLine( ) method: double money = Console.WriteLine( money); 12.50

output buffer Console.Out money Console.WriteLine display buffer You can combine string literals and numerical data using the placeholder {.. } to mark the place where the numerical data should be displayed. double money = Console.WriteLine( “You owe {0} to me”, money); You owe to me

output buffer Console.Out money Console.WriteLine display buffer You can use a format string to format the output double money = Console.WriteLine( “You owe {0:C} to me”, money); You owe $12.50 to me

Formatting Strings D or d: Display an integer value as a decimal number F or f: Display a real value - default is two digits after the decimal point C or c: Display a real value as currency

Formatting Strings Use a number to indicate how many digits to display int number = 23; Console.WriteLine(“The value is {0:D4}”, number) The value is 0023

Formatting Strings Use a number to indicate how many decimal digits to display double number = ; Console.WriteLine(“The value is {0:F2}”, number) The value is 23.98

Style When writing programs, in any programming language, it is helpful to use a consistent style. Good software development organizations will often dictate that programmers use a specific style. This makes it easier for everyone to read the code that is being developed. In this course, you are expected to follow certain style guidelines. They are available on the course web site.

Style - Identifiers Use names that have meaning. Avoid single character, very short, or very long names. Examples:Meaningful NamesBaffling Names amounta isFinishedxl Constants All upper case with words separated by an underscore Example:SIZE Classes and Method names Title case (capitalization of the first letter in each word) Example:SimpleCalc( ) Data Variables Lower case for the first word and title case for every word thereafter. Example:myAccount

Style - Braces Many C# language statements do not require braces; however, some statements such as conditionals and loops may or may not require braces and it is good programming practice to provide them. Use braces liberally to visually delimit the beginning and end of code blocks. Including braces now avoids the possibility of errors creeping into your code when you add additional statements at the last minute. Place the opening (left) brace { so that it lines up with the left side of class headers, function headers, conditional statements, or repetitive statements. Place the closing (right) brace } in the same column as the opening brace. Always enter braces in opening/closing pairs to avoid forgetting to add one or the other or both. For braces that span more than three to five lines, comment the ending brace to indicate its nature (e.g., // end if ).

Indentation As you moved from block to block, indent at least three spaces. Indentation makes code much more readable.

Example void reviewCode( ) { if ( meetsGuidelines ) { Console.WriteLine(““Proceed to the next assignment”); } else { Console.WriteLine(“Rework your documentation”); } // end if/else } // end reviewCode( )

Your Own Code Declaration Every source code file must contain the following declaration. Code that does not contain this declaration will not be graded! "I declare that the following source code was written solely by me. I understand that copying any source code, in whole or in part, constitutes cheating, and that I will receive a zero on this project if I am found in violation of this policy.

Magic Numbers A magic number is any numeric literal other than 1, 0, or –1 used in your program. However if 1, 0 and –1 are used to represent something other than the integers 1, 0, or –1 they will be considered magic numbers. Unfortunately, most code you will see in C# books or programming books in general will include magic numbers because it’s easier to code in the short run. In the long run, six months from today, you will be clueless as to what the number means. Therefore, DON’T USE MAGIC NUMBERS in your assignments.

Where are variables stored? If a variable is declared inside of the curly brackets, that define a method, then that variable is said to be local to the method. Only the code in the method can see a local variable. It is stored on the stack. If a variable is declared outside of the curly brackets, that define a method, then that variable is said to be a class level variable. It is stored in the data segment. It is available to any method in the program.

using System; Class Program { const double PI = ; static void Main( ) { double radius;... } Declared inside of curly braces – stack This is a local variable

using System; Class Program { const double PI = ; static void Main( ) { double radius;... } Declared outside of any method – data segment It is a global variable. It is available to any method in the class Program. This is a class level variable

Practice Name the simple C# data types.

Practice Here is some data stored in the memory of the computer What is it’s value?

Practice Suppose that you needed a Student object in a course registration program.  What attributes might a Student have?  What behaviors might a Student require?

Practice In which part of computer storage is each of the following stored? * A class level variable * A local variable

Practice Name one class that we have learned about in this lesson.

Practice Which method is used to convert data into its character representation and send it to the standard output device? To what class does this method belong?

Practice Name a method that is used to convert numerical data from its character representation and store it to in memory in its binary representation?

Practice Write a program that prints the message “Hello, my name is Hal”. Then the program will prompt the user for his or her first name. It then Will print “Hello, user name, how are you?”

Practice Write a program that prints the message “Hello, my name is Hal”. Then the program will prompt the user for his or her name. It then Will print “Hello, user name, how are you?” Prompt the user to type in their age. Then print “ user name, you are n years old”

Practice Write a program that prints the message “Hello, my name is Hal”. Then the program will prompt the user for his or her age. Then prompt the user to type in their name. Then print “ Hello user name, you are n years old”

Practice Write a program that works as follows: - It asks the user for the amount of a purchase - It calculates the state sales tax (5.5%) - It calculates the county sales tax (1.5%) - It displays the total tax and the amount due - It asks the user to enter the amount paid - It calculates and displays the change due.

Practice Write a program that does the following: Declares an integer, a double, and a character. In turn, asks the user to enter in an appropriate piece of data for each variable and stores it in that variable. Add together the integer and the double and the character. The result is stored in a double named sum. Print out the sum. Ask the user to type their name (first and last). Store their name in a string variable. Print out “Thank you (their name). Be able to discuss the results of your program.