Objects and Primitive Data

Transcript Objects and Primitive Data

Programs and Data

Topics

Data Simple data types Variables and constants Declarations Assignment Input and output Style

Objectives

At the completion of this topic, students should be able to: Describe the way that data is stored in the computer Describe the object model of programming 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

Data

Central to the idea of how a program works is the concept of data. Every program you will ever write will deal with some kind of data.

That data might be … • • • • • • • • • An image Some music A person’s name A person’s age A table of temperatures by week A list of scores for a gymnastic event A model of a molecule A model of a character in a game etc

Data is stored in the computer’s memory.

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) and each box has an address Some boxes hold really big things (like objects)

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.

12 X000054EA – hex 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.

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.

For example, we might have a piece of data that represents the area of a circle.

x would be a bad choice to name that variable areaOfCircle 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 …

Programs deal with all kinds of data.

This data can be put into two broad categories.

Simple Data the most basic forms of data - numbers and characters Objects more complex data – made up of many pieces of simple data.

For example, a person’s address is usually made up of a House number and a street name – 123 Main Street.

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

A Memory Chip

A Memory Chip Address: Where the data resides in memory

Size:

How many bits make up the data Shape: How the data is coded . . .00110001000110 . . .

binary digits (bits)

Number Systems

Decimal: Base 10

In all number systems, each position represents a power of the base, where the rightmost digit is the multiplied by the base 0 , the next digit to the left is multiplied by base 1 , and so on.

So 352 can be expressed in powers of 10 as (3 * 10 2 ) + (5 * 10 1 ) + (2 * 10 0 ) = 300 + 50 + 2 21

Binary: Base 2

In binary we only have two symbols with which to count, 0 and 1.

We call these binary symbols “bits”, short for “binary digits”.

If we group 8 of them together we have a byte, for example: 01001001.

What does this represent? Who knows? Maybe an integer, maybe a character, maybe an instruction or a pixel on your monitor or a tiny bit of music.

Binary: Base 2

Since the base is 2, the place value represents a power of 2.

So a number like 1100 means… (1 * 2 3 ) + (1 * 2 2 ) + (0 * 2 1 ) + (0 * 2 0 ) = 8 + 4 + 0 + 0 …which would translate to 12 in decimal 23

Hexadecimal: Base 16

Here we need 16 unique symbols to count with, so 0 – 9 aren’t enough.

To fill in the remaining equivalents of 10 – 15 in decimal, we use the letters A – F.

So to count in hex, we use 0, 1, 2…8, 9, A, B, C, D, E, F. That gives us 16 unique symbols, where A 16 = 10 10 , F 16 = 15 10 etc.

Note that in documentation, we often express the base with a subscript, as in 3D2F 16 (hex) or 1011 2 (binary).

Hexadecimal: Base 16

In hex, an number like C4 16 means… (C * 16 1 ) + (4 * 16 0 ) + …which could be converted to decimal with (C * 16) + (4 * 1) = (12 10 * 16) + 4 = 196 10 (Note that the symbols 0 -9 have the same value in decimal and hex) 25

Hexadecimal: Base 16

The convenient thing about hex (which is not true of decimal) is that the base (16) is a power of 2, specifically 2 4 As a result, hex is a concise shorthand for binary, because each hex digit represents four binary digits.

Equivalent Values

Decimal

0 1 2 3 4 5 6 7

Binary

0000 0001 0010 0011 0100 0101 0110 0111

Hex

0 1 2 3 4 5 6 7

Decimal

8 9 10 11 12 13 14 15

Binary

1000 1001 1010 1011 1100 1101 1110 1111

Hex

8 9 A B C D E F 27

Hexadecimal: Base 16

For example, we would write the binary equivalent of decimal 12 as 1100 2 , or C write the address in hex.

16 . For large binary numbers (like memory addresses), it is more convenient to So 0100 1011 0010 0101 2 can be shortened to 4B25 16 , which programmers often write as 0x4B25 or just x4B25.

That said, keep in mind that in memory, on your hard drive and on the Internet, binary is the only format that computers use! Decimal and hex are for humans!

Simple Data

Simple data elements all have a

data type

that defines its size and shape that is an alias for its address and an identifier 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 Numeric Data Types

The C# language defines a number of different kinds of data. In this course we will mainly use the following numeric data types: Type Storage Max Value

int double

32 bits 64 bits - 2,147,483,647 to 2,147,483,648 over 10 308

Integer Numbers

Integers are whole numbers they have no fractional part.

Integer operations include addition subtraction multiplication division remainder assignment

Examples of Integers

10 -5 327 2,905,301

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

10.5

-5.02

327.981

2,905,301.004

0.0000239897

-1.56 X 10 -4

Character Data

When interpreted as a character, certain bit patterns represent printable characters and control characters.

Different standards exist for encoding characters The ASCII standard, finalized in 1968, uses 7 bits for each character.

In the ASCII standard, 1000001 is interpreted as the character ‘A’.

The 8 bit ASCII standard was added later to increase the number of possible characters that could be encoded.

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

The ASCII Code Table

1 st hex digit x41 = ‘A’

Character Representation

Characters are stored in the computer’s memory in ASCII format. For example, using the standard ASCII code, the character ‘A’ would be stored as 0100 0001. C# actually uses a superset of ASCII called Unicode, that supports a multiple byte character code and The character ‘A’ is stored as 0000 0000 0100 0001.

Characters

* Characters are written in C# programs as 'A' , 'B' , etc.

* 'A' is stored as 0100 0001 in memory or hex 0X41 or dec 65 * 'B' is stored as 0100 0010 in memory or hex 0X42 or dec 66 * 'a' is stored as 0110 0001 in memory or hex 0X61 or dec 97 * 'b' is stored as 0110 0010 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 ' ' \t ' ' \n ' ' \r ' backspace tab new-line 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 0000 0000 0000 0000 0000 0000 0110 0010 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 0000 0000 0000 0000 0000 0000 0000 0101 This is the sign bit, and when this bit is zero, the number is positive

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: sign exponent Mantissa or Coefficient for example, the number 6,045.03 (0.604503 x 10 4 ) would have sign of 0 an exponent of 4 and a mantissa of .604503

The actual binary representation is beyond the scope of this course.

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 and 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 0011 0011 0001 1010 might mean something like take the byte stored in memory location 0024 and put it into the A register.

Integers Real Numbers Characters Instructions

Summary

straight binary representation split into sign, exponent and coefficient coded bytes – Unicode an ASCII superset coded bytes – machine language

Declaring a Variable

int someNumber; char firstLetter; bool theAnswer; 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 where the value is stored.

double density = 12.45; int hoursWorked = 14; char key = ‘g’; 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” .

data type (size & shape) int idata = 500; identifier (an alias for its address) 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

The keyword const means that this is a constant.

You cannot change the value after it is declared and initialized.

const int SCALE_VALUE = 14; 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 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” 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.

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 = 3.14159; 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.

Exceptions are 1, -1 and 0 const int MAX = 5; .

sum = a + MAX ;

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

Data

it also has * etc

behaviors

* when you turn the key it starts * when you press the brake it stops * when you push the horn it beeps

Methods

object-oriented languages

encapsulate

data into an the data and the methods that operate on that

object

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.

External Method an object’s methods ( behaviors ) manage specific pieces of data ( attributes ) inside the object.

color size

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

String

class.

A string is just a sequence of characters. “hello” “George” “12 East State Road”

Declare a string this way: string myName;

Declare a string and give it an initial value this way: string myName = “John Dolittle”;

Some Convenient Classes

The other important class we need to talk about is the

Console

class.

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

display buffer program keyboard buffer keyboard buffer Console.In

program

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 keyboard buffer Console.In

program

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

John Doe name

25 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 keyboard buffer the string “25” Console.In

Parse method 25 age

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( ) ); 12.50

keyboard buffer keyboard buffer The string “12.50” Console.In

Parse method 12.50

money

Console.WriteLine

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.

display buffer program

Console.WriteLine

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

Console.WriteLine

Numbers are automatically converted to strings by the WriteLine( ) method: double money = 12.50

Console.WriteLine( money); display buffer 12.50

money

Console.WriteLine

You can combine string literals and numerical data using the placeholder { .. } to mark the place where the numerical data should be displayed.

double money = 12.50

Console.WriteLine( “You owe {0} to me”, money); display buffer 12.50

money

Console.WriteLine

You can use a format string to format the output double money = 12.50

Console.WriteLine( “You owe {0:C} to me”, money); display buffer 12.50

money

Formatting Specifiers 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

With an integer you can 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

With a double you can use a number to indicate how many decimal digits to display double number = 23.98344; Console.WriteLine

(“The value is {0:F3}”, number) The value is 23.983

Formatting Strings

You can arrange things on the screen by using numbers to specify a field width and justify the output in the field.

double number = 23.98344; Console.WriteLine

(“The value is {0, 6:F2}”, number) The value is _ 23.98

2 3 . 9 8 6 character field

Data displayed on a Graphical User Interface must first be converted to a string. That string is then assigned to the Text Property of the control where you want to see it.

Suppose the name of this TextBox control is “outputTxtBox” and we have an integer named sum to show there.

outputTxtBox.Text = String.Format(“{0:d}”, sum);

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.

Program that do not conform to the style guidelines for this class will lose points.

Style - Identifiers

Use names that have meaning. Avoid single character, very short, or very long names.

Examples:

Meaningful Names Baffling Names

amount isFinished a xl

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 ) } else Console.WriteLine(““Proceed to the next assignment”); { 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.

Comments

Keep in mind that every program that you submit should contain comments that describe how the program works.

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 = 3.14149; } { static void Main( ) double radius; . . .

Declared inside of curly braces – stack This is a local variable

} using System; { Class Program const double PI = 3.14149; } { 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.

0000 0000 0000 1001 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 ask the user for his or her name.

Get the name and save it in a String object.

Then print the message “Hello,

user name

, how are you?” Prompt the user to type in their age. Save it in an Integer. Then print the message “

user name

, you are

years old”

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.