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.
X000054EA – hex address
12
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
20
Decimal: Base 10
In all number systems, each position represents a
power of the base, where the rightmost digit is the
multiplied by the base0, the next digit to the left is
multiplied by base1, and so on.
So 352 can be expressed in powers of 10 as
(3 * 102) + (5 * 101) + (2 * 100) = 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.
22
Binary: Base 2
Since the base is 2, the place value represents
a power of 2.
So a number like 1100 means…
(1 * 23) + (1 * 22) + (0 * 21) + (0 * 20) = 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 A16 = 1010,
F16 = 1510 etc.
Note that in documentation, we often express the base
with a subscript, as in 3D2F16 (hex) or 10112 (binary).
24
Hexadecimal: Base 16
In hex, an number like C416 means…
(C * 161) + (4 * 160) +
…which could be converted to decimal with
(C * 16) + (4 * 1) = (1210 * 16) + 4 = 19610
(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 24
As a result, hex is a concise shorthand for binary,
because each hex digit represents four binary digits.
26
Equivalent Values
Decimal
Binary
Hex
Decimal
Binary
Hex
0
0000
0
8
1000
8
1
0001
1
9
1001
9
2
0010
2
10
1010
A
3
0011
3
11
1011
B
4
0100
4
12
1100
C
5
0101
5
13
1101
D
6
0110
6
14
1110
E
7
0111
7
15
1111
F
27
Hexadecimal: Base 16
For example, we would write the binary equivalent of
decimal 12 as 11002, or C16. For large binary numbers
(like memory addresses), it is more convenient to
write the address in hex.
So 0100 1011 0010 01012 can be shortened to 4B2516,
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!
28
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 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
int
32 bits
double 64 bits
Max Value
- 2,147,483,647 to 2,147,483,648
over 10308
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
2nd hex digit
1st 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 104)
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.
Summary
Integers
straight binary representation
Real Numbers
split into sign, exponent and coefficient
Characters
coded bytes – Unicode an ASCII superset
Instructions
coded 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 where the value is stored.
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;
value
(value in memory)
identifier
(an alias for its address)
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;
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”
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.
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
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
Data
Methods
object-oriented languages
encapsulate the data and the
methods that operate on that
data into an object.
an object’s methods (behaviors)
manage specific pieces
of data (attributes) inside the 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
size
color
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
output buffer
Console.Out
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 = ReadLine( );
John Doe
keyboard buffer
keyboard buffer
Console.In
John Doe
When using the Console class, be sure to add
using static System.Console;
name
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(ReadLine( ) );
25
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(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
output
buffer
Console.Out
program
Console.WriteLine
string name = “Joe Coder”;
WriteLine( name );
display buffer
output
buffer
Console.Out
Joe Coder
name
Console.WriteLine
Numbers are automatically converted to
strings by the WriteLine( ) method:
double money = 12.50
WriteLine( money);
display buffer
output
buffer
Console.Out
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
WriteLine($ “You owe {money} to me.”);
display buffer
output
buffer
Console.Out
12.50
money
Console.WriteLine
You can use a format string to format the output
double money = 12.50
WriteLine($ “You owe {money:C} to me.”);
display buffer
output
buffer
Console.Out
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;
WriteLine($“The value is {number:D4}” );
The value is 0023
Formatting Strings
With a double you can use a number to indicate
how many decimal digits to display
double number = 23.98344;
WriteLine($“The value is {number:F3}“);
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 {number, 6:F2}” );
2
The value is _23.98
3
.
9
6 character field
8
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 = $”{sum:f2}”;
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
a
isFinished
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 )
{
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.
Comments
Keep in mind that every program that you submit should
contain comments that describe how the program works.
More on this later …
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 n 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.
• 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.