1. Visual Programming COMP-315
Chapter #2
Variables, C# Data Types, and IO, Operators & Program Control Statements

2. Variables A variable is a typed name for a location in memory
A variable must be declared, specifying the variable's name and the type of information that will be held in it
9200
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9212
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numberOfStudents:
average:
max:
total:
int numberOfStudents; …
int average, max;
data type
variable name
int total;
Which ones are valid variable names?
myBigVar VAR _test @test
99bottles namespace It’s-all-over

3. Assignment An assignment statement changes the value of a variable
The assignment operator is the = sign
int total; …
total = 55;
The value on the right is stored in the variable on the left
The value that was in total is overwritten
You can only assign a value to a variable that is consistent with the variable's declared type (more later)
You can declare and assign initial value to a variable at the same time, e.g., int total = 55;

4. Example static void Main(string[] args) { int total; total = 15;
System.Console.Write(“total = “);
System.Console.WriteLine(total);
total = ; }

5. constant int numberOfStudents = 42;
Constants
A constant is similar to a variable except that it holds one value for its entire existence
The compiler will issue an error if you try to change a constant
In C#, we use the constant modifier to declare a constant
constant int numberOfStudents = 42;
Why constants?
give names to otherwise unclear literal values
facilitate changes to the code
prevent inadvertent errors

6. C# Data Types There are 15 data types in C#
Eight of them represent integers:
byte, sbyte, short, ushort, int, uint, long,ulong
Two of them represent floating point numbers
float, double
One of them represents decimals:
decimal
One of them represents boolean values:
bool
One of them represents characters:
char
One of them represents strings:
string
One of them represents objects:
object

7. Numeric Data Types
The difference between the various numeric types is their size, and therefore the values they can store:
Type
byte
sbyte
short
ushort
int
uint
long
ulong
decimal
float
double
Storage
8 bits
16 bits
32 bits
64 bits
128 bits
Range
-32,
-2,147,483,648 – 2,147,483,647
0 – 4,294,967,295
-9 to 91018
0 – 1.81019
1.010-28; 7.91028 with significant digits
1.510-45; 3.41038 with 7 significant digits
5.010-324; 1.710308 with significant digits
Question: you need a variable to represent world population. Which type do you use?

8. Examples of Numeric Variables
int x = 1;
short y = 10;
float pi = 3.14f; // f denotes float
float f3 = 7E-02f; // 0.07
double d1 = 7E-100;
// use m to denote a decimal
decimal microsoftStockPrice = 28.38m;

9. Boolean A bool value represents a true or false condition
A boolean can also be used to represent any two states, such as a light bulb being on or off
The reserved words true and false are the only valid values for a boolean type
bool doAgain = true;

10. Characters A char is a single character from the a character set
A character set is an ordered list of characters; each character is given a unique number
C# uses the Unicode character set, a superset of ASCII
Uses sixteen bits per character, allowing for 65,536 unique characters
It is an international character set, containing symbols and characters from many languages
Code chart can be found at:
Character literals are represented in a program by delimiting with single quotes, e.g.,
'a‘ 'X‘ '7' '$‘ ',‘
char response = ‘Y’;

11. Common Escape Sequences

12. string message = “Hello World”;
A string represents a sequence of characters, e.g.,
string message = “Hello World”;
Question: how to represent this string: The double quotation mark is “
Question: how to represent this string: \\plucky.cs.yale.edu\zs9$\MyDocs
Strings can be created with verbatim string literals by starting e.g., string a2

13. Data Input Console.ReadLine()
Used to get a value from the user input
Example string myString = Console.ReadLine();
Convert from string to the correct data type
Int32.Parse()
Used to convert a string argument to an integer
Allows math to be preformed once the string is converted
Example: string myString = “1023”; int myInt = Int32.Parse( myString );
Double.Parse()
Single.Parse()

14. System.Console.WriteLine(“Price = {0:C}.”, price);
Output
Console.WriteLine(variableName) will print the variable
You can use the values of some variables at some positions of a string: System.Console.WriteLine(“{0} {1}.”, iAmVar0, iAmVar1);
You can control the output format by using the format specifiers: float price = 2.5f;
System.Console.WriteLine(“Price = {0:C}.”, price);
Price = $2.50.

15. Arithmetic Expressions
An expression is a combination of operators and operands
Arithmetic expressions (we will see logical expressions later) compute numeric results and make use of the arithmetic operators:
Addition +
Subtraction -
Multiplication *
Division /
Remainder %

16. Division and Remainder
If both operands to the division operator (/) are integers, the result is an integer (the fractional part is discarded)
14 / equals? 4
8 / equals?
The remainder operator (%) returns the remainder after dividing the second operand into the first
14 % equals? 2
8 % equals? 8

17. result = total + count / max - offset;
Operator Precedence
Operators can be combined into complex expressions
result = total + count / max - offset;
Operators have a well-defined precedence which determines the order in which they are evaluated
Precedence rules
Parenthesis are done first
Division, multiplication and modulus are done second
Left to right if same precedence (this is called associativity)
Addition and subtraction are done last
Left to right if same precedence

18. Precedence of Arithmetic Operations

19. Operator Precedence: Examples
What is the order of evaluation in the following expressions?
a + b + c + d + e
a + b * c - d / e 1 2 3 4 3 1 4 2
a / (b + c) - d % e 2 1 4 3
a / (b * (c + (d - e))) 4 3 2 1
Example: TemperatureConverter.cs

20. Data Conversions
Sometimes it is convenient to convert data from one type to another
For example, we may want to treat an integer as a floating point value during a computation
Conversions must be handled carefully to avoid losing information
Two types of conversions
Widening conversions are generally safe because they tend to go from a small data type to a larger one (such as a short to an int)
Q: how about int to long?
Narrowing conversions can lose information because they tend to go from a large data type to a smaller one (such as an int to a short)

21. Data Conversions In C#, data conversions can occur in three ways:
Assignment conversion
occurs automatically when a value of one type is assigned to a variable of another
only widening conversions can happen via assignment
Example: aFloatVar = anIntVar
Arithmetic promotion
happens automatically when operators in expressions convert their operands
Example: aFloatVar / anIntVar
Casting

22. Data Conversions: Casting
Casting is the most powerful, and dangerous, technique for conversion
Both widening and narrowing conversions can be accomplished by explicitly casting a value
To cast, the type is put in parentheses in front of the value being converted
For example, if total and count are integers, but we want a floating point result when dividing them, we can cast total:
result = (float) total / count;

23. Assignment
You can consider assignment as another operator, with a lower precedence than the arithmetic operators
First the expression on the right hand
side of the = operator is evaluated
answer = sum / 4 + MAX * lowest; 4 1 3 2
Then the result is stored in the
variable on the left hand side

24. Assignment
The right and left hand sides of an assignment statement can contain the same variable
First, one is added to the
original value of count
count = count + 1;
Then the result is stored back into count
(overwriting the original value)

25. Assignment Operators

26. Increment and Decrement Operators

27. Precedence and Associativity
high
low

28. Boolean Expressions: Basics
A condition often uses one of C#’s equality operators (==, !=) or relational operators (<, >, <=, >=), which all return boolean results:
== equal to
!= not equal to
< less than
> greater than
<= less than or equal to
>= greater than or equal to

29. Equality and Relational Operators
Note the difference between the equality operator (==) and the assignment operator (=)
Question: if (grade = 100) Console.WriteLine( “Great!” );

30. Comparing Characters
We can also use the relational operators on character data
The results are based on the Unicode character set
The following condition is true because the character '+' comes before the character 'J' in Unicode:
if ('+' < 'J')
Console.WriteLine("+ is less than J");
The uppercase alphabet (A-Z) and the lowercase alphabet (a-z) both appear in alphabetical order in Unicode

31. More Complex (Compound) Boolean Expressions: Logical Operators
Boolean expressions can also use the following logical and conditional operators:
! Logical NOT
& Logical AND
| Logical OR
^ Logical exclusive OR (XOR)
&& Conditional AND || Conditional OR
They all take boolean operands and produce boolean results

32. Logical and Conditional Operators

33. Logical and Conditional Operators

34. Comparison: Logical and Conditional Operators
Logical AND (&) and Logical OR (|)
Always evaluate both conditions
Conditional AND (&&) and Conditional OR (||)
Would not evaluate the second condition if the result of the first condition would already decide the final outcome.
Ex 1: false && (x++ > 10) no need to evaluate the 2nd condition
Ex 2: if (count != 0 && total /count) {
… }

35. More Interesting: Control Statements
Selection (a.k.a., conditional statements): decide whether or not to execute a particular statement; these are also called the selection statements or decision statements
if selection (one choice)
if/else selection (two choices)
Also: the ternary conditional operator e1?e2:e3
switch statement (multiple choices)
Repetition (a.k.a., loop statements): repeatedly executing the same statements (for a certain number of times or until a test condition is satisfied).
while structure
do/while structure
for structure
foreach structure (Chapter 12)

36. C# Control Structures: SelectionF
if structure (single selection)
if/else structure (double selection)
switch structure (multiple selections)
. .
break .

37. C# Control Structures: RepetitionF
while structure
do/while structure
for structure/foreach structure

38. The if Statement The if statement has the following syntax:
The condition must be a boolean expression.
It must evaluate to either true or false.
if is a C#
reserved word
if ( condition )
statement;
If the condition is true, the statement is executed.
If it is false, the statement is skipped.

39. if Statement The if statement if ( <test> )
<code executed if <test> is true> ;
The if statement
Causes the program to make a selection
Chooses based on conditional
<test>: any expression that evaluates to a bool type
True: perform an action
False: skip the action
Single entry/exit point
No semicolon after the condition

40. if/else Statement The if/else structure if ( <test> )
<code executed if <test> is true> ;
else
<code executed if <test> is false> ;
The if/else structure
Alternate courses can be taken when the statement is false
Rather than one action there are two choices
Nested structures can test many cases

41. Nested if/else Statements
The statement executed as a result of an if statement or else clause could be another if statement
These are called nested if /else statements
if (temperature > 90) //int temperature
if (sunny) // boolean sunny
System.out.println(“Beach”);
else
System.out.println(“IceCream Parlor”);
if (sunny)
System.out.println(“Tennis”);
System.out.println(“Volleyball”);
// beginning of the next statement

42. The if-else-if Ladder
A common programming construct that is based upon the nested if is the if-else-if ladder. It
looks like this:
The conditional expressions are evaluated from the top downward. As soon as a true condition is found, the statement associated with it is executed, and the rest of the ladder is bypassed.
if (studentGrade >= 90)
Console.WriteLine(“A”);
else if (studentGrade >= 80)
Console.WriteLine(“B”);
else if (studentGrade >= 70)
Console.WriteLine(“C”);
else if (studentGrade >= 60)
Console.WriteLine(“D”);
else
Console.WriteLine(“F”);
// beginning of the next statement

43. Ternary Conditional Operator (?:)
Conditional Operator (e1?e2:e3)
C#’s only ternary operator
Can be used to construct expressions
Similar to an if/else structure
string result;
int numQ;
…………
result = (numQ==1) ? “Quarter” : “Quarters”;
// beginning of the next statement

44. while Statement The while statement has the following syntax:
If the condition is true, the statement is executed.
Then the condition is evaluated again.
while is a
reserved word
while ( condition )
statement;
The statement (or a block of statements) is executed repetitively until the condition becomes false.

45. while Statement Repetition Structure while ( <test> ) {
<code to be repeated if <test> is true> ; }
Repetition Structure
An action is to be repeated
Continues while <test> is true
Ends when <test> is false
Contains either a line or a body of code

46. while Statement
Note that if the condition of a while statement is false initially, the statement is never executed
Therefore, the body of a while loop will execute zero or more times

47. Infinite Loops
The body of a while loop must eventually make the condition false
If not, it is an infinite loop, which will execute until the user interrupts the program
This is a common type of logical error
You should always double check to ensure that your loops will terminate normally

48. The do Statement The do statement has the following syntax: do{
statement; }
while ( condition );
Uses both
the do and
while
reserved
words
The statement is executed once initially, then the condition is evaluated
The statement is repetitively executed until the condition becomes false

49. Comparing the while and do Loops
The while loops vs. the do/while loops
Using a while loop
Condition is tested
The action is performed
Loop could be skipped altogether
Using a do/while loop
Action is performed
Then the loop condition is tested
Loop will be run at least once T F
while structure T F
do/while structure

50. The for Statement The for statement has the following syntax:
The initialization portion
is executed once
before the loop begins
The statement is
executed until the
condition becomes false
Reserved
word
for ( initialization ; condition ; increment )
statement;
The increment portion is executed at the end of each iteration

51. The for Statement
It is well suited for executing a specific number of times that can be determined in advance
Increment/Decrement
When incrementing
In most cases < or <= is used
When decrementing
In most cases > or >= is used

52. The flexibility of the for Statement
Each expression in the header of a for loop is optional
If the initialization is left out, no initialization is performed
If the condition is left out, it is always considered to be true, and therefore creates an infinite loop
If the increment is left out, no increment operation is performed
Both semi-colons are always required in the for loop header
for ( ; ; ) {
// do something }

53. Statements break and continue
Used to alter the flow of control
The break statement
Used to exit a loop early
The continue statement
Used to skip the rest of the statements in a loop and restart at the first statement in the loop
Programs can be completed without their usage; use with caution.

54. The foreach Loop
// Use the foreach loop.
using System;
class ForeachDemo {
static void Main() {
int sum = 0;
int[] nums = new int[10];
// Give nums some values.
for(int i = 0; i < 10; i++)
nums[i] = i;
// Use foreach to display and sum the values.
foreach(int x in nums) {
Console.WriteLine("Value is: " + x);
sum += x; }
Console.WriteLine("Summation: " + sum);
The foreach loop cycles through the elements of a collection. A collection is a group of objects.
C# defines several types of collections, of which one is an array.

55. The switch Statement
The switch statement provides another means to decide which statement to execute next
The switch statement evaluates an expression, then attempts to match the result to one of several possible cases
Each case contains a value and a list of statements
The flow of control transfers to statement list associated with the first value that matches

56. The switch Statement: Syntax
The general syntax of a switch statement is:
switch
and
case
default
are
reserved
words
switch ( expression ) {
case value1 :
statement-list1
case value2 :
statement-list2
case ...
default :
statement-list }
If expression
matches value2,
control jumps
to here

57. The switch Statement
The expression of a switch statement must result in an integral data type, like an integer or character or a string
Note that the implicit boolean condition in a switch statement is equality - it tries to match the expression with a value

58. The switch Statement
A switch statement can have an optional default case as the last case in the statement
The default case has no associated value and simply uses the reserved word default
If the default case is present, control will transfer to it if no other case value matches
If there is no default case, and no other value matches the expression, control falls through to the statement after the switch
A break statement is used as the last statement in each case's statement list
A break statement causes control to transfer to the end of the switch statement