OOP with C# by Dr.hakim

1. Control
Statements (1)
Lecture 3
Dr. Hakem Beitollahi
Computer Engineering Department
Soran University

2. Objectives of this lecture
 In this chapter you will learn:
 Algorithms
 Pseudocode
 Control Structures
if Selection Structure
if/else Selection Structure
 switch statement
 List of keywords in C#
Control Statements— 2

3. Algorithms
 Any computing problem can be solved by
executing a series of actions in a specific order.
 Two important items:
 The actions to be executed and
 The order in which these actions are to be executed.
 Example: rise-and-shine algorithm
 One junior executive for getting out of bed and going
to work
1. get out of bed
2. take off pajamas
3. take a shower
4. get dressed
5. eat breakfast
6. carpool to work
Control Statements— 3

4. Pseudocode
 Pseudocode is an artificial and informal
language that helps programmers develop
algorithms.
 Pseudocode is similar to everyday English; it is
convenient and user-friendly
 it is not an actual computer programming
language
 Pseudocode is not executed on computers.
 pseudocode helps the programmer “think out” a
program before attempting to write it in a
programming language
Control Statements— 4

5. Control Statements
Control Statements— 5

6. Control Statements
 Three control structures
 Sequence structure
o Programs executed sequentially by default
 Selection structures
o if, if…else, switch
 Repetition structures
o while, do…while, for
Control Statements— 6

7. if Selection Statement (I)
 Selection statements
 Choose among alternative courses of action
 Pseudocode example
o If (student’s grade is greater than or equal to 60)
Print “Passed”
 If the condition is true
 Print statement executes, program continues to
next statement
 If the condition is false
 Print statement ignored, program continues
 Indenting makes programs easier to read
 C# ignores white-space characters
7/31

8. if Selection Statement (II)
 Example:
if ( grade >= 60 )
Console.WriteLine("Passed“);
grade >= 60
False
True
Console.WriteLine (“passed”
8/31

9. Good Programming Practice 1
 Indent both body statements of an if/else
structure.
Control Statements— 9

10. if…else Double-Selection Statement (I)
 if
 Performs action if condition true
 if…else
 Performs one action if condition is true, a different action if it is
false
 C# code
 if ( grade >= 60 )
Console.WriteLine("Passed“);
else
Console.WriteLine("Failed“);
10/31

11. if…else Double-Selection Statement (II)
 If-else flowchart
Expression
true false
Action1 Action2
11/31

12. if…else Double-Selection Statement (III)
Ternary conditional operator (?:)
 Three arguments (condition, value if true,
value if false)
Code could be written:
Console.WriteLine(studentGrade >= 60 ? "Passed" : "Failed");
Condition
If(studentGr
ade >= 60)
Value if true
Console.writeline
(“passed”)
Value if false
Console.writel
ine(“faild”)
12/31

13. If…else (example)
// Control Statement example via random generation
using System;
class Conditional_logical
{
static void Main( string[] args )
{
int magic; /* magic number */
string guess_str; /* user's guess string*/
int guess; /* user's guess */
Random randomObject = new Random();
magic = randomObject.Next(100); //this code generates a magic number between 0 to 100 */
Console.Write("Guess the magic number: ");
guess_str = Console.ReadLine();
guess = Int32.Parse(guess_str);
Console.WriteLine("Computer's guess is: " + magic);
if (guess == magic)
Console.WriteLine("** Right **");
else
Console.WriteLine("** Wrong **");
}// end method Main
} // end class
13/31
Define a randomObject
Next method: generate a random
number between 0 to 100
if-else statement

14. if…else Double-Selection Statement (IV)
 Nested if…else statements
 One inside another, test for multiple cases
 Once a condition met, other statements are skipped
 Example
o if ( Grade >= 90 )
Console.Write(“A“);
else
if (Grade >= 80 )
Console.Write("B“);
else
if (Grade >= 70 )
Console.Write("C“);
else
if ( Grade >= 60 )
Console.Write("D“);
else
Console.Write("F“);
14/31

15. if…else Double-Selection Statement (V)
 Previous example can be written as
follows as well
15/31
if (Grade >= 90)
Console.WriteLine("A");
else if (Grade >= 80)
Console.WriteLine("B");
else if (Grade >= 70)
Console.WriteLine("C");
else if (Grade >= 60)
Console.WriteLine("D");
else
Console.WriteLine("F");

16. Good Programming Practice 2
A nested if...else statement can perform
much faster than a series of single-selection if
statements because of the possibility of early exit
after one of the conditions is satisfied.
In a nested if... else statement, test the
conditions that are more likely to be true at the
beginning of the nested if...else statement.
This will enable the nested if...else
statement to run faster and exit earlier than
testing infrequently occurring cases first.
16/31

17. if…else double-selection statement (VI)
 Dangling-else problem
 Compiler associates else with the immediately
preceding if
 Example
o if ( x > 5 )
if ( y > 5 )
Console.WriteLine("x and y are > 5“);
else
Console.WriteLine("x is <= 5“);
 Compiler interprets as
o if ( x > 5 )
if ( y > 5 )
Console.WriteLine("x and y are > 5“);
else
Console.WriteLine("x is <= 5“);
17/31

18. if…else double-selection
statement (VII)
 Dangling-else problem (Cont.)
 Rewrite with braces ({})
o if ( x > 5 )
{
if ( y > 5 )
Console.WriteLine ("x and y are > 5“);
}
else
Console.WriteLine ("x is <= 5“);
 Braces indicate that the second if statement is in the body of
the first and the else is associated with the first if statement
18/31

19. if…else double-selection
statement (VIII)
 Compound statement
 Also called a block
o Set of statements within a pair of braces
o Used to include multiple statements in an if
body
 Example
o if ( Grade >= 60 )
Console.WriteLine ("Passed.“);
else
{
Console.WriteLine ("Failed.“);
Console.WriteLine ("You must take this course again.“);
}
 Without braces,
Console.WriteLine ("You must take this course again.)";
always executes
19/31

20. Good Programming Practice 3
Always putting the braces in an if...else
statement (or any control statement) helps
prevent their accidental omission, especially
when adding statements to an if or else
clause at a later time. To avoid omitting one or
both of the braces, some programmers prefer to
type the beginning and ending braces of blocks
even before typing the individual statements
within the braces.
Control Statements— 20

21. Switch Statement
Control Statements— 21

22. Switch statement (I)
 C/C++/C# has a built-in multiple-branch selection statement, called
switch, which successively tests the value of an expression against
a list of integer or character constants.
 When a match is found, the statements associated with that
constant are executed
 The general form of the switch statement is
switch (expression) {
case constant1:
statement sequence
break;
case constant2:
statement sequence
break;
case constant3:
statement sequence
break;
default
statement sequence
break;
} 22/31

23. Switch statement (II)
 The expression must evaluate to a character or integer
value
 Floating-point expressions, for example, are not
allowed.
 When a match is found, the statement sequence
associated with that case is executed until the break
statement or the end of the switch statement is reached
 The default statement is executed if no matches are
found.
 The default is optional and, if it is not present, no action
takes place if all matches fail.
23/31

24. Switch statement (III)
// switch statement example
using System;
class Conditional_logical
{
static void Main( string[] args )
{
int num;
Console.Write("Enter a number between 0 to 4: ");
num = Int32.Parse(Console.ReadLine());
switch(num) {
case 1:
Console.WriteLine("You enterd number 1");
break;
case 2:
Console.WriteLine("You entered number 2");
break;
case 3:
Console.WriteLine("You enterd number 3");
break;
default:
Console.WriteLine("Either your number is less than 1 or bigger than 3");
break;
}
}// end method Main
} // end class
24/31
Prompt user
Read user’s number and convert it to int
Switch based on num
You must have break after each case

25. Switch statement (IV)
 There are three important things to know about the
switch statement:
 The switch differs from the if in that switch can only
test for equality, whereas if can evaluate any type of
relational or logical expression.
o E.g., case (A>10)
Incorrect
 Switch works only for character and integer numbers.
It does not work for floating and double
o E.g., case (A=2.23)
Incorrect
 No two case constants in the same switch can have
identical values. Of course, a switch statement
enclosed by an outer switch may have case constants
that are the same.
25/31

26. Common Programming Error 1
Not including a break statement at the end
of each case in a switch is a syntax
error. The exception to this rule is the
empty case.
Control Statements— 26

27. Common Programming Error 2
Specifying an expression including variables
(e.g., a + b) in a switch statement’s case
label is a syntax error.
Control Statements— 27

28. Common Programming Error 3
Providing identical case labels in a switch
statement is a compilation error. Providing case
labels containing different expressions that
evaluate to the same value also is a compilation
error. For example, placing case 4 + 1: and
case 3 + 2: in the same switch statement is
a compilation error, because these are both
equivalent to case 5:.
Control Statements— 28

29. List of keywords in C#
Control Statements— 29

30. List of keywords in C# (I)
Control Statements— 30

31. List of keywords in C# (II)
Control Statements— 31