1. Thanachat Thanomkulabut
Selection Statement
Thanachat Thanomkulabut

2. Outline Boolean expression if statement nested if statement
switch case statement
Flowchart

3. Boolean Expression Operators Comparison Boolean Equal == Not equal !=
Less <
Greater >
Less than or equal to <=
Greater than or equal to >=
Boolean
And &&
Or ||
Not !
! Exclamation mark
0 and 0 = 0
0 and 1 = 0
1 and 0 = 0
1 and 1 = 1
0 or 0 = 0
0 or 1 = 1
1 or 0 = 1
1 or 1 = 1
not 0 = 1
not 1 = 0

4. Boolean Expression Example
From the equation: X2+9X+10 = 0
How can we check that value of X is the answer for above equation?
Condition: Is value Y even number?
((X*X +9*X +10) == 0) //true if X is the answer
(Y%2 == 0) //true if Y is even OR
(Y%2 != 1) //true if Y is even

5. Example: Boolean Expressions
double x = 4.0;
Expression Value
x < ___________
x > ___________
x <= ___________
5.0 == x ___________
x != ___________
(3!=4)&&(7<5) ___________
(4>4)||(5<=10) ___________
true
false
true
false
true
false
true

6. Outline Boolean expression if statement nested if statement
switch case statement
Flowchart

7. if statement
if statement
Execute the specific statement when the ”condition” becomes true
Syntax:
true
if (condition)
statement;
if (condition) {
statement1;
statement2; }
true
if (condition) {
statement; }

8. if statement if (condition) statement; condition if (condition) {
False
True
Statement
if (condition) {
statement; }

9. if statement if (condition) { statement1; statement2; condition } True
False
True
Statement1
Statement2

10. if statement example BMI (Body Mass Index) Weight in Kilograms BMI =10
BMI (Body Mass Index)
BMI =
Weight in Kilograms
(Height in Meters) X (Height in Meters)
BMI
Weight Status
Below 18.5
Underweight
18.5 – 24.9
Normal
25.0 – 29.9
Overweight
30.0 and Above
Obese (Extremely Fat)

11. if statement example double BMI = W /(H*H); if(BMI<18.5)
Weight Status
Below 18.5
Underweight
18.5 – 24.9
Normal
25.0 – 29.9
Overweight
30.0 and Above
Obese (Extremely Fat) 11
BMI =
Weight in Kilograms
(Height in Meters) X (Height in Meters)
double BMI = W /(H*H);
if(BMI<18.5)
Console.WriteLine(“Underweight”);
if(BMI>=18.5 && BMI<=24.9)
Console.WriteLine(“Normal”);
if(BMI>=25.0 && BMI<=29.9)
Console.WriteLine(“Overweight”);
if(BMI>=30.0)
Console.WriteLine(“Obese”);

12. Test I Selfish Ratio Recieving Selfish Ratio = Giving Ratio Output
if statement
Test I 12
Selfish Ratio
Selfish Ratio =
Recieving
Giving
Ratio
Output
More than 1
You are selfish 1
You break even
Less than 1
You are giver

13. if…else… statement If condition is true  execute statement1
if statement
if…else… statement 13
If condition is true  execute statement1
If condition is false  execute statement2
Syntax:
if (condition)
statement1; //true
else
statement2; //false
if (condition)
statement1; //true
else {
statement2; //false
statement3; //false }

14. if…else… statement if (condition) statement1; //true else
if statement
if…else… statement
condition
False
True
Statement2
Statement1
if (condition)
statement1; //true
else
statement2; //false

15. if…else… statement if (condition) statement1; //true else {
if statement
if…else… statement
condition
False
True
Statement2
Statement1
Statement3
if (condition)
statement1; //true
else {
statement2; //false
statement3; //false }

16. if…else… statement example
if statement
if…else… statement example
Write the program which check input number.
input : integer number
output : message to inform that number is odd or even.
Value in N
Output
Even Number
It’s even number.
Odd Number
It’s odd number.

17. if…else… statement example
if statement
if…else… statement example
Value in N
Output
Even Number
It’s even number.
Odd Number
It’s odd number.
if(n%2 == 0)
Console.WriteLine(“It’s even number”);
else
Console.WriteLine(“It’s odd number”);

18. Test II Write the program which find the value of function
if statement
Test II
Write the program which find the value of function
input : number
output :
f(x) x
x2+1
x<0
x+2
x≥0

19. Outline Boolean expression if statement nested if statement
switch case statement
FlowChart

20. Nested if statement int N; N = int.Parse(Console.ReadLine());20
int N;
N = int.Parse(Console.ReadLine());
if (N >= 0) {
if (N==0)
Console.WriteLine(“N is zero number”);
else
Console.WriteLine(“N is positive number”); }
Console.WriteLine(“N is negative number”);
if#1
if#2

21. Nested if statement int N; N = int.Parse(Console.ReadLine());21
int N;
N = int.Parse(Console.ReadLine());
if (N > 0)
Console.WriteLine(“N is positive number”);
else
if (N==0)
Console.WriteLine(“N is zero number”);
Console.WriteLine(“N is negative number”);
if#1
if#2

22. Nested if statement   int N; N = int.Parse(Console.ReadLine());3 -5 -5 3
N is positive number
N is zero number
N is negative number 22
int N;
N = int.Parse(Console.ReadLine());
if (N >= 0) {
if (N==0)
Console.WriteLine(“N is zero number”);
else
Console.WriteLine(“N is positive number”); }
Console.WriteLine(“N is negative number”);
3 >= 0
0 >= 0
-5 >= 0 
False
True 
False
True

23. Nested if statement int N; N = int.Parse(Console.ReadLine());3 -4 3 -4
N is negative number
N is zero number
N is positive number 23
int N;
N = int.Parse(Console.ReadLine());
if (N > 0)
Console.WriteLine(“N is positive number”);
else
if (N==0)
Console.WriteLine(“N is zero number”);
Console.WriteLine(“N is negative number”);
3 > 0
0 > 0
-4 > 0
True
False
True
False

24. Exercise 1: Separated IF (simple)24
1. Have you eaten lunch?
2. Do you like noodle?
if (eaten==true)
{Console.WriteLine(“ I’ve already eaten”);}
else
{Console.WriteLine(“ I’ve not eat yet”);}
if (like_noodle==true)
{Console.WriteLine(“I like noodle”);}
{Console.WriteLine(“I don’t like noodle”);}

25. Exercise 2: Related two IF (full version)25
1. Do you like noodle?
2. If you don’t like noodle, do you like fried rice?
if (like_noodle==true)
{Console.WriteLine(“I like noodle”);}
else
{ Console.WriteLine(“I don’t like noodle”);
if (like_friedrice==true)
{Console.WriteLine(“I like friedrice”);}
{Console.WriteLine(“I don’t like friedrice”);} }

26. Exercise 2: Related two IF (short version)26
1. Do you like noodle?
2. If you don’t like noodle, do you like fried rice?
if (like_noodle==true)
{Console.WriteLine(“I like noodle”);}
else if (like_friedrice==true)
{Console.WriteLine(“I like friedrice”);}
else
{Console.WriteLine(“I don’t like friedrice”);}

27. Exercise 3: Nested two IF (short version)27
1. Do you like noodle?
1.1 If you like noodle, do you love Sen-Yai?
1.2 If you don’t like noodle, do you like fried rice?
if (like_noodle==true)
{Console.WriteLine(“I like noodle”);
if (love_SenYai==true)
{Console.WriteLine(“I love Sen-Yai”);}
else
{Console.WriteLine(“I don’t love Sen-Yai”);} }
else if (like_friedrice==true)
{Console.WriteLine(“I like fried rice”);}
{Console.WriteLine(“I don’t like fried rice”);}

28. Nested if statement example
Write the program which show student’s grade
input : score of student
output :
Score
Grade
80 – 100 A
70 – 79 B
60 – 69 C
50 – 59 D
0 – 49 F

29. Nested if statement
Test III
Write the program which calculation value of following function
input : value of x
output : function value of x according with ...
f(x) =
2x+10, x ≤ 5
x2+10, < x ≤ 20
x3+10, x > 20

30. Outline Boolean expression if statement nested if statement
switch case statement
Flowchart

31. เปรียบเทียบ “Read” แต่ละแบบ
Console.Read()
ผู้ใช้ใส่สตริงใดๆเข้ามา รอจนกว่าผู้ใช้จะเคาะ Return แล้วค่อยทำคำสั่งต่อไป
ให้ค่า int กลับมา เป็น int ที่แทนรหัส ASCII ของ character ตัวหน้าสุด
Console.Read() ถูกเรียกอีกครั้ง จะให้ค่า ASCII ของ character ถัดไป เมื่อถึงตัวสุดท้าย รอผู้ใช้เคาะ Return อีกครั้งจึงจะทำคำสั่งต่อไป
Console.ReadLine()
ให้ค่าสตริงตามที่ผู้ใช้พิมพ์เข้ามา รอจนกว่าเคาะ Return แล้วทำคำสั่งต่อไป
Console.ReadKey()
ให้ค่า character ที่ผู้ใช้พิมพ์ และทำคำสั่งต่อไปทันที

32. using System;     class Program     {         public static void Main(string[] args)         {             Console.Write("Input any string: ");             char ch = (char)(Console.Read());             Console.WriteLine(ch);             ch = (char)(Console.Read());             Console.WriteLine(ch);             ch = (char)(Console.Read());             Console.WriteLine(ch);             ch = (char)(Console.Read());             Console.WriteLine(ch);             ch = (char)(Console.Read());             Console.WriteLine(ch);             Console.WriteLine("Hello World!");             Console.ReadKey(true);         }     }

33. switch…case statement33
For selecting a statement where its label corresponds to the value of the switch expression.
switch (<expression>) {
case <constant-expression> :
<statements>;
break;
[default:
break;] }
<expression> must be int, char, string

34. Example: switch-case (1)
switch...case statement
Example: switch-case (1) 34
int day_num;
Console.Write("Input the day");
day_num =
int.Parse(Console.ReadLine());
switch(day_num)
{case 1:
Console.Write ("Today is Sunday");
break;
case 2:
Console.Write("Today is Monday"); :
default :
Console.Write ("I don’t know"); }
int day_num;
day name 1
Sunday 2
Monday 3
Tuesday 4
Wednesday 5
Thursday 6
Friday 7
Saturday 2 1 30
<expression>
<constant-expression>

35. Test III Write the program which show numbers of day in each months
switch...case statement
Test III
Write the program which show numbers of day in each months
input : Abbreviation of months
output : numbers of day in each months
Input
Output
Jan 31
Feb
28 or 29
Mar
Api 30
May
...

36. Example: switch-case (2)
switch...case statement
Example: switch-case (2) 36
string month;
Console.Write("Input Month");
month = Console.ReadLine();
switch(month)
{case “Jan”:
case “Mar”:
case “May”:
Console.Write("This month has 31days");
break;
case “Apr”:
case “Jun”:
Console.Write("This month has 30days");
default :
Console.Write (“Wrong Input"); }
Input
Output
Jan 31
Feb
28 or 29
Mar
Api 30
May
...

37. Example: switch-case (3)
switch...case statement
Example: switch-case (3) 37
<expression> must be int, char, string
char version
int version
char op;
Console.Write("Select + - / * :");
op=char.Parse(Console.ReadLine());
switch(op) {
case '+':
Console.Write("{0}+{1}={2}",
x,y,x+y);
break;
case '-':
Console.Write("{0}-{1}={2}",
x,y,x-y); :
default:
Console.Write("Try again"); }
int day_num;
day_num=
int.Parse(Console.ReadLine());
switch(day_num ) {
case 1:
Console.Write ("Sunday");
break;
case 2:
console.Write("Monday"); :
default :
Console.Write(“Try again"); }
<expression>
<constant-expression>

38. Example: switch-case (4)
switch...case statement
Example: switch-case (4) 38
<expression> must be int, char, string
string version
string op;
Console.Write("Select + - / * :");
op=Console.ReadLine();
switch(op) {
case “+”:
Console.Write("{0}+{1}={2}",
x,y,x+y);
break;
case “-”:
Console.Write("{0}-{1}={2}",
x,y,x-y); :
default:
Console.Write("Try again"); }
<expression>
<constant-expression>

39. Convert switch-case to if else
switch...case statement
Convert switch-case to if else 39
switch version with default
if (a == 1 || a == 2)
Console.WriteLine("Hi");
else if (a == 3 || a == 4)
Console.WriteLine("Hello");
else
Console.WriteLine("Bye");
if else version
int a; a=
int.Parse(Console.ReadLine());
switch (a) {
case 1 :
case 2 :
Console.WriteLine("Hi");
break;
case 3 :
case 4 :
Console.WriteLine("Hello");
default :
Console.WriteLine("Bye"); }
switch (a) {
case 1 :
case 2 :
Console.WriteLine("Hi");
break; }
switch version without default

40. Outline Boolean expression if statement nested if statement
switch case statement
Flowchart

41. Flowchart Symbols Overview41
Graphical representation
Terminator
Process
Input/output
Condition
ข้าวหลามตัด = diamond
Connector
Flow line

42. Program Flowchart Example42
Start
Statement1
Statement2
Statement3
Statement4
End

43. if statement flowchart
Start 43
statement1;
if (condition)
statement2; //true
else {
statement3; //false }
statement4;
Statement1
Condition
true
false
Statement2
Statement3
Statement4
End

44. if statement flowchart
Start
n= int.Parse(Console.ReadLine());
if (n>0)
n= 2*n+5;
else {
Console.Write(“Go”);
n = n%4; }
n=int.Parse(Console.ReadLine());
n>0
false
true
Console. Write(“Go”);
n=2*n+5;
n=n%4;
End

45. Test IV Write flow chart which accord to following program
n= int.Parse(Console.ReadLine());
n = n%5;
if (n==2)
Console.WriteLine(“Remainder is 2”);
else {
n = n*10; }
Console.WriteLine(“Program End”);

46. Any question?