1. Repetition (While-Loop) [ccd@ku version]
Computer and Programming
Department of Computer Engineering Kasetsart University
Cliparts are taken from
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2. Outline While loop structure Two types of controlled loops
Do-while loop structure

3. Code.org Revisited: Artist
The Artist must be ordered to draw a hexagon by repeating the forward and turn-right commands 6 times!
static void Main() {     Forward(100);     TurnRight(60);
    Forward(100);     TurnRight(60);     Forward(100);     TurnRight(60);     Forward(100);     TurnRight(60);     Forward(100);     TurnRight(60);     Forward(100);     TurnRight(60); }
Try it at

4. Using a Loop Structure The previous code has a lot of repetitions
Want to change the size of the hexagon, 6 places need change
With a loop structure, the code can be made much shorter, clearer, and more flexible
E.g., what if we want to draw a 10-gon, or n-gon instead
static void Main() {     int repeat = 0;     while (repeat < 6) {       Forward(100);        TurnRight(60);        repeat = repeat + 1;     } }

5. Fundamental Control Structures
We have already learned and used
these three control structures.
Sequence
Subroutine
Selection (or Branching)
Repetition (or Iteration or Loop)

6. Fundamental Control Structures
Sequence
Subroutine
Selection
Repetition

7. Repetition (or Iteration or Loop)
Within a method, we can alter the flow of control using either selections or loops.
In C#, the loop statements while, do-while, for, and foreach allow us execute a statement or a block repeatedly.
Like selections, a loop uses a boolean expression to control how the loop repeats.

8. Task: Hello World n times
Write a program to write Hello World! n times, where n  0 is the input. After that, write Goodbye! once.
Sample Run
Sample Run
Sample Run
How many times? 4
Hello World!
Goodbye!
How many times? 0
Goodbye!
How many times? 1
Hello World!
Goodbye!

9. Hello World n times – Topmost Level
The Main method:
Reads a nonnegative integer n.
Write Hello World n times.
Write Goodbye!
 static void Main()  {
     int n = ReadInt("How many times? ");
     WriteHello(n);
Console.WriteLine("Goodbye!");
     Console.ReadLine();  }

10. Hello World n times – Input
Our good old ReadInt() method:
static int ReadInt(string prompt) {    Console.Write(prompt);      return int.Parse(Console.ReadLine()); }

11. Hello World n times – Program (so far)
using System;
class Hello {    static void Main() {      int n = ReadInt("How many times? ");      WriteHello(n);      Console.WriteLine("Goodbye!");      Console.ReadLine();    }
   static int ReadInt(string prompt) {      Console.Write(prompt);
return int.Parse(Console.ReadLine());     }
   static void WriteHello(int n) {
   } }
Code for the WriteHello()

12. WriteHello() method – Steps
The WriteHello() method:
receives n as its parameter.
repeats writing Hello World! n times.
How to do that?
Let’s devise a “counter” scheme.

13. WriteHello() method – Steps
Designing a counter-controlled loop:
A variable count records the number of times Hello World has been written so far.
Devise the scheme and experiment with it.
Let’s say
n is 2
round n
count
next round?
action
updated
Pre-loop 2 #1
Yes
Write once 1 #2
Yes
Write once 2 #3 No
So, the loop exits

14. WriteHello() – Constructing a loop
Pre-loop statements
round n
count
next
round?
action
updated
Pre-loop 2 #1
Yes
Write once 1 #2 #3 No
n is given
count = 0
count < n
Write
“Hello World”
count = count+1 F T
Write once
loop condition
This column implies that the condition for doing another round is _________
Loop-body statements
count < n

15. WriteHello() – Check Boundary Cases
round n
count
next
round?
action
updated
Pre-loop
n is given
count = 0
count < n
Write
“Hello World”
count = count+1 F T
# No
Yes, it works!
The loop-body statements will never be executed at all.
We should also check the boundary cases when n is 0 (or negative) to see if our loop still works.

16. WriteHello() – Code it in C#
n is given
count = 0
count < n
Write
“Hello World”
count = count+1 F T
To express the loop in C#,
we need a new statement:
The while statement
static void WriteHello(int n)
{   int count = 0; }
while (count < n)
{   Console.WriteLine("Hello World!");   count = count+1; }

17. The while Statement while (condition) statement;
C# Syntax
Semantics
while (condition)
statement;
condition F T
statement
condition must be a boolean expression
The while statement as a whole is actually a single C# statement

18. A block of statements within while
C# Syntax
Semantics
while (condition)
condition F T
statement
{ statement; .
. statement; }    

19. Example: while statement in WriteHello()
n is given
count = 0
count < n
Write
“Hello World”
count = count+1 F T
count < n
Write
“Hello World”
count = count+1 F T
while (count < n)
{   Console.WriteLine("Hello World!");
  count = count+1; }

20. Types of Controlled Loop
Counter-controlled loop
A counter variable controls the number of times the loop will repeat.
Event-controlled loop
The loop continues to repeat until some event occurs.

21. Counter-controlled Loop
The loop uses a counter variable to control the number of times the loop will repeat.
The number of times the loop will repeat can be determined before the loop starts.

22. Counter-controlled Loop - Pattern
Initialize the counter
counter is within the limit?
Update the counter F T
Some computations
Initialize the counter;
while (counter is within the limit)
{  
  Update the counter; }
Some computations

23. WriteHello() uses a counter-controlled Loop
Initialize the counter;
while (counter is within the limit )
{  
  Update the counter; }
Some computations
static void WriteHello(int n)
{ int count = 0;
while ( count < n ) {
  Console.WriteLine("Hello World!");  
count = count+1; } }

24. counter-controlled loops
More Example
Another example on
counter-controlled loops
ณ. บัดนาว!

25. Task: Print a Fahrenheit-to-Celcius Table
Print a conversion table from Fahrenheit to Celcius from 212 F to 32 F, decreasing by 20 F.
Sample Run

26. Print a Fahrenheit-to-Celcius Table- Ideas
The formula to convert fahrenheit to celcius:
celcius = (5/9)*(fahrenheit-32)
We’ll write a method PrintFahToCel() to do the task.
We’ll construct a counter-controlled loop in which the fahrenheit value itself is the counter.
Note that for each repetition through the loop, the counter is decreasing by 20 from 212 to 32.

27. PrintFahToCel() method – Loop Design
Designing a counter-controlled loop:
A variable fah that stores the value of farenheit is the counter and is also used in the computation.
So, the condition for doing another round is …………...
Devise the loop algorithm
fah >= 32
round
fah
next round?
action
updated
Pre-loop
212 #1
212
Yes
Compute for 212
192 #2
192
Yes
Compute for 192
172
… … … … …
#Last 32
Yes
Compute for 32 12
#Last+1 12 No
So, the loop exits

28. PrintFahToCel() – from design to code
round
fah
next round?
action
updated
Pre-loop
212 #1
Yes
Compute for 212
192 #2
Compute for 192
172 …
#Last 32
Compute for 32 12
#Last+1 No
 fah = 212;
while (fah >= 32.0) {
cel = (5.0/9)*(fah-32);      Console.WriteLine("{0} {1}", fah, cel);
     fah = fah - 20;  }

29. PrintFahToCel() – The Complete Code
static void PrintFahToCel() {    double cel, fah;    Console.WriteLine("{0,12}{1,12}", "fahrenheit", "celcius");    Console.WriteLine("{0,12}{1,12}", " ", " ");    fah = 212.0;    while (fah >= 32.0) {         cel = (5.0 / 9) * (fah - 32);         Console.WriteLine("{0,12:f1}{1,12:f1}", fah, cel);         fah = fah - 20;    }    Console.WriteLine("{0,12}{1,12}", " ", " "); }
Please write the Main method by yourself.

30. Event-Controlled Loop
The loop continues to repeat until some event occurs, which breaks the loop conditon.
The number of times the loop will repeat is hard (or impossible) to predict before the loop starts.

31. Event-Controlled Loop - Pattern
Pre-loop setup
Condition (still holds?) F T
Some computations
Pre-loop setup;
while (Condition)
{   }
Some computations
Computation inside the loop must eventually trigger an event that breaks the loop condition.
Otherwise, the loop will repeat infinitely.

32. event-controlled loops
Example
An example on
event-controlled loops
ณ. บัดนาว!

33. Task: Score Average
We want to write a program to read all students’ scores and compute the average of them.
Suppose we don’t know in advance how many students there are. How can the program know that the input has ended?
One possible solution: use a special value called a sentinel to indicate “the end of data entry”
We’ll use -1 as the sentinel in this program.
The sentinel must be chosen so that it cannot be a normal input value.

34. Score Average – I/O Specification
Enter score (-1 to end): 75
Enter score (-1 to end): 94
Enter score (-1 to end): 97
Enter score (-1 to end): 88
Enter score (-1 to end): 70
Enter score (-1 to end): 64
Enter score (-1 to end): -1
The average of 6 score(s) is 81.33
Sample Run
Sample Run
Enter score (-1 to end): 64
Enter score (-1 to end): -1
The average of 1 score(s) is 64.00
Sample Run
Enter score (-1 to end): -1
No score entered

35. Score Average – Topmost Steps
The Main() method’s Algorithm:
Calls the method ReadScoresAndFindAverage() to
read all the scores, then
compute and return the number of input scores and the score average.
Print the number of input scores and the average. Also take care of the case when there are no scores entered.

36. Score Average – Topmost Steps
static void Main() {      double average; // the score average      int nscores; // the number of input scores     
ReadScoresAndFindAverage(out nscores, out average);
     if (nscores > 0)
         Console.WriteLine(
"The average of {0} score(s) is {1:f2}",
nscores, average);
     else          Console.WriteLine("No score entered");             
Console.ReadLine(); // you know why by now. }

37. ReadScoresAndFindAverage() – Design
The method ReadScoresAndFindAverage():
What has to be done by this method:
It has two output parameters, count and average, that will send back the number of input scores and the average, respectively.
It uses a loop to read a new score into the variable score repeatedly until -1 is entered.
Since average = (the sum of all scores)/count, it needs another variable sum to store the sum of all scores.
Each iteration of the loop must add the newly read score into sum.
Important variables

38. ReadScoresAndFindAverage() – Loop Design
Use an event-controlled loop:
The event that breaks the condition of the loop is when the input is -1.
Let’s say
The inputs are 20, 12, 15, -1
Devise the loop algorithm:
round
score
next round?
count
sum
(read)
Pre-loop 20 #1 20
Yes 1
(0)+20 12 #2 12
Yes 2
(0+20)+12 15 #3 15
Yes 3
( )+15 -1 #4 -1 No
So, the condition for doing another round is …………...
So, the loop exits
score != -1

39. ReadScoresAndFindAverage() – Loop Design
Check a boundary case:
The inputs are 20, -1
round
score
next round?
count
sum
(read)
Pre-loop 20 #1 20
Yes 1
(0)+20 -1 #2 -1 No
It works perfectly!
loop exits
Check a boundary case:
The input is only -1
round
score
next round?
count
sum
(read)
Pre-loop -1 #1 -1 No
loop exits

40. ReadScoresAndFindAverage() – From Design to Code
round
score
next round?
count
sum
(read)
Score
Pre-loop 20 #1
Yes 1 #2 12
(0)+20 #3 -1 No 2
(0+20)+12 15 #4 3
( )+15
count = 0; int sum = 0; int score = ReadInt("Enter score (-1 to end): ");
while (score != -1) {
   count = count + 1;     sum = sum + score;     score = ReadInt("Enter score (-1 to end): "); }

41. ReadScoresAndFindAverage() – Complete Code
static void ReadScoresAndFindAverage(out int count, out double average) {         count = 0;         int sum = 0;         int score = ReadInt("Enter score (-1 to end): ");         while (score != -1) {             count = count+1;             sum = sum+score;             score = ReadInt("Enter score (-1 to end): ");         }         if (count > 0)             average = (double)sum / count;         else             average = 0; }

42. Caveats: Infinite Loop
int i = 0;
int n = ReadInt("Enter a input n: "); while (i < n)
{   Console.WriteLine(i);   i = i - 1; }
What is the output if user enters 5?
i becomes -1, -2, -3,…
The infinite loop occurs when the condition of the loop is always true.

43. Next Topic ณ. บัดนาว! What if some computation
is to be executed repeatedly
but at least once?
ณ. บัดนาว!

44. Code for the CheckPassword() method
Task: Check Password
Write the method CheckPassword() that keeps asking for a password until the correct one is entered
Enter password: I hate coding
Enter password: Coding is hard
Enter password: In code we trust
Password correct
using System; class Program {   static void Main() {     CheckPassword("In code we trust");     Console.WriteLine(“Password correct");     Console.ReadKey(true);   }
  static void CheckPassword(string password) {
  } } 1: 2: 3: 4: 5: 6: 7: 8: 9: .
Code for the CheckPassword() method

45. CheckPassword() – Steps: version 1
Read input from user
Show a prompt
BEGIN
END
true
false
input != password?
We need to read the input once before we can check the condition.

46. CheckPassword() – Code: version 1
static void CheckPassword(string password)
{     string input;     Console.Write("Enter password: ");     input = Console.ReadLine();     while (input != password) {       Console.Write("Enter password: ");        input = Console.ReadLine();           } }
Code gets
repeated

47. Loop: do-while Statement
C# Syntax
Semantics do
statement;
while (condition);
condition
False
True
statement
Condition must be a boolean expression

48. A block of Statements within do-while
C# Syntax
Semantics
condition
False
True
statement do
while (condition);
{ statement; .
. statement; }    

49. CheckPassword() – Steps: version2
Use a do-while structure
Read input from user
Show a prompt
BEGIN
END
true
false
input != password

50. CheckPassword() – code: version 2
static void CheckPassword(string password)
{     string input;     do {       Console.Write("Enter password: ");        input = Console.ReadLine();     } while (input != password); }
Code becomes slimmer. No duplicates.

51. Compare two kinds of loops
int i = 0; while (i < 5)
{    Console.WriteLine(i);    i = i + 1; }
Console.WriteLine("->{0}", i); 1 2 3 4
->5
Output:
int i = 0; do
{    Console.WriteLine(i);    i = i + 1; } while (i < 5);
Console.WriteLine("->{0}", i); 1 2 3 4
->5
Output:

52. Compare two kinds of loops
int i = 5; while (i < 5)
{    Console.WriteLine(i);    i = i + 1; }
Console.WriteLine("->{0}", i);
Output:
->5
int i = 5; do
{    Console.WriteLine(i);    i = i + 1; } while (i < 5);
Console.WriteLine("->{0}", i);
Output: 5
->6

53. Conclusion Comparing the two loop structures:
Types of controlled loops
Counter-controlled loop
Event-controlled loop
while
do-while
Condition is checked before each iteration.
Condition is checked after each iteration.
Execute loop body zero or more times.
Execute loop body one or more times.
No ending semicolon if loop body is a block.
Always ending with a semicolon.