Loops

1. LoopsLoops
Execute Blocks of Code MultipleTimesExecute Blocks of Code MultipleTimes
Svetlin NakovSvetlin Nakov
Telerik CorporationTelerik Corporation
www.telerik.comwww.telerik.com

2. Table of ContentsTable of Contents
 What is a Loop?What is a Loop?
 Loops in C#Loops in C#
 whilewhile loopsloops
 dodo …… whilewhile loopsloops
 forfor loopsloops
 foreachforeach loopsloops
 Nested loopsNested loops

3. What Is Loop?What Is Loop?
 AA looploop is a control statement that allowsis a control statement that allows
repeating execution of a block of statementsrepeating execution of a block of statements
May execute a code block fixed number of timesMay execute a code block fixed number of times
May execute a code block while given conditionMay execute a code block while given condition
holdsholds
May execute a code block for each member of aMay execute a code block for each member of a
collectioncollection
 Loops that never end are called anLoops that never end are called an infiniteinfinite
loopsloops

4. UsingUsing while(…)while(…) LoopLoop
Repeating a Statement WhileRepeating a Statement While
Given Condition HoldsGiven Condition Holds

5. How To Use While Loop?How To Use While Loop?
 The simplest and most frequently used loopThe simplest and most frequently used loop
 The repeat conditionThe repeat condition
Returns a boolean result ofReturns a boolean result of truetrue oror falsefalse
Also calledAlso called loop conditionloop condition
while (condition)while (condition)
{{
statements;statements;
}}

6. While Loop – How It Works?While Loop – How It Works?
truetrue
conditioncondition
statementstatement
falsefalse

7. While Loop – ExampleWhile Loop – Example
int counter = 0;int counter = 0;
while (counter < 10)while (counter < 10)
{{
Console.WriteLine("Number : {0}", counter);Console.WriteLine("Number : {0}", counter);
counter++;counter++;
}}

8. while(…)while(…)
ExamplesExamples

9. Sum 1..N – ExampleSum 1..N – Example
 Calculate and print the sum of the first NCalculate and print the sum of the first N
natural numbersnatural numbers
Console.Write("n = ");Console.Write("n = ");
int n = int.Parse(Console.ReadLine());int n = int.Parse(Console.ReadLine());
int number = 1;int number = 1;
int sum = 1;int sum = 1;
Console.Write("The sum 1");Console.Write("The sum 1");
while (number < n)while (number < n)
{{
number++;number++;
sum += number ;sum += number ;
Console.Write("+{0}", number);Console.Write("+{0}", number);
}}
Console.WriteLine(" = {0}", sum);Console.WriteLine(" = {0}", sum);

10. Calculating Sum 1..NCalculating Sum 1..N
Live DemoLive Demo

11. Prime Number – ExamplePrime Number – Example
 Checking whether a number is prime or notChecking whether a number is prime or not
Console.Write("Enter a positive integer number: ");Console.Write("Enter a positive integer number: ");
uint number = uint.Parse(Console.ReadLine());uint number = uint.Parse(Console.ReadLine());
uint divider = 2;uint divider = 2;
uint maxDivider = (uint) Math.Sqrt(number);uint maxDivider = (uint) Math.Sqrt(number);
bool prime = true;bool prime = true;
while (prime && (divider <= maxDivider))while (prime && (divider <= maxDivider))
{{
if (number % divider == 0)if (number % divider == 0)
{{
prime = false;prime = false;
}}
divider++;divider++;
}}
Console.WriteLine("Prime? {0}", prime);Console.WriteLine("Prime? {0}", prime);

12. Checking Whether aChecking Whether a
Number Is PrimeNumber Is Prime
Live DemoLive Demo

13. UsingUsing breakbreak OperatorOperator
 breakbreak operator exits the inner-most loopoperator exits the inner-most loop
static void Main()static void Main()
{{
int n = Convert.ToInt32(Console.ReadLine());int n = Convert.ToInt32(Console.ReadLine());
// Calculate n! = 1 * 2 * ... * n// Calculate n! = 1 * 2 * ... * n
int result = 1;int result = 1;
while (true)while (true)
{{
if(n == 1)if(n == 1)
break;break;
result *= n;result *= n;
n--;n--;
}}
Console.WriteLine("n! = " + result);Console.WriteLine("n! = " + result);
}}

14. Calculating FactorialCalculating Factorial
Live DemoLive Demo

15. dodo {{ …… }}
whilewhile ((……))
LoopLoop

16. Using Do-While LoopUsing Do-While Loop
 Another loop structure is:Another loop structure is:
 The block of statements is repeatedThe block of statements is repeated
While the boolean loop condition holdsWhile the boolean loop condition holds
 The loop is executed at least onceThe loop is executed at least once
dodo
{{
statements;statements;
}}
while (condition);while (condition);

17. Do-While StatementDo-While Statement
truetrue
conditioncondition
statementstatement
falsefalse

18. dodo {{ …… }}
whilewhile ((……))
ExamplesExamples

19. Factorial – ExampleFactorial – Example
 Calculating N factorialCalculating N factorial
static void Main()static void Main()
{{
int n = Convert.ToInt32(Console.ReadLine());int n = Convert.ToInt32(Console.ReadLine());
int factorial = 1;int factorial = 1;
dodo
{{
factorial *= n;factorial *= n;
n--;n--;
}}
while (n > 0);while (n > 0);
Console.WriteLine("n! = " + factorial);Console.WriteLine("n! = " + factorial);
}}

20. Factorial withFactorial with BigIntegerBigInteger ––
ExampleExample
 Calculating N factorialCalculating N factorial withwith BigIntegerBigInteger
using System.Numerics;using System.Numerics;
static void Main()static void Main()
{{
int n = 1000;int n = 1000;
BigIntegerBigInteger factorial = 1;factorial = 1;
dodo
{{
factorial *= n;factorial *= n;
n--;n--;
}}
while (n > 0);while (n > 0);
Console.WriteLine("n! = " + factorial);Console.WriteLine("n! = " + factorial);
}}
Don't forget to addDon't forget to add
reference toreference to
System.Numerics.dlSystem.Numerics.dl
ll..

21. Factorial (Factorial (dodo ...... whilewhile))
Live DemoLive Demo

22. Product[N..M] – ExampleProduct[N..M] – Example
 Calculating the product of all numbers in theCalculating the product of all numbers in the
interval [n..m]:interval [n..m]:
int n = int.Parse(Console.ReadLine());int n = int.Parse(Console.ReadLine());
int m = int.Parse(Console.ReadLine());int m = int.Parse(Console.ReadLine());
int number = n;int number = n;
decimal product = 1;decimal product = 1;
dodo
{{
product *= number;product *= number;
number++;number++;
}}
while (number <= m);while (number <= m);
Console.WriteLine("product[n..m] = " + product);Console.WriteLine("product[n..m] = " + product);

23. Product of the NumbersProduct of the Numbers
in the Interval [n..m]in the Interval [n..m]
Live DemoLive Demo

24. forfor LoopsLoops

25. For LoopsFor Loops
 The typicalThe typical forfor loop syntax is:loop syntax is:
 Consists ofConsists of
Initialization statementInitialization statement
Boolean test expressionBoolean test expression
Update statementUpdate statement
Loop body blockLoop body block
for (initialization; test; update)for (initialization; test; update)
{{
statements;statements;
}}

26. The Initialization ExpressionThe Initialization Expression
 Executed once, just before the loop is enteredExecuted once, just before the loop is entered
Like it is out of the loop, before itLike it is out of the loop, before it
 Usually used to declare a counter variableUsually used to declare a counter variable
for (for (int number = 0int number = 0; ...; ...); ...; ...)
{{
// Can use number here// Can use number here
}}
// Cannot use number here// Cannot use number here

27. The Test ExpressionThe Test Expression
 Evaluated before each iteration of the loopEvaluated before each iteration of the loop
IfIf truetrue, the loop body is executed, the loop body is executed
IfIf falsefalse, the loop body is skipped, the loop body is skipped
 Used as aUsed as a loop conditionloop condition
for (int number = 0;for (int number = 0; number < 10number < 10; ...); ...)
{{
// Can use number here// Can use number here
}}
// Cannot use number here// Cannot use number here

28. The Update ExpressionThe Update Expression
 Executed at each iterationExecuted at each iteration afterafter the body ofthe body of
the loop is finishedthe loop is finished
 Usually used to update the counterUsually used to update the counter
for (int number = 0; number < 10;for (int number = 0; number < 10; number++number++))
{{
// Can use number here// Can use number here
}}
// Cannot use number here// Cannot use number here

29. forfor LoopLoop
ExamplesExamples

30. SimpleSimple forfor Loop – ExampleLoop – Example
 A simple for-loop to print the numbersA simple for-loop to print the numbers 00……99::
for (int number = 0; number < 10; number++)for (int number = 0; number < 10; number++)
{{
Console.Write(number + " ");Console.Write(number + " ");
}}
 A simple for-loop to calculate n!:A simple for-loop to calculate n!:
decimal factorial = 1;decimal factorial = 1;
for (int i = 1; i <= n; i++)for (int i = 1; i <= n; i++)
{{
factorial *= i;factorial *= i;
}}

31. ComplexComplex forfor Loop – ExampleLoop – Example
31
 ComplexComplex forfor-loops could have several counter-loops could have several counter
variables:variables:
for (int i=1, sum=1; i<=128; i=i*2, sum+=i)for (int i=1, sum=1; i<=128; i=i*2, sum+=i)
{{
Console.WriteLine("i={0}, sum={1}", i, sum);Console.WriteLine("i={0}, sum={1}", i, sum);
}}
i=1, sum=1i=1, sum=1
i=2, sum=3i=2, sum=3
i=4, sum=7i=4, sum=7
i=8, sum=15i=8, sum=15
......
 Result:Result:

32. For LoopsFor Loops
Live DemoLive Demo

33. N^M – ExampleN^M – Example
 CalculatingCalculating nn to powerto power mm (denoted as(denoted as n^mn^m):):
static void Main()static void Main()
{{
int n = int.Parse(Console.ReadLine());int n = int.Parse(Console.ReadLine());
int m = int.Parse(Console.ReadLine());int m = int.Parse(Console.ReadLine());
decimal result = 1;decimal result = 1;
for (int i=0; i<m; i++)for (int i=0; i<m; i++)
{{
result *= n;result *= n;
}}
Console.WriteLine("n^m = " + result);Console.WriteLine("n^m = " + result);
}}

34. Calculating N^MCalculating N^M
Live DemoLive Demo

35. UsingUsing continuecontinue OperatorOperator
 continuecontinue operator ends the iteration of theoperator ends the iteration of the
inner-most loopinner-most loop
 Example: sum all odd numbers inExample: sum all odd numbers in [1,[1, n]n]
that are not divisors ofthat are not divisors of 77::
int n = int.Parse(Console.ReadLine());int n = int.Parse(Console.ReadLine());
int sum = 0;int sum = 0;
for (int i = 1; i <= n; i += 2)for (int i = 1; i <= n; i += 2)
{{
if (i % 7 == 0)if (i % 7 == 0)
{{
continue;continue;
}}
sum += i;sum += i;
}}
Console.WriteLine("sum = {0}", sum);Console.WriteLine("sum = {0}", sum);

36. UsingUsing continuecontinue OperatorOperator
Live DemoLive Demo

37. foreachforeach LoopLoop
Iteration over a CollectionIteration over a Collection

38. For LoopsFor Loops
 The typicalThe typical foreachforeach loop syntax is:loop syntax is:
 Iterates over all elements of a collectionIterates over all elements of a collection
TheThe elementelement is the loop variable that takesis the loop variable that takes
sequentially all collection valuessequentially all collection values
TheThe collectioncollection can be list, array or othercan be list, array or other
group of elements of the same typegroup of elements of the same type
foreach (Type element in collection)foreach (Type element in collection)
{{
statements;statements;
}}

39. foreachforeach Loop – ExampleLoop – Example
 Example ofExample of foreachforeach loop:loop:
string[] days = new string[] {string[] days = new string[] {
"Monday", "Tuesday", "Wednesday", "Thursday","Monday", "Tuesday", "Wednesday", "Thursday",
"Friday", "Saturday", "Sunday" };"Friday", "Saturday", "Sunday" };
foreach (String day in days)foreach (String day in days)
{{
Console.WriteLine(day);Console.WriteLine(day);
}}
 The above loop iterates of the array of daysThe above loop iterates of the array of days
 The variableThe variable dayday takes all its valuestakes all its values

40. foreachforeach LoopLoop
Live DemoLive Demo

41. Nested LoopsNested Loops
Using Loops Inside a LoopUsing Loops Inside a Loop

42. What Is Nested Loop?What Is Nested Loop?
 A composition of loops is called aA composition of loops is called a nested loopnested loop
A loop inside another loopA loop inside another loop
 Example:Example:
for (initialization; test; update)for (initialization; test; update)
{{
for (initialization; test; update)for (initialization; test; update)
{{
statements;statements;
}}
……
}}

43. Nested LoopsNested Loops
ExamplesExamples

44. Triangle – ExampleTriangle – Example
 Print the following triangle:Print the following triangle:
11
1 21 2
……
1 2 3 ... n1 2 3 ... n
int n = int.Parse(Console.ReadLine());int n = int.Parse(Console.ReadLine());
for(int row = 1; row <= n; row++)for(int row = 1; row <= n; row++)
{{
for(int column = 1; column <= row; column++)for(int column = 1; column <= row; column++)
{{
Console.Write("{0} ", column);Console.Write("{0} ", column);
}}
Console.WriteLine();Console.WriteLine();
}}

45. TriangleTriangle
Live DemoLive Demo

46. Primes[N, M]Primes[N, M] – Example– Example
 Print all prime numbers in the interval [n, m]:Print all prime numbers in the interval [n, m]:
int n = int.Parse(Console.ReadLine());int n = int.Parse(Console.ReadLine());
int m = int.Parse(Console.ReadLine());int m = int.Parse(Console.ReadLine());
for (int number = n; number <= m; number++)for (int number = n; number <= m; number++)
{{
bool prime = true;bool prime = true;
int divider = 2;int divider = 2;
int maxDivider = Math.Sqrt(num);int maxDivider = Math.Sqrt(num);
while (divider <= maxDivider)while (divider <= maxDivider)
{{
if (number % divider == 0)if (number % divider == 0)
{{
prime = false;prime = false;
break;break;
}}
divider++;divider++;
}}
if (prime)if (prime)
{{
Console.Write("{0} ", number);Console.Write("{0} ", number);
}}
}}

47. Primes in Range [n, m]Primes in Range [n, m]
Live DemoLive Demo

48. C# Jump StatementsC# Jump Statements
 Jump statements are:Jump statements are:
breakbreak,, continuecontinue,, gotogoto
 HowHow continuecontinue woks?woks?
InIn whilewhile andand do-whiledo-while loops jumps to theloops jumps to the
test expressiontest expression
InIn forfor loops jumps to the update expressionloops jumps to the update expression
 To exit an inner loop useTo exit an inner loop use breakbreak
 To exit outer loops useTo exit outer loops use gotogoto with a labelwith a label
Avoid usingAvoid using gotogoto! (it is considered harmful)! (it is considered harmful)

49. C# Jump Statements – ExampleC# Jump Statements – Example
int outerCounter = 0;int outerCounter = 0;
for (int outer = 0; outer < 10; outer++)for (int outer = 0; outer < 10; outer++)
{{
for (int inner = 0; inner < 10; inner++)for (int inner = 0; inner < 10; inner++)
{{
if (inner % 3 == 0)if (inner % 3 == 0)
continue;continue;
if (outer == 7)if (outer == 7)
break;break;
if (inner + outer > 9)if (inner + outer > 9)
goto breakOut;goto breakOut;
}}
outerCounter++;outerCounter++;
}}
breakOut:breakOut:
LabelLabel

50. Loops – More ExamplesLoops – More Examples

51. Nested Loops – ExamplesNested Loops – Examples
 Print all four digit numbers in formatPrint all four digit numbers in format ABCDABCD
such thatsuch that AA++BB == CC++DD (known as happy numbers)(known as happy numbers)
static void Main()static void Main()
{{
for (int a =1 ; a <= 9; a++)for (int a =1 ; a <= 9; a++)
for (int b = 0; b <= 9; b++)for (int b = 0; b <= 9; b++)
for (int c = 0; c <= 9; c++)for (int c = 0; c <= 9; c++)
for (int d = 0; d <= 9; d++)for (int d = 0; d <= 9; d++)
if (a + b == c + d)if (a + b == c + d)
Console.WriteLine("{0}{1}{2}{3}",Console.WriteLine("{0}{1}{2}{3}",
a, b, c, d);a, b, c, d);
}}
Can you improveCan you improve
this algorithm tothis algorithm to
use 3 loops only?use 3 loops only?

52. Happy NumbersHappy Numbers
Live DemoLive Demo

53. Nested Loops – ExamplesNested Loops – Examples
 Print all combinations from TOTO 6/49Print all combinations from TOTO 6/49
static void Main()static void Main()
{{
int i1, i2, i3, i4, i5, i6;int i1, i2, i3, i4, i5, i6;
for (i1 = 1; i1 <= 44; i1++)for (i1 = 1; i1 <= 44; i1++)
for (i2 = i1 + 1; i2 <= 45; i2++)for (i2 = i1 + 1; i2 <= 45; i2++)
for (i3 = i2 + 1; i3 <= 46; i3++)for (i3 = i2 + 1; i3 <= 46; i3++)
for (i4 = i3 + 1; i4 <= 47; i4++)for (i4 = i3 + 1; i4 <= 47; i4++)
for (i5 = i4 + 1; i5 <= 48; i5++)for (i5 = i4 + 1; i5 <= 48; i5++)
for (i6 = i5 + 1; i6 <= 49; i6++)for (i6 = i5 + 1; i6 <= 49; i6++)
Console.WriteLine("{0} {1} {2} {3} {4} {5}",Console.WriteLine("{0} {1} {2} {3} {4} {5}",
i1, i2, i3, i4, i5, i6);i1, i2, i3, i4, i5, i6);
}}
Warning:Warning:
execution of thisexecution of this
code could takecode could take
too long time.too long time.

54. TOTO 6/49TOTO 6/49
Live DemoLive Demo

55. SummarySummary
 C# supports four types of loops:C# supports four types of loops:
whilewhile
do-whiledo-while
forfor loopsloops
foreachforeach loopsloops
 Nested loops can be used to implement moreNested loops can be used to implement more
complex logiccomplex logic
 The operatorsThe operators continuecontinue,, breakbreak && gotogoto cancan
control the loop executioncontrol the loop execution

56. Questions?Questions?
LoopsLoops
http://academy.telerik.com

57. ExercisesExercises
1.1. Write a program that prints all the numbers from 1Write a program that prints all the numbers from 1
to N.to N.
2.2. Write a program that prints all the numbers from 1Write a program that prints all the numbers from 1
to N, that are not divisible by 3 and 7 at the sameto N, that are not divisible by 3 and 7 at the same
time.time.
3.3. Write a program that reads from the console aWrite a program that reads from the console a
sequence of N integer numbers and returns thesequence of N integer numbers and returns the
minimal and maximal of them.minimal and maximal of them.
4.4. Write a program that calculates N!/K! for given NWrite a program that calculates N!/K! for given N
and K (1<N<K).and K (1<N<K).
5.5. Write a program that calculates N!*K! / (K-N)! forWrite a program that calculates N!*K! / (K-N)! for
given N and K (1<N<K).given N and K (1<N<K).

58. Exercises (2)Exercises (2)
6.6. Write a program that, for a given two integerWrite a program that, for a given two integer
numbersnumbers NN andand XX, calculates the sum, calculates the sum
S = 1 + 1!S = 1 + 1!/X/X ++ 2!/X2!/X22
++ …… ++ N!/XN!/XNN
7.7. Write a program that reads a number N andWrite a program that reads a number N and
calculates the sum of the first N members of thecalculates the sum of the first N members of the
sequence of Fibonacci:sequence of Fibonacci: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34,0, 1, 1, 2, 3, 5, 8, 13, 21, 34,
55, 89, 144, 233, 377, …55, 89, 144, 233, 377, …
Each member of the Fibonacci sequence (except theEach member of the Fibonacci sequence (except the
first two) is a sum of the previous two members.first two) is a sum of the previous two members.
6.6. Write a program that calculates the greatestWrite a program that calculates the greatest
common divisor (GCD) of given two numbers. Usecommon divisor (GCD) of given two numbers. Use
the Euclidean algorithm (find it in Internet).the Euclidean algorithm (find it in Internet).

59. Exercises (3)Exercises (3)
10.10. In the combinatorial mathematics, the CatalanIn the combinatorial mathematics, the Catalan
numbers are calculated by the following formula:numbers are calculated by the following formula:
Write a program to calculate the NWrite a program to calculate the Nthth
Catalan numberCatalan number
by given N.by given N.
11.11. Write a program that prints all possible cards from aWrite a program that prints all possible cards from a
standard deck of 52 cards (without jokers). Thestandard deck of 52 cards (without jokers). The
cards should be printed with their English names.cards should be printed with their English names.
Use nestedUse nested forfor loops andloops and switch-caseswitch-case..

60. Exercises (4)Exercises (4)
60
12.12. Write a program that reads from the console aWrite a program that reads from the console a
positive integer number N (N < 20) and outputs apositive integer number N (N < 20) and outputs a
matrix like the following:matrix like the following:
N = 3N = 3 N = 4N = 4
11 22 33
22 33 44
33 44 55
11 22 33 44
22 33 44 55
33 44 55 66
44 55 66 77

61. Exercises (5)Exercises (5)
13.13. * Write a program that calculates for given N how* Write a program that calculates for given N how
many trailing zeros present at the end of themany trailing zeros present at the end of the
number N!. Examples:number N!. Examples:
N = 10N = 10  N! = 36288N! = 362880000  22
N = 20N = 20  N! = 243290200817664N! = 24329020081766400000000  44
Does your program work for N = 50 000?Does your program work for N = 50 000?
Hint: The trailing zeros in N! are equal to theHint: The trailing zeros in N! are equal to the
number of its prime divisors of valuenumber of its prime divisors of value 55. Think why!. Think why!
61

62. Exercises (6)Exercises (6)
14.14. * Write a program that reads a positive integer* Write a program that reads a positive integer
number N (N < 20) from console and outputs in thenumber N (N < 20) from console and outputs in the
console the numbers 1 ... N numbers arranged as aconsole the numbers 1 ... N numbers arranged as a
spiral.spiral.
Example for N = 4Example for N = 4
62
11 22 33 44
1212 1313 1414 55
1111 1616 1515 66
1010 99 88 77