LoopsLoopsExecute Blocks of Code Multiple TimesExecute Blocks of Code Multiple Times

Svetlin NakovSvetlin NakovTelerik CorporationTelerik Corporationwww.telerik.comwww.telerik.com

Table of ContentsTable of Contents

What is a Loop?What is a Loop? Loops in C#Loops in C#

whilewhile loops loops

dodo … … whilewhile loops loops

forfor loops loops

foreachforeach loops loops

Nested loopsNested loops

What Is Loop?What Is Loop? A A looploop is a control statement that allows is 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 condition May execute a code block while given condition holdsholds

May execute a code block for each member of a May execute a code block for each member of a collectioncollection

Loops that never end are called an Loops that never end are called an infinite infinite loopsloops

Using Using while(…)while(…) Loop LoopRepeating a Statement While Repeating a Statement While

Given Condition HoldsGiven Condition Holds

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 of Returns a boolean result of truetrue or or falsefalse

Also called Also called loop conditionloop condition

while (condition)while (condition){{ statements;statements;}}

While Loop – How It Works?While Loop – How It Works?

truetrue

conditioncondition

statementstatement

falsefalse

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++;}}

while(…)while(…)ExamplesExamples

Sum 1..N – ExampleSum 1..N – Example Calculate and print the sum of the first N Calculate and print the sum of the first N

natural numbersnatural numbersConsole.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);

Calculating Sum 1..NCalculating Sum 1..NLive DemoLive Demo

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);

Checking Whether a Checking Whether a Number Is PrimeNumber Is Prime

Live DemoLive Demo

Using Using breakbreak Operator Operator breakbreak operator exits the inner-most loop operator 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); }}

Calculating FactorialCalculating FactorialLive DemoLive Demo

dodo { { … … }} while while ((……))

LoopLoop

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);

Do-While StatementDo-While Statement

truetrue

conditioncondition

statementstatement

falsefalse

dodo { { … … }} while while ((……))

ExamplesExamples

Factorial – ExampleFactorial – Example

Calculating N factorialCalculating N factorialstatic 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);}}

Factorial with Factorial with BigIntegerBigInteger – – ExampleExample

Calculating N factorialCalculating N factorial with with 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 add Don't forget to add reference to reference to

System.Numerics.dlSystem.Numerics.dlll..

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

Product[N..M] – ExampleProduct[N..M] – Example

Calculating the product of all numbers in the Calculating 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);

Product of the Numbers Product of the Numbers in the Interval [n..m]in the Interval [n..m]

Live DemoLive Demo

forfor Loops Loops

For LoopsFor Loops

The typical The 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;}}

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

The Test ExpressionThe Test Expression

Evaluated before each iteration of the loopEvaluated before each iteration of the loop

If If truetrue, the loop body is executed, the loop body is executed

If If falsefalse, the loop body is skipped, the loop body is skipped

Used as a Used 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

The Update ExpressionThe Update Expression

Executed at each iteration Executed at each iteration afterafter the body of the 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

forfor Loop LoopExamplesExamples

Simple Simple forfor Loop – Example Loop – Example A simple for-loop to print the numbers A 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;}}

Complex Complex forfor Loop – Example Loop – Example

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Complex Complex 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=1i=2, sum=3i=2, sum=3i=4, sum=7i=4, sum=7i=8, sum=15i=8, sum=15......

Result:Result:

For LoopsFor LoopsLive DemoLive Demo

N^M – ExampleN^M – Example Calculating Calculating nn to power to 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);}}

Calculating N^MCalculating N^MLive DemoLive Demo

Using Using continuecontinue Operator Operator continuecontinue operator ends the iteration of the operator ends the iteration of the

inner-most loopinner-most loop Example: sum all odd numbers in Example: sum all odd numbers in [1,[1, n]n]

that are not divisors of that 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);

Using Using continuecontinue Operator OperatorLive DemoLive Demo

foreachforeach Loop LoopIteration over a CollectionIteration over a Collection

For LoopsFor Loops

The typical The typical foreachforeach loop syntax is: loop syntax is:

Iterates over all elements of a collectionIterates over all elements of a collection

The The elementelement is the loop variable that takes is the loop variable that takes sequentially all collection valuessequentially all collection values

The The collectioncollection can be list, array or other can 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;}}

foreachforeach Loop – Example Loop – Example

Example of Example 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 variable The variable dayday takes all its values takes all its values

foreachforeach Loop LoopLive DemoLive Demo

Nested LoopsNested LoopsUsing Loops Inside a LoopUsing Loops Inside a Loop

What Is Nested Loop?What Is Nested Loop? A composition of loops is called a A 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; }} … …} }

Nested LoopsNested LoopsExamplesExamples

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();}}

TriangleTriangleLive DemoLive Demo

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); }}}}

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

C# Jump StatementsC# Jump Statements Jump statements are:Jump statements are:

breakbreak, , continuecontinue, , gotogoto

How How continuecontinue woks? woks?

In In whilewhile and and do-whiledo-while loops jumps to the loops jumps to the test expressiontest expression

In In forfor loops jumps to the update expression loops 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 label with a label

Avoid using Avoid using gotogoto! (it is considered harmful)! (it is considered harmful)

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

Loops – More ExamplesLoops – More Examples

Nested Loops – ExamplesNested Loops – Examples Print all four digit numbers in format Print all four digit numbers in format ABCDABCD

such that such 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 improve Can you improve this algorithm to this algorithm to use 3 loops only?use 3 loops only?

Happy NumbersHappy NumbersLive DemoLive Demo

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 this execution of this code could take code could take too long time.too long time.

TOTO 6/49TOTO 6/49Live DemoLive Demo

SummarySummary C# supports four types of loops:C# supports four types of loops:

whilewhile

do-whiledo-while

forfor loops loops

foreachforeach loops loops

Nested loops can be used to implement more Nested loops can be used to implement more complex logiccomplex logic

The operators The operators continuecontinue,, breakbreak & & gotogoto can can control the loop executioncontrol the loop execution

Questions?Questions?

LoopsLoops

http://academy.telerik.com

ExercisesExercises1.1. Write a program that prints all the numbers from 1 Write a program that prints all the numbers from 1

to N.to N.

2.2. Write a program that prints all the numbers from 1 Write a program that prints all the numbers from 1 to N, that are not divisible by 3 and 7 at the same to N, that are not divisible by 3 and 7 at the same time.time.

3.3. Write a program that reads from the console a Write a program that reads from the console a sequence of N integer numbers and returns the sequence 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 N Write 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)! for Write a program that calculates N!*K! / (K-N)! for given N and K (1<N<K).given N and K (1<N<K).

Exercises (2)Exercises (2)1.1. Write a program that, for a given two integer Write a program that, for a given two integer

numbers numbers NN and and XX, calculates the sum, calculates the sumS = 1 + 1!S = 1 + 1!/X/X + + 2!/X2!/X22 + + …… + + N!/XN!/XNN

2.2. Write a program that reads a number N and Write a program that reads a number N and calculates the sum of the first N members of the calculates 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 the Each 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.

4.4. Write a program that calculates the greatest Write a program that calculates the greatest common divisor (GCD) of given two numbers. Use common divisor (GCD) of given two numbers. Use the Euclidean algorithm (find it in Internet).the Euclidean algorithm (find it in Internet).

Exercises (3)Exercises (3)

1.1. In the combinatorial mathematics, the Catalan In 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 Catalan number by given N.number by given N.

5.5. Write a program that prints all possible cards from a Write a program that prints all possible cards from a standard deck of 52 cards (without jokers). The standard deck of 52 cards (without jokers). The cards should be printed with their English names. cards should be printed with their English names. Use nested Use nested forfor loops and loops and switch-caseswitch-case..

Exercises (4)Exercises (4)

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1.1. Write a program that reads from the console a Write a program that reads from the console a positive integer number N (N < 20) and outputs a positive 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

Exercises (5)Exercises (5)

1.1. * 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 the many trailing zeros present at the end of the number N!. Examples:number N!. Examples:

N = 10 N = 10 N! = 36288 N! = 362880000 2 2

N = 20 N = 20 N! = 243290200817664 N! = 24329020081766400000000 4 4

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 the Hint: The trailing zeros in N! are equal to the number of its prime divisors of value number of its prime divisors of value 55. Think why!. Think why!

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Exercises (6)Exercises (6)

1.1. * 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 the number N (N < 20) from console and outputs in the console the numbers 1 ... N numbers arranged as a console the numbers 1 ... N numbers arranged as a spiral.spiral.

Example for N = 4Example for N = 4

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11 22 33 44

1212 1313 1414 55

1111 1616 1515 66

1010 99 88 77