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LoopsLoopsExecute Blocks of Code Multiple TimesExecute Blocks of Code Multiple Times
Svetlin NakovSvetlin NakovTelerik Telerik
CorporationCorporationwww.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 is a control statement that
allows repeating execution of a allows repeating execution of a block of statementsblock of statements May execute a code block fixed May execute a code block fixed
number of timesnumber of times May execute a code block while May execute a code block while
given condition holdsgiven condition holds May execute a code block for each May execute a code block for each
member of a collectionmember of a collection Loops that never end are called an Loops that never end are called an
infinite loopsinfinite loops
Using Using while(…)while(…) LoopLoopRepeating a Statement Repeating a Statement
While Given Condition While Given Condition HoldsHolds
How To Use While How To Use While Loop?Loop?
The simplest and most frequently The simplest and most frequently used loopused 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 While Loop – How It Works?Works?
truetrue
conditioncondition
statementstatement
falsfalsee
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 Calculate and print the sum of the
first N natural numbersfirst N natural 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 Calculating Sum 1..N1..N
Live DemoLive Demo
Prime Number – Prime Number – ExampleExample
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 Checking Whether a Whether a Number Is Number Is
PrimePrimeLive 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 Calculating FactorialFactorial
Live 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 The block of statements is repeatedrepeated While the boolean loop condition While the boolean loop condition
holdsholds 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
falsfalsee
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 – Example – Example
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 Don't forget to add reference to add reference to System.Numerics.dSystem.Numerics.d
llll..
Factorial (Factorial (dodo ... ... whilewhile))Live DemoLive Demo
Product[N..M] – Product[N..M] – ExampleExample
Calculating the product of all Calculating the product of all numbers in the interval [n..m]: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());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 Product of the Numbers in the Numbers in the Interval [n..m]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 The Initialization ExpressionExpression
Executed once, just before the loop is Executed once, just before the loop is enteredentered Like it is out of the loop, before itLike it is out of the loop, before it
Usually used to declare a counter Usually used to declare a counter variablevariable
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 Evaluated before each iteration of the loopthe 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 loop is finishedthe body of the loop is finished
Usually used to update the Usually used to update the countercounter
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 – Loop – ExampleExample
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 – Loop – ExampleExample
31
Complex Complex forfor-loops could have -loops could have several counter variables:several counter 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, Console.WriteLine("i={0}, sum={1}", i, sum);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 OperatorOperator
continuecontinue operator ends the iteration of operator ends the iteration of the inner-most loopthe inner-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 OperatorOperator
Live 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 Iterates over all elements of a collectioncollection The The elementelement is the loop variable that is the loop variable that
takes sequentially all collection valuestakes sequentially all collection values The The collectioncollection can be list, array or can be list, array or
other group of elements of the same other group of elements of the same typetype
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 The above loop iterates of the array of daysarray of days The variable The variable dayday takes all its takes all its
valuesvalues
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 nested
looploop 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, Primes in Range [n, m]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 loops jumps to the test expressionthe test expression
In In forfor loops jumps to the update loops jumps to the update expressionexpression
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 ! (it is considered harmful)harmful)
C# Jump Statements – C# Jump Statements – ExampleExample
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:
LabeLabell
Loops – More Loops – More ExamplesExamples
Nested Loops – Nested Loops – ExamplesExamples
Print all four digit numbers in Print all four digit numbers in format 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}Console.WriteLine("{0}{1}{2}{3}",{3}", a, b, c, d);a, b, c, d);}}
Can you Can you improve this improve this algorithm to algorithm to use 3 loops use 3 loops
only?only?
Happy NumbersHappy NumbersLive DemoLive Demo
Nested Loops – Nested Loops – ExamplesExamples
Print all combinations from TOTO Print all combinations from TOTO 6/496/49static 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} Console.WriteLine("{0} {1} {2} {3} {4} {5}",{5}", i1, i2, i3, i4, i5, i6);i1, i2, i3, i4, i5, i6);}}
Warning: Warning: execution of execution of
this code this code could take could take too long too long
time.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 Nested loops can be used to implement more complex logicimplement more complex logic
The operators The operators continuecontinue,, breakbreak & & gotogoto can control the loop execution can control the loop execution
QuestionsQuestions??
LoopsLoops
http://academy.telerik.com
ExercisesExercises1.1. Write a program that prints all the Write a program that prints all the
numbers from 1 to N.numbers from 1 to N.
2.2. Write a program that prints all the Write a program that prints all the numbers from 1 to N, that are not numbers from 1 to N, that are not divisible by 3 and 7 at the same time.divisible by 3 and 7 at the same time.
3.3. Write a program that reads from the Write a program that reads from the console a sequence of N integer console a sequence of N integer numbers and returns the minimal and numbers and returns the minimal and maximal of them.maximal of them.
4.4. Write a program that calculates N!/K! for Write a program that calculates N!/K! for given N and K (1<N<K).given N and K (1<N<K).
5.5. Write a program that calculates N!*K! / Write a program that calculates N!*K! / (K-N)! for given N and K (1<N<K).(K-N)! for given N and K (1<N<K).
Exercises (2)Exercises (2)6.6. 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
7.7. 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 calculates the sum of the first N members of the sequence of Fibonacci: the sequence of Fibonacci: 0, 1, 1, 2, 3, 5, 8, 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, …13, 21, 34, 55, 89, 144, 233, 377, …
Each member of the Fibonacci sequence (except Each member of the Fibonacci sequence (except the first two) is a sum of the previous two the first two) is a sum of the previous two members.members.
6.6. Write a program that calculates the greatest Write a program that calculates the greatest common divisor (GCD) of given two numbers. common divisor (GCD) of given two numbers. Use the Euclidean algorithm (find it in Use the Euclidean algorithm (find it in Internet).Internet).
Exercises (3)Exercises (3)
10.10. In the combinatorial mathematics, the In the combinatorial mathematics, the Catalan numbers are calculated by the Catalan numbers are calculated by the following formula:following formula:
Write a program to calculate the NWrite a program to calculate the Nthth Catalan number by given N.Catalan number by given N.
11.11. Write a program that prints all possible Write a program that prints all possible cards from a standard deck of 52 cards cards from a standard deck of 52 cards (without jokers). The cards should be (without jokers). The cards should be printed with their English names. Use printed with their English names. Use nested nested forfor loops and loops and switch-caseswitch-case..
Exercises (4)Exercises (4)
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12.12. Write a program that reads from the Write a program that reads from the console a positive integer number N console a positive integer number N (N < 20) and outputs a matrix like the (N < 20) and outputs a matrix like the following: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)
13.13. * Write a program that calculates for * Write a program that calculates for given N how many trailing zeros given N how many trailing zeros present at the end of the number N!. present at the end of the number N!. Examples: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 Does your program work for N = 50 000?000?
Hint: The trailing zeros in N! are equal Hint: The trailing zeros in N! are equal to the number of its prime divisors of to the number of its prime divisors of value value 55. Think why!. Think why!
61
Exercises (6)Exercises (6)
14.14. * Write a program that reads a positive * Write a program that reads a positive integer number N (N < 20) from integer number N (N < 20) from console and outputs in the console the console and outputs in the console the numbers 1 ... N numbers arranged as a 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