09. Methods

MethodsMethodsSubroutines in Computer ProgrammingSubroutines in Computer Programming

Svetlin NakovSvetlin NakovTelerik Telerik

CorporationCorporationwww.telerik.com

Table of ContentsTable of Contents

1.1. Using MethodsUsing Methods What is a Method? Why to Use What is a Method? Why to Use

Methods?Methods? Declaring and Creating MethodsDeclaring and Creating Methods Calling MethodsCalling Methods

2.2. Methods with ParametersMethods with Parameters Passing ParametersPassing Parameters Returning ValuesReturning Values

3.3. Best PracticesBest Practices2

What is a Method?What is a Method? A A methodmethod is a kind of building block is a kind of building block

that solves a small problemthat solves a small problem

A piece of code that has a name and A piece of code that has a name and can be called from the other codecan be called from the other code

Can take parameters and return a Can take parameters and return a valuevalue

Methods allow programmers to Methods allow programmers to construct large programs from simple construct large programs from simple piecespieces

Methods are also known as Methods are also known as functionsfunctions, , proceduresprocedures, and , and subroutinessubroutines 3

Why to Use Methods?Why to Use Methods? More manageable programmingMore manageable programming

Split large problems into small piecesSplit large problems into small pieces Better organization of the programBetter organization of the program Improve code readabilityImprove code readability Improve code understandabilityImprove code understandability

Avoiding repeating codeAvoiding repeating code Improve code maintainabilityImprove code maintainability

Code reusabilityCode reusability Using existing methods several timesUsing existing methods several times

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Declaring and Declaring and Creating Creating MethodsMethods

Declaring and Creating Declaring and Creating MethodsMethods

Each method has a Each method has a namename It is used to call the methodIt is used to call the method

Describes its purposeDescribes its purpose

static void PrintLogo()static void PrintLogo(){{ Console.WriteLine("Telerik Corp.");Console.WriteLine("Telerik Corp."); Console.WriteLine("www.telerik.com");Console.WriteLine("www.telerik.com");}}

MethMethod od

namename

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Declaring and Creating Declaring and Creating Methods (2)Methods (2)

Methods declared Methods declared staticstatic can be can be called by any other method (static called by any other method (static or not)or not) This will be discussed later in detailsThis will be discussed later in details

The keyword The keyword voidvoid means that the means that the method does not return any resultmethod does not return any result


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Each method has a Each method has a bodybody It contains the programming It contains the programming

codecode Surrounded by Surrounded by {{ and and }}

MethMethod od

bodybody

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using System;using System;

class MethodExampleclass MethodExample{{ static void PrintLogo()static void PrintLogo() {{ Console.WriteLine("Telerik Corp.");Console.WriteLine("Telerik Corp."); Console.WriteLine("www.telerik.com");Console.WriteLine("www.telerik.com"); }}

static void Main()static void Main() {{ // ...// ... }}} }


Methods are always declared inside Methods are always declared inside a a classclass

Main()Main() is also a method like all is also a method like all othersothers

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Calling MethodsCalling Methods

Calling MethodsCalling Methods To call a method, simply use:To call a method, simply use:

1.1. The method’s nameThe method’s name

2.2. Parentheses (don’t forget them!)Parentheses (don’t forget them!)

3.3. A semicolon (A semicolon (;;))

This will execute the code in the This will execute the code in the method’s body and will result in method’s body and will result in printing the following:printing the following:

PrintLogo();PrintLogo();

Telerik Corp.Telerik Corp.www.telerik.comwww.telerik.com

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Calling Methods (2)Calling Methods (2) A method can be called from:A method can be called from:

TheThe Main()Main() method method

Any other methodAny other method

Itself (process known as Itself (process known as recursionrecursion))

static void Main()static void Main(){{ // ...// ... PrintLogo();PrintLogo(); // ...// ...}}

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Declaring and Declaring and Calling Calling

MethodsMethodsLive DemoLive Demo

Methods with Methods with ParametersParametersPassing Parameters and Returning Passing Parameters and Returning

ValuesValues

Method ParametersMethod Parameters To pass information to a method, you can To pass information to a method, you can

use use parameters parameters (also known as (also known as argumentsarguments))

You can pass zero or several input valuesYou can pass zero or several input values

You can pass values of different typesYou can pass values of different types

Each parameter has name and typeEach parameter has name and type

Parameters are assigned to particular Parameters are assigned to particular values when the method is calledvalues when the method is called

Parameters can change the method Parameters can change the method behavior depending on the passed valuesbehavior depending on the passed values

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Defining and Using Defining and Using Method ParametersMethod Parameters

Method’s behavior depends on its Method’s behavior depends on its parametersparameters

Parameters can be of any typeParameters can be of any type intint, , doubledouble, , stringstring, etc., etc. arrays (arrays (intint[][], , double[]double[], etc.), etc.)

static void PrintSign(int number)static void PrintSign(int number){{ if (number > 0)if (number > 0) Console.WriteLine("Positive");Console.WriteLine("Positive"); else if (number < 0)else if (number < 0) Console.WriteLine("Negative");Console.WriteLine("Negative"); elseelse Console.WriteLine("Zero");Console.WriteLine("Zero");}}

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Defining and Using Defining and Using Method Parameters (2)Method Parameters (2)

Methods can have as many Methods can have as many parameters as needed:parameters as needed:

The following syntax is not valid:The following syntax is not valid:

static void PrintMax(float number1, float number2)static void PrintMax(float number1, float number2)

{{

float max = number1;float max = number1;

if (number2 > number1)if (number2 > number1)

max = number2;max = number2;

Console.WriteLine("Maximal number: {0}", max);Console.WriteLine("Maximal number: {0}", max);

}}

static void PrintMax(float number1, number2)static void PrintMax(float number1, number2)

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Calling MethodsCalling Methodswith Parameterswith Parameters

To call a method and pass values To call a method and pass values to its parameters:to its parameters: Use the method’s name, followed by Use the method’s name, followed by

a list of expressions for each a list of expressions for each parameterparameter

Examples:Examples:PrintSign(-5);PrintSign(-5);PrintSign(balance);PrintSign(balance);PrintSign(2+3);PrintSign(2+3);

PrintMax(100, 200);PrintMax(100, 200);PrintMax(oldQuantity * 1.5, quantity * 2);PrintMax(oldQuantity * 1.5, quantity * 2);

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Calling MethodsCalling Methodswith Parameters (2)with Parameters (2)

Expressions must be of the same Expressions must be of the same type as method’s parameters (or type as method’s parameters (or compatible)compatible)

If the method requires a If the method requires a floatfloat expression, you can pass expression, you can pass intint instead instead

Use the same order like in method Use the same order like in method declarationdeclaration

For methods with no parameters do For methods with no parameters do not forget the parenthesesnot forget the parentheses

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ExamplesExamples

Using Using Methods Methods

With With ParametersParameters

Methods Parameters – Methods Parameters – ExampleExample

static void PrintSign(int number)static void PrintSign(int number){{ if (number > 0)if (number > 0) Console.WriteLine("The number {0} is positive.", Console.WriteLine("The number {0} is positive.", number);number); else if (number < 0)else if (number < 0) Console.WriteLine("The number {0} is negative.", Console.WriteLine("The number {0} is negative.", number);number); elseelse Console.WriteLine("The number {0} is zero.", Console.WriteLine("The number {0} is zero.", number);number);}}

static void PrintMax(float number1, float number2)static void PrintMax(float number1, float number2){{ float max = number1;float max = number1; if (number2 > number1)if (number2 > number1) {{ max = number2;max = number2; }} Console.WriteLine("Maximal number: {0}", max);Console.WriteLine("Maximal number: {0}", max);}}

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Live DemoLive Demo

Method ParametersMethod Parameters

Months – ExampleMonths – Example

Display the period between two Display the period between two months in a user-friendly waymonths in a user-friendly wayusing System;using System;

class MonthsExampleclass MonthsExample{{ static void SayMonth(int month)static void SayMonth(int month) {{ string[] monthNames = new string[] {string[] monthNames = new string[] { "January", "February", "March", "January", "February", "March", "April", "May", "June", "July", "April", "May", "June", "July", "August", "September", "October","August", "September", "October", "November", "December"};"November", "December"}; Console.Write(monthNames[month-1]);Console.Write(monthNames[month-1]); }}

(the example continues)(the example continues)

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Months – Example (2)Months – Example (2)

static void SayPeriod(int startMonth, int endMonth)static void SayPeriod(int startMonth, int endMonth)

{{

int period = endMonth - startMonth;int period = endMonth - startMonth;

if (period < 0)if (period < 0)

{{

period = period + 12;period = period + 12;

// From December to January the// From December to January the

// period is 1 month, not -11!// period is 1 month, not -11!

}}

Console.Write("There are {0} " + months from ", Console.Write("There are {0} " + months from ",

period);period);

SayMonth(startMonth);SayMonth(startMonth);

Console.Write(" to ");Console.Write(" to ");

SayMonth(endMonth);SayMonth(endMonth);

}}

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Live DemoLive Demo

MonthMonthss

Printing Triangle – Printing Triangle – ExampleExample

Creating a program for printing Creating a program for printing triangles as shown below:triangles as shown below:

1111 1 21 21 21 2 1 2 31 2 31 2 31 2 3 1 2 3 41 2 3 41 2 3 41 2 3 4 1 2 3 4 51 2 3 4 5

n=5 n=5 1 2 3 4 51 2 3 4 5 n=6 n=6 1 2 3 1 2 3 4 5 64 5 61 2 3 41 2 3 4 1 2 3 4 51 2 3 4 51 2 31 2 3 1 2 3 41 2 3 41 21 2 1 2 31 2 311 1 21 2

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Printing Triangle – Printing Triangle – ExampleExample

static void Main()static void Main(){{ int n = int.Parse(Console.ReadLine());int n = int.Parse(Console.ReadLine());

for (int line = 1; line <= n; line++)for (int line = 1; line <= n; line++) PrintLine(1, line);PrintLine(1, line); for (int line = n-1; line >= 1; line--)for (int line = n-1; line >= 1; line--) PrintLine(1, line);PrintLine(1, line);}}

static void PrintLine(int start, int end)static void PrintLine(int start, int end){{ for (int i = start; i <= end; i++)for (int i = start; i <= end; i++) {{ Console.Write(" {0}", i);Console.Write(" {0}", i); }} Console.WriteLine();Console.WriteLine();}}

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Printing TrianglePrinting TriangleLive DemoLive Demo

Optional ParametersOptional Parameters C# 4.0 supports optional parameters C# 4.0 supports optional parameters

with default values:with default values:

The above method can be called in The above method can be called in several ways:several ways:

static void PrintNumbers(int start=0; int end=100){{ for (int i=start; i<=end; i++)for (int i=start; i<=end; i++) {{ Console.Write("{0} ", i);Console.Write("{0} ", i); }}}}

PrintNumbers(5, 10);PrintNumbers(15);PrintNumbers();PrintNumbers(end: 40, start: 35);

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Optional ParametersOptional ParametersLive DemoLive Demo

Returning Returning Values From Values From

MethodsMethods

Returning Values From Returning Values From MethodsMethods

A method can A method can returnreturn a value to its a value to its callercaller

Returned value:Returned value: Can be assigned to a variable:Can be assigned to a variable:

Can be used in expressions:Can be used in expressions:

Can be passed to another method:Can be passed to another method:

string message = Console.ReadLine();string message = Console.ReadLine();// Console.ReadLine() returns a string// Console.ReadLine() returns a string

float price = GetPrice() * quantity * 1.20;float price = GetPrice() * quantity * 1.20;

int age = int.Parse(Console.ReadLine());int age = int.Parse(Console.ReadLine());

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Defining Defining Methods That Methods That

Return a ValueReturn a Value Instead of Instead of voidvoid, specify the type of data to , specify the type of data to

returnreturn

Methods can return any type of data (Methods can return any type of data (intint, , stringstring, array, etc.), array, etc.)

voidvoid methods do not return anything methods do not return anything The combination of method's name, The combination of method's name,

parameters and return value is called parameters and return value is called method signaturemethod signature

Use Use returnreturn keyword to return a result keyword to return a result

static int Multiply(int firstNum, int secondNum)static int Multiply(int firstNum, int secondNum){{ return firstNum * secondNum;return firstNum * secondNum;}}

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The The returnreturn Statement Statement

The The returnreturn statement: statement: Immediately terminates method’s Immediately terminates method’s

executionexecution

Returns specified expression to the callerReturns specified expression to the caller

Example:Example:

To terminate To terminate voidvoid method, use just: method, use just:

Return can be used several times in a Return can be used several times in a method bodymethod body

return -1;

return;

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ExamplesExamples

Returning Returning Values From Values From

MethodsMethods

Temperature Temperature Conversion – Conversion –

ExampleExample Convert temperature from Convert temperature from

Fahrenheit to Celsius:Fahrenheit to Celsius:static double FahrenheitToCelsius(double degrees)static double FahrenheitToCelsius(double degrees){{ double celsius = (degrees - 32) * 5 / 9;double celsius = (degrees - 32) * 5 / 9; return celsius;return celsius;}}

static void Main()static void Main(){{ Console.Write("Temperature in Fahrenheit: ");Console.Write("Temperature in Fahrenheit: "); double t = Double.Parse(Console.ReadLine());double t = Double.Parse(Console.ReadLine()); t = FahrenheitToCelsius(t);t = FahrenheitToCelsius(t); Console.Write("Temperature in Celsius: {0}", t);Console.Write("Temperature in Celsius: {0}", t);}}

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Live DemoLive Demo

Temperature Temperature ConversionConversion

Positive Numbers – Positive Numbers – ExampleExample

Check if all numbers in a sequence Check if all numbers in a sequence are positive:are positive:

static bool ArePositive(int[] sequence)static bool ArePositive(int[] sequence){{ foreach (int number in sequence)foreach (int number in sequence) {{ if (number <= 0)if (number <= 0) {{ return false;return false; }} }} return true;return true;}}

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Live DemoLive Demo

Positive Positive NumbersNumbers

Data Validation – Data Validation – ExampleExample

Validating input data:Validating input data:using System;using System;

class ValidatingDemoclass ValidatingDemo{{ static void Main()static void Main() {{ Console.WriteLine("What time is it?");Console.WriteLine("What time is it?"); Console.Write("Hours: ");Console.Write("Hours: "); int hours = int.Parse(Console.ReadLine());int hours = int.Parse(Console.ReadLine());

Console.Write("Minutes: ");Console.Write("Minutes: "); int minutes = int.Parse(Console.ReadLine());int minutes = int.Parse(Console.ReadLine());

// (The example continues on the next slide)// (The example continues on the next slide)

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Data Validation – Data Validation – ExampleExample

bool isValidTime = bool isValidTime = ValidateHours(hours) &&ValidateHours(hours) && ValidateMinutes(minutes);ValidateMinutes(minutes); if (isValidTime)if (isValidTime) Console.WriteLine("It is {0}:{1}",Console.WriteLine("It is {0}:{1}", hours, minutes);hours, minutes); elseelse Console.WriteLine("Incorrect time!");Console.WriteLine("Incorrect time!"); }}

static bool ValidateMinutes(int minutes)static bool ValidateMinutes(int minutes) {{ bool result = (minutes>=0) && (minutes<=59);bool result = (minutes>=0) && (minutes<=59); return result;return result; }}

static bool ValidateHours(int hours) { ... }static bool ValidateHours(int hours) { ... }}}

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Live DemoLive Demo

Data Data ValidationValidation

Methods – Best Methods – Best PracticesPractices

Each method should perform a single,Each method should perform a single, well-defined task well-defined task

Method’s name should describe that Method’s name should describe that task in a clear and non-ambiguous waytask in a clear and non-ambiguous way Good examples: Good examples: CalculatePriceCalculatePrice, , ReadNameReadName

Bad examples: Bad examples: ff, , g1g1, , ProcessProcess

In C# methods should start with capital In C# methods should start with capital letterletter

Avoid methods longer than one screenAvoid methods longer than one screen Split them to several shorter methodsSplit them to several shorter methods

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SummarySummary Break large programs into simple Break large programs into simple

methods that solve small sub-methods that solve small sub-problemsproblems

Methods consist of declaration and Methods consist of declaration and bodybody

Methods are invoked by their nameMethods are invoked by their name Methods can accept parametersMethods can accept parameters

Parameters take actual values when Parameters take actual values when calling a methodcalling a method

Methods can return a value or Methods can return a value or nothingnothing

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QuestionsQuestions??

MethodsMethods

http://academy.telerik.com

ExercisesExercises

1.1. Write a method that asks the user for his Write a method that asks the user for his name and prints “Hello, <name>” (for name and prints “Hello, <name>” (for example, “Hello, Peter!”). Write a program example, “Hello, Peter!”). Write a program to test this method.to test this method.

2.2. Write a method Write a method GetMax()GetMax() with two with two parameters that returns the bigger of two parameters that returns the bigger of two integers. Write a program that reads 3 integers. Write a program that reads 3 integers from the console and prints the integers from the console and prints the biggest of them using the method biggest of them using the method GetMax()GetMax()..

3.3. Write a method that returns the last digit of Write a method that returns the last digit of given integer as an English word. Examples: given integer as an English word. Examples: 512 512 "two", 1024 "two", 1024 "four", 12309 "four", 12309 "nine". "nine".

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Exercises (2)Exercises (2)

4.4. Write a method that counts how many times Write a method that counts how many times given number appears in given array. Write given number appears in given array. Write a test program to check if the method is a test program to check if the method is working correctly.working correctly.

5.5. Write a method that checks if the element at Write a method that checks if the element at given position in given array of integers is given position in given array of integers is bigger than its two neighbors (when such bigger than its two neighbors (when such exist).exist).

6.6. Write a method that returns the index of the Write a method that returns the index of the first element in array that is bigger than its first element in array that is bigger than its neighbors, or -1, if there’s no such element.neighbors, or -1, if there’s no such element.

Use the method from the previous exercise.Use the method from the previous exercise.48


7.7. Write a method that reverses the digits of Write a method that reverses the digits of given decimal number. Example: 256 given decimal number. Example: 256 652 652

8.8. Write a method that adds two positive integer Write a method that adds two positive integer numbers represented as arrays of digits (each numbers represented as arrays of digits (each array element array element arr[iarr[i]] contains a digit; the last contains a digit; the last digit is kept in digit is kept in arr[0]arr[0]). Each of the numbers ). Each of the numbers that will be added could have up to 10 000 that will be added could have up to 10 000 digits.digits.

9.9. Write a method that return the maximal Write a method that return the maximal element in a portion of array of integers element in a portion of array of integers starting at given index. Using it write another starting at given index. Using it write another method that sorts an array in ascending / method that sorts an array in ascending / descending order.descending order.

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10.10. Write a program to calculate Write a program to calculate n!n! for each for each nn in in the range the range [1..100][1..100]. Hint: Implement first a . Hint: Implement first a method that multiplies a number method that multiplies a number represented as array of digits by given represented as array of digits by given integer number. integer number.

11.11. Write a method that adds two polynomials. Write a method that adds two polynomials. Represent them as arrays of their Represent them as arrays of their coefficients as in the example below:coefficients as in the example below:

xx22 + 5 = 1 + 5 = 1xx22 + 0 + 0xx + 5 + 5

10.10. Extend the program to support also Extend the program to support also subtraction and multiplication of subtraction and multiplication of polynomials.polynomials.

55 00 11

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Exercises (5)Exercises (5)13.13. Write a program that can solve these Write a program that can solve these

tasks:tasks: Reverses the digits of a numberReverses the digits of a number

Calculates the average of a sequence of Calculates the average of a sequence of integersintegers

Solves a linear equation Solves a linear equation aa * * xx + + bb = 0 = 0

Create appropriate methods.Create appropriate methods.

Provide a simple text-based menu for Provide a simple text-based menu for the user to choose which task to solve.the user to choose which task to solve.

Validate the input data:Validate the input data: The decimal number should be non-negativeThe decimal number should be non-negative

The sequence should not be emptyThe sequence should not be empty

aa should not be equal to should not be equal to 00 51

09. Methods

Technology

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