Methods (Functions) CSE 1310 – Introduction to Computers and Programming Vassilis Athitsos University of Texas at Arlington 1

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Methods (Functions)

CSE 1310 – Introduction to Computers and ProgrammingVassilis Athitsos

University of Texas at Arlington

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Terminology: Methods/Functions• In Java, the term methods is the one mainly used.• In some other languages, the term functions is more

common.• In some languages (e.g., C++) methods and functions

have minor differences in meaning.• In this course, I will use the two terms (methods and

functions) as synonyms.

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A First Example

• Here we see our first example of a function.

• It is called square.– Its job is to

compute the square of a number.

import java.util.Scanner;

public class example1{ public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N,

N_square); }}

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What Is a Function• A function is a piece of

code that does a specific job.

• Usually (but not always) the job is to compute something.

• Examples:– Computing the square

of a number.– Determining if an

number is prime.– Computing the number

of digits of a number.– Counting the number of

vowels in a string.



N_square); }}

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How to Write a Function• Specify a name, like

square.import java.util.Scanner;


N_square); }}

function name

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square.• Specify the inputs

(called arguments).– The arguments are the

input that the function needs in order to compute an output.

– For example, what does the square function take as input?



N_square); }}

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(called arguments).– The arguments are the

input that the function needs in order to compute an output.

– For example, what does the square function take as input?

• A number.– We can have functions

with 0, 1, or more arguments. We will see examples later.



N_square); }}

argument(s)

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(called arguments).• Specify the return type

of the function.– What is the type of the

value that the function computes?

– In other words, what values are legal and illegal for the output of the function?



N_square); }}

return type

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of the function.• Specify what the

function does. This is what we call the body of the function.



N_square); }}

body

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of the function.• Specify what the

function does. This is what we call the body of the function.

• Specify what value the function returns, using one or more return statements.



N_square); }}

return statement

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Using a Function (Function Calls)• Once you have written

a function, you can use it anywhere in your code, by doing a function call.



N_square); }}

function call

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• When you call a function, you must:– Provide the appropriate

argument(s).



N_square); }}

argument(s)

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• When you call a function, you must:– Provide the appropriate

argument(s).– Use the return value

appropriately.



N_square); }}

storing the return value

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Using the Return Value• In the square function, the

return value is a double.• You can use this value in

any way that you can use any double number. Examples:

• You can store the return value in a variable.



N_square); }}

storing the return value in a variable

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• You can print the return value directly.


public class example1{ public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); System.out.printf("%.2f squared = %.2f\n", N,

square(N)); }}

printing the return value directly

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• You can use it as part of a more complicated expression.


public class example1{ public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble();

double my_var = 18 + square(N); System.out.printf("result = %.2f\n", my_var); }}

using the return value in an expression

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• You can use it as part of a more complicated expression.

• You can use it as an argument for another function call.


public class example1{ public static double square(double number) { double result = number * number; return result; }

public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble();

double my_var = 18 + square(square(N)); System.out.printf("result = %.2f\n", my_var); }}

the return value is used as argument for another function call

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Function Calls

• You can call a function as many times as you like.• On the next slide, we call the square function three

times.

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public class example1{ public static double square(double number) { double result = number * number; return result; }

public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number M: "); double M = in.nextDouble(); System.out.printf("Please enter a number N: "); double N = in.nextDouble();

double M_square = square(M); System.out.printf("%.2f squared = %.2f\n", M, M_square); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); double MN_square = square(M*N); System.out.printf("%.2f squared = %.2f\n", M*N, M*N_square); }}

Example: Calling the square function three times.

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Example: Printing Prime Numbers• Write a program

that:– Asks the user to

enter an integer N.– Prints all prime

integers from 2 up to (and including) N.

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• First step:– Write the basic

structure of the program.

– Put placeholders where more detail is needed.


public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt();

for (int i = 2; i <= N; i++) { if i is prime { System.out.printf("%d\n", i); } } }}

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• Second step:– Identify functions

that can be used to complete the program.




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• We can use a function is_prime to check if i is prime.

• Arguments?

• Return type?




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• We can use a function is_prime to check if i is prime.

• Arguments? an integer

• Return type? boolean




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• Step 3: use the function.– We have not written

the function yet, but that is OK.

– Obviously, the program will not run until we write the function.



for (int i = 2; i <= N; i++) { if (is_prime(i)) { System.out.printf("%d\n", i); } } }}

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public class example1 { public static boolean is_prime(int N) { for (int i = 2; i < N; i++) { if (N % i == 0) { return false; } } return true; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt();

for (int i = 2; i <= N; i++) { if (is_prime(i)) { System.out.printf("%d\n", i); }}}}

• Write a program that:– Asks the user to



• Step 4: write the function.

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Output:

Enter an integer: 802357111317192329313741434753596167717379


public class example1 { public static boolean is_prime(int N) { for (int i = 2; i < N; i++) { if (N % i == 0) { return false; } } return true; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt();

for (int i = 2; i <= N; i++) { if (is_prime(i)) { System.out.printf("%d\n", i); }}}}

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Why Do We Need Functions• To write better code:

– More correct, easier to read/write/change.

• To write complicated code.• Functions help us organize code.

YOU CANNOT WRITE NON-TRIVIAL PROGRAMS IF YOU DO NOT USE FUNCTIONS

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Example: Prime Numbers, Again• Write a program that:

– Asks the user to enter an integer N.

– Prints out the smallest prime number greater than or equal to N.

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• First step:– Write the basic

structure of the program.

– Put placeholders where more detail is needed.



int i = N; while (i is not prime) { i++; } System.out.printf("%d\n", i); }}

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• Second step:– Identify functions that

can be used to complete the program.

• We can use the is_prime function again!!!



int i = N; while (is_prime(i) == false) { i++; } System.out.printf("%d\n", i); }}

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• Write a program that:– Asks the user to enter

an integer N.– Prints out the smallest

prime number greater than or equal to N.

• To complete the program, we can just use the is_prime function we already have.

• Functions make it really easy to re-use code!!!


public class example1{ public static boolean is_prime(int N) { for (int i = 2; i < N; i++) { if (N % i == 0) { return false; } } return true; }

public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); while (is_prime(N) == false) { N++; } System.out.printf("%d\n", N); }}

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public class example1{ public static boolean is_prime(int N) { for (int i = 2; i < N; i++) { if (N % i == 0) { return false; } } return true; }

public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); while (is_prime(N) == false) { N++; } System.out.printf("%d\n", N); }}

Example Output:

Enter an integer: 10000001000003

Example Output:

Enter an integer: 123456789123456791

Example Output:

Enter an integer: 75007507

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Making Code Easier to Read• In lots of programs, we need to perform a task many

times.• Copying and pasting code can work, but has

disadvantages:• The resulting code can look long and ugly.• If we make a mistake, we end up copying and pasting

the mistake many times.– Fixing such mistakes, that have been copied and pasted

many times, can be a pain.

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Example: Input Validation• Write a program that:

– Asks the user to enter integers A, B, C.• if the user enters an invalid input, the program should keep asking

the user to enter a valid integer,

– Prints out the sum of the three integers.

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• A first version of the solution can be seen on the next slide.

import java.util.Scanner;public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int A; while (true) { System.out.printf("Enter integer A: "); String s = in.next(); try { A = Integer.parseInt(s); } catch (Exception e) { System.out.printf("Invalid input\n"); continue; } break; } int B; while (true) { System.out.printf("Enter integer B: "); String s = in.next(); try { B = Integer.parseInt(s); }

catch (Exception e) { System.out.printf("Invalid input\n"); continue; } break; } int C; while (true) { System.out.printf("Enter integer C: "); String s = in.next(); try { C = Integer.parseInt(s); } catch (Exception e) { System.out.printf("Invalid input\n"); continue; } break; } System.out.printf("A + B + C = %d\n", A +

B + C); }}

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• A first version of the solution can be seen on the previous slide.

• What is wrong with that code?

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Example: Input Validation• Solution:

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Example: Input Validation• Solution: write a function for getting an integer from

the user.• Name: user_integer• Arguments:

• Return value:

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Example: Input Validation• Solution: write a function for getting an integer from

the user.• Name: user_integer• Arguments:

– One argument - what we print to the user.– Type of the argument: String

• Return value:– int


public class example1{ public static int user_integer(String message) { Scanner in = new Scanner(System.in); int result; while (true) { System.out.printf(message); String s = in.next(); try { result = Integer.parseInt(s); } catch (Exception e) { System.out.printf("%s is not a valid integer.\n\n", s); continue; } return result; } }

public static void main(String[] args) { int A = user_integer("Enter integer A: "); int B = user_integer("Enter integer B: "); int C = user_integer("Enter integer C: "); System.out.printf("A + B + C = %d\n", A + B + C); }}

• Here we see a solution that defines and uses the user_integer function.

• The code is shorter.

• Unlike the previous version, code is not duplicated.

• The main function is short and easy to read.

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Example: Repeated Printing• Write a program that:

– Asks the user to enter a string S.– Asks the user to enter a number N.– Prints N times string S.

Enter string S: helloEnter integer N: 5hellohellohellohellohello

Example Output:

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Example: Repeated Printing• Write a program that:

– Asks the user to enter a string S.– Asks the user to enter a number N.– Prints N times string S.

• Let's write a function, that:– takes as input a

string S and an integer N.

– prints the stringN times.

import java.util.Scanner;public class example1{ public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter string S: "); String S = in.next(); System.out.printf("Enter integer N: "); int N = in.nextInt(); print String S repeatedly, N times. }}

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Example: Repeated Printing• The complete

program is shown on the right.

• Function repeat_print has two features we have not seen before:

• It takes two arguments.

• It returns nothing.

import java.util.Scanner;public class example1{ public static void repeat_print(String message,

int times) { for (int i = 0; i < times; i++) { System.out.printf("%s\n", message); } } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter string S: "); String S = in.next(); System.out.printf("Enter integer N: "); int N = in.nextInt(); repeat_print(S, N); }}

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A Function That Returns Nothing• Function repeat_print

does something useful.• However, we do not

want any value returned from the function.

• In that case, we specify the return type of the function as void.

import java.util.Scanner;public class example1{ public static void repeat_print(String message,

int times) { for (int i = 0; i < times; i++) { System.out.printf("%s\n", message); } } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter string S: "); String S = in.next(); System.out.printf("Enter integer N: "); int N = in.nextInt(); repeat_print(S, N); }}

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The main Function• When Java executes a program, how does Java know

where to start?• Every program must have a function called main, such

that:– It takes one argument, of type String []. We will understand

this type in our next topic, when we do arrays. – the return type is void.

• Until we saw how to write functions, all our code used to go to main.

• Now that we have started using functions, the main code will be a relatively small part of the program.

• Functions will do the bulk of the work.

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Function Arguments• Functions have arguments.• To call a function XYZ, you write something like :

XYZ(argument_1, …, argument_N)

• How would you know how many arguments to use?

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Function Arguments• Functions have arguments.• To call a function XYZ, you write something like:

XYZ(argument_1, …, argument_N)

• How would you know how many arguments to use?– From the function definition, which (among other things)

defines EXACTLY:• how many arguments to use.• the type of each argument.• the order of the arguments.

public static type XYZ(type 1 arg_1, …, type_N arg_N)

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Executing a Function Call• When the body of the function starts executing, the

only variables visible to the function are:– the arguments.– variables defined in the body of the function.

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What Will This Program Do?public class example1{ public static void foo(String var1,

String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); }}

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Step-by-step Execution

Current line

Main Namespace:public class example1{ public static void foo(String var1,


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First line to execute:

variables in main:public class example1{ public static void foo(String var1,


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variables in main:

var1 = "hello"

public class example1{ public static void foo(String var1,


First line creates variable var1

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Next line to execute:

variables in main:

var1 = "hello"



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variables in main:

var1 = "hello"var2 = goodbye



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variables in main:

var1 = "hello"var2 = "goodbye"var3 = "earth"



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Next line to execute:function call

variables in main:

var1 = "hello"var2 = "goodbye"var3 = "earth"var4 = "moon"



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Next line to execute:function call.First, assign values to arguments.

variables in main:




variables in foo:

var1 = ???var2 = ???

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Next line to execute:function call.First, assign values to arguments.

variables in main:




variables in foo:

var1 = " earth "var2 = " moon "

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Next line to execute:How does Java know which var1 to print?

variables in main:




variables in foo:


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variables in main:




variables in foo:


Currently executing the body of foo. Only the variables of foo are visible. "hello" is printed.

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variables in main:




variables in foo:


"moon" is printed.

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variables in main:




variables in foo:


Which line comes next?

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variables in main:




Done with the function call.The variables of foo disappear.

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variables in main:




"goodbye" is printed.

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Summary of Program Output

Output:

var1 = "earth"var2 = "moon"var2 = "goodbye"



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Methods (Functions) CSE 1310 – Introduction to Computers and Programming Vassilis Athitsos University of Texas at Arlington 1