CSE 110 Review Session

Hans Hovanitz, Kate Kincade, and Ian Nall

Quick Intro• What is a program?

o List of instructions a computer can follow

• Variable: is a name that refers to a location in memory

o They store values which can be different types called Data Types

• Variables must be declared and initialized

• Primary data types are:

o Integer String

o Float Char

o Double Boolean

If and If-Else Statements

• The If and If-Else statements use decision logic

• In order to properly use if and if-else statements, relational operators must be used.

o == : equal

o > : one value is greater than another

o < : one value is less than another

o != : not equal

o compareTo() : method used to compare multiple Strings

• Whenever relational operators are used, a boolean value is returned

Quick Exampleif(x==5){ //if the variable x has a value of 5

System.out.print(5); }

else{ //if the above if statement is not true

System.out.print(“Not a 5”);}

Sample ProblemWhat is the output of this code?

int i = 6;

if(i > 10){

System.out.println(“Big number!”);

}else if(i ==10){

System.out.println(“Almost big.”);}

else

{System.out.println(“Small number!”);}

Small Number!

Loops

• Unlike If-Else statements, loops use iterative logic

o Meaning, loops are used in order to perform the same operation multiple times

• Every loop requires four components:

o initialization of a variable local to the loop

o a stopping condition

o incrementation or a modification statement

o The operation or operations to be done within the loop

Quick Example

for(int i = 0; i < 10: i++){

System.out.print(i + “ ,”);

}

What is the output of this loop?

0, 1, 2, 3, 4, 5, 6, 7, 8, 9

Sample ProblemWhat is the output of the following code? Additionally, where is the incrementation statement, initialization, stopping condition, and operation to be done within the loop?

int i = 10;

while(i>0){

System.out.print(i + “, ”);

i = i-2;

}

Initialization

Stopping Condition

Incrementation Statement

Operation to be done within the loop

Output: 10, 8, 6, 4, 2

Methods - Review

What is a method?

● A method is a sequence of code instructions that are given a name

● A method exists within a class● Several methods can exist within a class● A method takes in 0 to many arguments (inputs)● A method returns 0 or 1 return values (output)● Code inside the method uses the arguments

What is a method good for? ● Methods are useful when you need to execute the same set of

instructions many times in your program (not in a loop)● Methods are useful for breaking up code into smaller, more

manageable logical chunks

Methods - Header/Definition and Example

public static int sum (int num1, int num2) {

int result = num1 + num2;return result;

}

A method header (also known as a definition) specifies four things: ● The name of the method● The parameters of the method and their types● The return type of the method● Visibility and other modifiers

Method Practice

How can we create a method that takes an integer as a parameter and returns the sum of all the numbers before and including that number -- e.g., 5+4+3+2+1 = 15

public static int countDownSum(int num){

int total = 0;int startCountDown = num;

for (int i = 0; i < startCountDown; i++){

total = total + num;num -= 1; //This is the same as num = num -1;

}

return total;}

Creating a Method - You Try

Create a method header with the following properties:

• Its name is “rateMovie”

• It takes 2 arguments. One argument is a String corresponding to the name of a movie. The second is an integer value which is a rating of that movie.

• Nothing is returned.

Solution:

public static void rateMovie(String movieName, int rating)

Creating a Method - You Try

Create a method that will calculate the volume of a box and return the sentence “Volume is large.” if the volume is greaterthan or equal to 100.0. If the volume is less than 100.0 your method should return the sentence “Volume is small.” Your method should also print the volume of the box to the console. Don’t worry about the units of the box.

• What parameters does the method require? Hint: There are 3• What is the return data type? • Do you need to print out the volume of the box before or after the return

statement?

Creating a Method - Possible Solutionpublic static void main (String [] args) {

System.out.println(boxSize(5.0, 7.0, 2.0));}public static String boxSize(double length, double width, double height) {

double volume = length * width * height;String result = “”;System.out.println(volume);if (volume >= 100) {

result = “Volume is large.”}else {

result = “Volume is small.”}return result;

}

Output of this program is:70.0Volume is small.

Classes and Objects

• Class: Defines the object type

• Object: an instance of a class

• An object has:

o Variables

o Methods

• Classes are used to define these for objects of its type

• Meaning, a class is a blueprint for an object of that class!

Brief Examplepublic Class Rectangle{

public int width, height;

public Rectangle(int height1, int width1){

width= width1;

height = height1}

public int perimeter(){

int perimeter = 2*length + 2*height

return perimeter;

}

}

Make Your Own Class!

Make your own dog class that has:

• A constructor with two arguments: one String which will contain the name of the dog and an integer which will contain its age

• A method that will determine whether or not the dog is old based on the age (if age > 5 then the dog is old). This method doesn’t need to return anything.

• Once you are finished making the class, write the code that would create an object of the dog class.

Possible Solutionpublic Class Dog{

public String name = “ ”;

public int age = 0;

public Dog(String thisName, int thisAge){

name = thisName;

age = thisAge;}

public void isOld(){

if(age > 5){System.out.println(“This is an old dog”);}

else{ System.out.println(“This dog isn’t old!”);}

}

}

Dog fido = new Dog(“Fido”, 2);

Arrays - ReviewAn array has these properties:

• Elements - These are the values or objects that make up the array. All of the elements in an array must be of the same type

• Length - The number of elements that make up the array.

• An array is fixed in length - Once you set the size of an array it cannot be changed

• The first element of an array is located at index 0

Array Syntax:<type> [] <variable name> = new <type>[<size>];

Elements unknown: int [] scores = new int[5];

Elements known: int [] scores = {90, 80, 75, 85, 90}

Arrays - Review

int [] scores = new int [5];

scores[1] = 90;scores [2] = 80;int exam3 = 85;scores [3] = exam3;

What element is at each index?

0 90 80 85 0

0 1 2 3 4

What happens if you try this?

int exam5 =scores[5]grades[5] = 100;

//Out of bounds exception!//Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException

Array Practice

How can we create an array with 3 elements and ask the user to fill the array with movie titles? We should then print each element of the array unless the title is “Star Wars”. If the title is “Star Wars” we should print “Please be awesome”.

Array Practice - Possible Solution Scanner scan = new Scanner(System.in);String [] movies = new String[3];

movies[0] = scan.nextLine();movies[1] = scan.nextLine();movies[2] = scan.nextLine();

for (int i = 0; i < movies.length; i++{ if (movies[i].equals(“Star Wars”))

{System.out.println(“Please be awesome”);

}else{

System.out.println(movies[i]);}

}

Array Practice - You TryWhat is the output of this program?

int [] numbers = {1, 2, 3, 4, 5};int var1 = 2;int var2 = 4;if (numbers[4] == variable 2) { numbers[1] = numbers[2];}else { numbers[2] = var1; var2 = numbers[3];}while (var2 > var1) { numbers[1] = numbers[1] +2; var1++}numbers[4] = (numbers[3])/2;for (int i = 1; numbers.length > i; i++) { System.out.println(numbers[i])}

index 0 1 2 3 4numbers = [1], [2], [3], [4], [5]var1 = 2 var2 = 4

numbers = [1], [2], [2], [4], [5]var2 = 4

numbers = [1], [4], [2], [4], [5] : numbers = [1], [6], [2], [4], [5]var1 = 3 : var1 = 4

numbers = [1], [6], [2], [4], [2]

Output:6242

ArrayLists

ArrayLists expand and shrink dynamically as neededSome commonly used methods:

• add• get• size• set

ArrayList Syntax:ArrayList<type> <name> = new ArrayList<type>();

Arraylist<Integer> scores = new ArrayList<Integer>();

ArrayLists - You TryWhat is the output of this program?

Arraylist<Integer> scores = new ArrayList<Integer>();scores.add(50);scores.add(75);System.out.println(scores.size());int removedScore = scores.remove(1);scores.add(removedScore + 10);scores.set(0, 100);scores.add(2, 85);for (int i = 0; i <scores.size(); i++){

System.out.println(scores.get(i));}

Output:21008585

Recursive Methods• A recursive method is a method which calls itself

• A couple of items are required for a recursive method:o Base case

o Code that will aid in getting closer to the base case

• Quick example:public int factorial(int number)

{if(number ==1 || number == 0){

return 1;}

else{return factorial(number-1)*number;}

}

Write your Own Recursive Method!

Write a recursive method that will calculate the sum of all numbers up to a specific number (meaning, if the number is 5, calculate 5+4+3+2+1). What is the base case? How can you get closer to the base case given a larger “size n” problem?

Possible Solutionpublic int addition(int number)

{ if(number == 0){

return 0;}

if(number ==1 |){

return 1;}

else{return addition(number-1)+number;}

}

Questions?

Extra Questions

Loops - Practice public static void main (String[] args){

for (int i = 0; i < 3; i++){

for (int j = 3; j > i; j--){

System.out.println(j)}System.out.println();

}}

What is the output of this program?

Solution

321

32

3

Methods - Practice

Create a method that determines whether or not a positive integer number is even or odd. If the number is odd, your method should return -1. If it is even, your method should return 1. If the number is invalid (not positive), then your method should return 0.

Solutionpublic static int isEven(int number) { int returnValue = 0; if (number > 0)

{if (number % 2 == 0)

{returnValue = 1;

}else{

returnValue = -1;}

} return returnValue;}

Arrays - Practice

Write a program that will find the maximum and minimum value of an array that contains the values 0.12, 0.81, 0.45, 0.03, 0.80. Then print out the max and min values.

Solution public static void main(String [] args) {

double [] values = {0.12, 0.81, 0.45, 0.03, 0.80};double max = values[0];double min = values[0];

for (int i = 0; i < values.length; i++) { if (values[i] > max) {

max = values[i]; }

if (values[i] < min) {

min = values[i] } System.out.println(“The maximum value is “ + max);

System.out.println(“The minimum value is “ + min); }

}

Recursion-Practice

What does this method do? When this method is called, will it return anything?

public int addition(int number){

return addition(number-1)+number;

}

It won’t return anything and it will run forever since there is no stopping condition!