2 Arrays Array is a data structure that represents a collection of the same type of data. A "fixed length list".

3 Declaring and Creating Arrays datatype[] arrayRefVar = new datatype[arraySize]; double[] myList = new double[10];  The new keyword is a Java operator that creates the object.  new calls a constructor, which initializes the new object.  Everything in java is class-based. So we need to make objects (instances) for anything that isn’t a primitive type.  ints, doubles, chars are primitive types  Arrays aren’t

4 Declaring, creating, initializing Using the Shorthand Notation double[] myList = {1.9, 2.9, 3.4, 3.5}; This shorthand notation is equivalent to the following statements: double[] myList = new double[4]; myList[0] = 1.9; myList[1] = 2.9; myList[2] = 3.4; myList[3] = 3.5;

Can do: int[] anArr = {1, 9, 4, 3}; int[] anArr = new int[4]; anArr[0] = 1; anArr[1] = 9; anArr[2] = 4; anArr[3] = 3; Can’t do: int[] anArr = new int[4]; anArr = {1, 9, 4, 3}; //can’t initialize // after initializing anArr[4] = 5; //Arrays are FIXED LENGTH!

Structural Repetition using Loops Often used when executing code for each element in a collection of data double[] data = {3.2, 4.1, 2.1}; double sum = 0; for (double echidnas : data) { sum += echidnas; } System.out.println(sum);

 What if we want to add values to the end of our array? public static int[] addingarrays(int[] arr1, int[] arr2){ int x = arr1.length; int y = arr2.length; int[] newarr = new int[x+y]; for (int ind=0;ind<arr1.length;ind++) { newarr[ind]= arr1[ind]; System.out.println(newarr[ind]); } for (int ind=0;ind<arr2.length; ind++){ newarr[x+ind] = arr2[ind]; System.out.println(newarr[x+ind]); } System.out.println(newarr); //will this work? return newarr; }

java.util.Arrays import java.util.Arrays; public class ArrayMethods { public static void print(int[] newarr) { System.out.println(Arrays.toString(newarr)); } //... }  Contains useful methods for dealing with Arrays, including:  Sort  Arrays.sort(values);  binarySearch  int index = Arrays.binarySearch(values,3);  Equals  Arrays.equals(values,array2) // sees if values inside of array are equal  Fill  int[] array = new int[5];  Arrays.fill(array,0);  toString No sum!! You have to write your own.

To Try: 1.Write a method that takes as input parameters two arrays of ints, each array being the same length (obtained by arr.length). The method creates a new array, twice as long, with the two arrays joined so that: the first value is arr[0], the second value is arr2[0], the third value is arr[1], the fourth is arr2[1], etc. It returns that new array. 1.Write a method that takes as input an array of integers, in order, and a number. The method should create a new array, with the number inserted into the correct (ordered) location. It should return this array.

public static void main (String[] args) { int [] k = {3,2,5,7}; int[] m = {8,6,4,9}; int[] x = f1(k,m); System.out.println(Arrays.toString(x)); int n[] = {3,7,12,18,27,47,52}; int[] y = f2(n,42); System.out.println(Arrays.toString(y)); } public static int[] f1(int[] arr1, int[] arr2){ int x = arr1.length; int[] newarr = new int[x*2]; for (int i=0;i<arr1.length;i++) { newarr[i*2]= arr1[i]; newarr[i*2+1] = arr2[i]; } return newarr; } public static int[] f2(int[] arr1, int x){ int y = arr1.length; int[] newarr = new int[y+1]; int a = 0; for (int i=0;i<arr1.length;i++) { if ((arr1[i] > x) && (a==0)) { newarr[i] = x; a = 1; } newarr[i+a]= arr1[i]; } return newarr; }

Recursion: Looping public static int r1(int x) { if (x == 0) { return(x); } else { return(x + r1(x-1)); } System.out.println(r1(5)); Remember – Every function call can be replaced with what the function returns If what is returned includes another function call, we must calculate what that function call returns before we know what is returned from the original function call All initialization must happen OUTSIDE the function (method) The only thing that happens only once in a recursive function is the stopping condition

Look at what is returned from each function call What is our stopping condition? public static int r4(int x, int y) { if (x == 1) { return(0); } else if (y%x == 0) { return(1 + r4(x-1,y)); } else { return(0+ r4(x-1,y)); } System.out.println(r4(11,12));

public static int r3(int x, int[] y, int z) { if (z == y.length) { return(0); } else if (y[z] == x) { return(1 + r3(x,y,z+1)); } else { return(r3(x,y,z+1)); } int[] vb = {1,0,1,1,0,0,1,0,1}; System.out.println(r3(1,vb,0));

Strings A type, but extended A class that encompasses a character array and provides many useful behaviors Chapter 9 (for the book people) Strings are IMMUTABLE

15 String Accessors message = "Welcome"; message.length() (gives us what?)

16 Retrieving Characters in a String Cannot use message[0] (strings are immutable) Use message.charAt(index) Index starts from 0

17 String Comparisons String s1 = new String("Welcome“); String s2 = "Welcome"; if (s1 == s2) { // whaddaya think? } if (s1.equals(s2)){ // do s1 and s2 have the same contents? }

18 String Comparisons, cont. compareTo(Object object) String s1 = new String("Welcome“); String s2 = "hello"; if (s1.compareTo(s2) > 0) { // is s1 greater than s2? } else if (s1.compareTo(s2) == 0) { // do s1 and s2 have the same contents? } else { // s1 is lexicographically less than s2 }

Strings are Immutable! Immutable means we can’t change them. Can’t do: String str = “hello”; str[3] = ‘b’; So then why does this work? String str = "Hello"; System.out.println(str); //Prints Hello str = "Help!"; System.out.println(str); //Prints Help!

Why can I do this? (Or, isn’t this mutating the string?) String str = "Mississippi"; System.out.println(str); //Prints Mississippi str = str.replace("i", "!"); System.out.println(str); //Prints M!ss!ss!pp!

String methods: charAt(int index) – gets the character at the index within the string and returns the char indexOf(char c)- sees if a character is within a string and returns an int representing the index indexOf(char c, int fromindex) - sees if a character is within a string from fromindex onwards length() – returns number of characters in a string (note that for strings this is a method. What is it for arrays?) substring(int beginindex) – extracts and returns a substring from a string, starting at beginindex and going to the end of the string substring(int beginindex, int endindex) – extracts and returns a substring from beginindex to endindex toLowerCase() – returns a string that is the lower case version of the string toUpperCase() – returns a string that is the upper case version of the string trim() - removes white space (space char) from both ends of a string and returns that string toCharArray() – converts string to character array and returns that array equalsIgnoreCase(String otherstring) – compares string with otherstring and returns Boolean value (true if equals, false if doesn’t) compareToIgnoreCase(String otherstring) – compares string with otherstring, and returns int (>0 if string is “greater”, 0 if equal, and <0 if “less than”.

toString() method Printing objects: – If you print an object that doesn’t have a toString() method, you print out the hash representation of the object Not what we want. – All java built-in objects have a toString method implemented. E.g., double[] x = {3.2, 7.1, 8.83, 2.5};//Color is a built-in java class -RGB System.out.println(Arrays.toString(v)); Will print out: [3.2, 7.1, 8.83, 2.5] (this is the string that the toString method explicitly created for Arrays and returned.)

toString() (Better Example) Classes should all have there own toString method written So if we create an object of the class, we can print out the object. Example: Java has a built-in class for Color objects. Every color object has 3 fields: the amount of red, the amount of green, and the amount of blue. To make an object of type color: Color x = new Color( 255, 0, 0 ); The color class has a built in toString method (all Java classes do) So we can now print out color objects using System.out.println(x) We’ll get: java.awt.Color[r=255,g=0,b=0] Also happens with concatenation, e.g.,: Color x = new Color( 255, 0, 0 ); String str = “This is color: “; str += x; toString() is a method that you should write when you create a class definition. will automatically be used as above.

Try Write a function that takes as an input parameter a string, and prints out every value in the string backwards. So, for instance, if the input string is “yranib”, the function should print out: b i n a r y