Arrays Clark Savage Turner, J.D., Ph.D. Copyright © 2000 by C Scheftic. All rights reserved. These notes do rely heavily on ones originally developed  by John Lewis and William Loftus to accompany  by John Lewis and William Loftus to accompany D Java Software Solutions © Addison-Wesley  by Ralph Morelli to accompany  by Ralph Morelli to accompany D Java Java Java © Prentice-Hall  and on the textbook by Bailey & Bailey  and on the textbook by Bailey & Bailey D Java Elements © McGraw Hill

CSC-101Arrays D An array is an ordered list of values An array of size N is indexed from zero to N-1 scores The entire array has a single name Each value has a numeric index This array holds 10 values that are indexed from 0 to 9 write & save on board

CSC-101Arrays D A particular value in an array is referenced using the array name followed by the array name followed by the index (position)in brackets the index (position)in brackets D For example, the expression scores[2] scores[2] refers to the value 94 (which is the 3rd value in the array) D That expression represents a place to store a single integer, and can be used wherever an integer variable can be used D For example, it can be assigned a value, printed, or used in a calculation

CSC-101 Referring to Array Elements Suppose j is 3 and k is 5 and you use valid scores[ 4 ]// scores[4] is 67 scores[ 4 ]// scores[4] is 67 scores[ j ]// scores[3] is 82 scores[ j ]// scores[3] is 82 scores[ k ]// scores[5] is 98 scores[ k ]// scores[5] is 98 scores[ j + k ]// scores[8] is 74 scores[ j + k ]// scores[8] is 74 scores[ k % j ]// scores[2] is 94 scores[ k % j ]// scores[2] is 94invalid scores[ 4.0 ]// double, not int scores[ 4.0 ]// double, not int scores[ ' 4 ' ]// char, not int scores[ ' 4 ' ]// char, not int scores[ " 4 " ]// String, not int scores[ " 4 " ]// String, not int scores[ j * k ]// 15 is out of bounds scores[ j * k ]// 15 is out of bounds scores[ 2 * k ]// 10 is also out, by 1 scores[ 2 * k ]// 10 is also out, by 1

CSC-101Arrays D An array stores multiple values of the same type That type can be a primitive type or an object type That type can be a primitive type or an object type D Therefore, we can create an array of integers, or an array of integers, or an array of characters, or an array of characters, or an array of String objects, or an array of String objects, or an array our own objects, an array our own objects, etc. etc. D In Java, the array itself is an object, so the name of the array is an object reference variable, & the name of the array is an object reference variable, & the array itself is instantiated separately. the array itself is instantiated separately.

CSC-101 Declaring Arrays  The scores array could be declared as follows: int[] scores = new int[10]; int[] scores = new int[10]; D This is the same as: int[] scores; int[] scores; scores = new int[10];  The variable scores is declared to be of type int[] (that is, an array of integers) D It is set to a new array object that can hold 10 integers D Note that the type of the array does not specify its size, but each object of that type has a specific size  See BasicArray.java (page 270) BasicArray.java

CSC-101 Declaring Arrays D Some examples of array declarations: float[] prices = new float[500]; float[] prices = new float[500]; boolean[] flags; boolean[] flags; flags = new boolean[20]; flags = new boolean[20]; char[] codes = new char[1750]; char[] codes = new char[1750];

CSC-101 Initializing Arrays D When arrays are declared, each cell is always initialized. D With primitive data types: You may initialize them yourself: You may initialize them yourself: int[ ] numbers = { 1, 2, 3, 4, 5} If you don’t, initial values are established for you: If you don’t, initial values are established for you: int[ ] digits = new int [5]; // set each to 0 (zero) D With objects: You may initialize them yourself: You may initialize them yourself: String[ ] articles = { " a ", " an ", " the " } If you don’t, initial values are established for you: If you don’t, initial values are established for you: String[ ] words = new String [5]; // set each to null But it is a semantic error (at best) to refer to an uninstantiated (null) array element But it is a semantic error (at best) to refer to an uninstantiated (null) array element

CSC-101 Bounds Checking D Once an array is created, it has a fixed size D An index used in an array reference must specify a valid element The index value must be in bounds (0 to N-1) The index value must be in bounds (0 to N-1) The Java interpreter will throw an exception if an array index is out of bounds The Java interpreter will throw an exception if an array index is out of bounds D This is called automatic bounds checking, and we have encountered this in another form already String myString = "something"; System.out.print(charAt(myString.length( )-1));

CSC-101 Bounds Checking  For example, if the array codes can hold 100 values, it can only be indexed using the numbers 0 to 99  If count has the value 100, then the following reference will cause an ArrayOutOfBoundsException : System.out.println (codes[count]); D It’s common to introduce off-by-one errors when using arrays for (int index=0; index <= 100; index++) codes[index] = index*50 + epsilon; problem

CSC-101 Bounds Checking  Each array object has a public constant (an instance variable) called length that stores the size of the array Note that length holds the number of elements, not the largest index Note that length holds the number of elements, not the largest index Like any other object, it is referenced via the array name: Like any other object, it is referenced via the array name: scores.length scores.length D D Compare and contrast this with the String method myString.length() myString.length()  See ReverseNumbers.java (page 272) ReverseNumbers.java  See LetterCount.java (page 274) LetterCount.java

CSC-101 Initializer Lists D An initializer list can be used to instantiate and initialize an array in one step D The values are delimited by braces and separated by commas D Examples: int[] units = {147, 323, 89, 933, 540, int[] units = {147, 323, 89, 933, 540, 269, 97, 114, 298, 476}; 269, 97, 114, 298, 476}; char[] letterGrades = {'A', 'B', 'C', 'D', 'F'}; char[] letterGrades = {'A', 'B', 'C', 'D', 'F'};

CSC-101 Initializer Lists D Note that when an initializer list is used: the new operator is not used the new operator is not used (an initializer list can be used only in the declaration of an array) no size value is specified no size value is specified (the size of the initialized array is determined by the number of items in the initializer list)  See Primes.java (page 278) Primes.java

CSC-101 Another example, viewed several ways D int daysInMonth[ ] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; D int daysInMonth [ ] = new int [12]; for ( i = 0 ; i < 12 ; i++) { daysInMonth[i] = 31; } daysInMonth [ 1] = 28; // except in leap years… daysInMonth [3] = daysInMonth[5] = daysInMonth [8] = daysInMonth[10] = 30; D int daysInMonth [ ] = new int [13]; for ( i = 1 ; i <= 12 ; i++) { daysInMonth[i] = 31; } daysInMonth [ 2] = 28; // except in leap years… daysInMonth [4] = daysInMonth[6] = daysInMonth [9] = daysInMonth[11] = 30; daysInMonth[0] = 0; D int daysInMonth[ ] = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; Ask this, have students try it, then review the options together.

CSC-101 And a companion example D String monthNames[ ] = {"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"}; versus D String monthNames[ ] = {null, "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"}; Ask this, have students try it, then review the options together.

CSC-101 Try using those examples System.out.println(”The longest months are:"); for (i = 0; i < 12; i++) { if (daysInMonth[i] == 31) System.out.println(monthNames[i]); } if (daysInMonth[i] == 31) System.out.println(monthNames[i]); } System.out.print("A shortest month is "); shortest = 0; for (i = 1; i < 12; i++) { if (daysInMonth[i] < daysInMonth[shortest]) if (daysInMonth[i] < daysInMonth[shortest]) shortest = i; } shortest = i; }System.out.println(monthNames[shortest]+".");

CSC-101 Array Declarations Revisited D Technically, the brackets of the array type can be associated with the array’s element type, or its element type, or its name name D Therefore the following declarations are equivalent: float[] prices; float[] prices; float prices[]; float prices[]; D The first format is generally more readable, and preferred.

CSC-101 Arrays as Parameters D An entire array can be passed to a method as a parameter D Like any other object, the reference to the array is passed, making aliases of the formal and actual parameters. Thus: changing an array element in the method changes the original changing an array element in the method changes the original D An array element can be passed to a method as well. Then it will follow the parameter passing rules of that element's type it will follow the parameter passing rules of that element's type

CSC-101 More on Arrays of Objects D The elements of an array can be object references  The following declaration reserves space to store 25 references to String objects String[] words = new String[25]; String[] words = new String[25];  It does NOT create the String objects themselves Each object stored in an array must be instantiated separately. Each object stored in an array must be instantiated separately. Until then, recall that the cell contains a null reference. Until then, recall that the cell contains a null reference.  See GradeRange.java (page 280) GradeRange.java

CSC-101 Command-Line Arguments  The signature of the main method indicates that it takes an array of String objects as a parameter D These values come from command-line arguments that are provided when the interpreter is invoked  For example, the following invocation of the interpreter passes an array of three String objects into main: > java DoIt pennsylvania texas california D These strings are stored at indexes 0-2 of the parameter  See NameTag.java (page 281) NameTag.java D CS: I am omitting this: doesn’t work this way on Mac and I won’t test ‘em on it so why bother with it at this point. I will just be prepared to answer questions from those who have read this part.

CSC-101 Arrays of Objects D Objects can have arrays as instance variables D Therefore, fairly complex structures can be created simply with arrays and objects D The software designer must carefully determine an organization of data and objects that makes sense for the situation  See Tunes.java (page 282) Tunes.java  See CDCollection.java (page 284) CDCollection.java  See CD.java (page 286) CD.java

CSC-101 Two-Dimensional Arrays D A one-dimensional array stores a simple list of values D A two-dimensional array can be thought of as a table of values, with rows and columns D A two-dimensional array element is referenced using two index values D To be precise, a two-dimensional array in Java is an array of arrays  See TwoDArray.java (page 299) TwoDArray.java

CSC-101 Multidimensional Arrays D An array can have as many dimensions as needed, creating a multidimensional array D Each dimension subdivides the previous one into the specified number of elements  Each array dimension has its own length constant D Because each dimension is an array of array references, the arrays within one dimension could be of different lengths