Why are arrays useful? We can use arrays to store a large amount of data without declaring many variables. Example: Read in a file of 1000 numbers, then print out the numbers in reverse order. Arrays can help us to group related data into elements. Example: For a given school exam, open a file full of exam scores and count up how many students got each score from 0 through 100. Arrays let us hold on to data and access it in random order. Example: Read a file full of weather data, store each month's weather stats as an element in a large array, and then examine it later to find overall weather statistics.

Array initialization statement Quick array initialization, general syntax: <type> [] <name> = {<value>, <value>, ..., <value>}; Example: int[] numbers = {12, 49, -2, 26, 5, 17, -6}; This syntax is useful when you know in advance what the array's elements will be. You don't explicitly specify the array's size when you use this syntax -- the Java compiler figures this out by looking at the number of values written. index 1 2 3 4 5 6 value 12 49 -2 26 17 -6

Array practice problem What are the contents of the array after the following code? int[] a = {2, 5, 1, 6, 14, 7, 9}; for (int i = 1; i < a.length; i++) { a[i] += a[i - 1]; } index 1 2 3 4 5 6 value 7 8 14 28 35 44 index 1 2 3 4 5 6 value

The Arrays class The Arrays class in package java.util has several useful static methods for manipulating arrays: Method name Description binarySearch(array, value) returns the index of the given value in this array (-1 if not found) equals(array1, array2) whether the two given arrays contain exactly the same elements in the same order fill(array, value) sets every element in the array to have the given value sort(array) arranges the elements in the array into ascending order toString(array) returns a String representing the array, such as "[10, 30, 17]"

Arrays.toString The Arrays.toString method is very useful when you want to print an array's elements separated by commas. Arrays.toString accepts an array as a parameter and returns the String representation, which you can then print. Example: int[] a = {2, 5, 1, 6, 14, 7, 9}; for (int i = 1; i < a.length; i++) { a[i] += a[i - 1]; } System.out.println("a is " + Arrays.toString(a)); Output: a is [2, 7, 8, 14, 28, 35, 44]

Traversal algorithms suggested reading: 7.1, 7.2, 4.4

Array traversal traversal: An examination of each element of an array. Traversal algorithms often take the following form: for (int i = 0; i < <array>.length; i++) { do something with <array> [i]; }

Printing array elements Example (print each element of an array on a line): int[] list = {4, 1, 9, 7}; for (int i = 0; i < list.length; i++) { System.out.println(i + ": " + list[i]); } Output: 0: 4 1: 1 2: 9 3: 7 How could we change the code to print the following? 4, 1, 9, 7 (do not use Arrays.toString.)

Examining array elements Example (find largest even integer in an array): int[] list = {4, 1, 2, 7, 6, 3, 2, 4, 0, 9}; int largestEven = 0; for (int i = 0; i < list.length; i++) { if (list[i] % 2 == 0 && list[i] > largestEven) { largestEven = list[i]; } System.out.println("Largest even: " + largestEven); Output: Largest even: 6

String traversal Strings are like arrays of chars; they also use 0-based indexes. We can write algorithms to traverse strings to compute information: // string stores voters' votes // (R)epublican, (D)emocrat, (I)ndependent String votes = "RDRDRRIDRRRDDDDIRRRDRRRDIDIDDRDDRRDRDIDD"; int[] counts = new int[3]; // R -> 0, D -> 1, I -> 2 for (int i = 0; i < votes.length(); i++) { char c = votes.charAt(i); if (c == 'R') { counts[0]++; } else if (c == 'D') { counts[1]++; } else { // c == 'I') counts[2]++; } System.out.println(Arrays.toString(counts)); Output: [17, 18, 5]

Section attendance problem Imagine an input file containing attendance data. Each line represents a section. Each section has 5 students. Every 5 numbers represent 1 week's attendance record. The first of the 5 marks whether the first student attended. The second of the 5 marks whether the second student attended. ... A mark of 0 means the student did not attend, and a mark of 1 means the student did attend. Example (contains 3 sections and a 9-week attendance record): 111111101011111101001110110110110001110010100 111011111010100110101110101010101110101101010 110101011011011011110110101011010111011010101 week1 week2 week3 week4 week5 week6 week7 week8 week9 11111 11010 11111 10100 11101 10110 11000 11100 10100 week2 student1 student2 student3 student4 student5 1 1 0 1 0

Section attendance problem Write a program that reads the preceding section data file and produces output such as the following: Section #1: Sections attended: [9, 6, 7, 4, 3] Student scores: [20, 18, 20, 12, 9] Student grades: [100.0, 90.0, 100.0, 60.0, 45.0] Section #2: Sections attended: [6, 7, 5, 6, 4] Student scores: [18, 20, 15, 18, 12] Student grades: [90.0, 100.0, 75.0, 90.0, 60.0] Section #3: Sections attended: [5, 6, 5, 7, 6] Student scores: [15, 18, 15, 20, 18] Student grades: [75.0, 90.0, 75.0, 100.0, 90.0] Assume the input file exists and is valid.

Array transformations Problems like the preceding one are examples where we use data in one form to compute new data in another form. We sometimes call this transforming the data. Often each transformation is stored into its own array. Many transformation problems require some sort of mapping between the original data and array indexes. Sometimes the mapping is a tally effect (if the input value is the integer i, store it into array index i ) Sometimes the mapping relates to the position of the original data (store each i th value we read into index i ) Sometimes the mapping must be done explicitly (count occurrences of "X" into index 0, count occurrences of "O" into index 1)