CS163 - Chapter 7

Finding the largest element

double max = myList[0]; for (int i = 1; i < myList.length; i++) { if (myList[i] > max) max = myList[i]; }

Summing all elements

double total = 0; for (int i = 0; i < myList.length; i++) { total += myList[i]; }

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; Caution: using the shorthand notation, you have to do it all in one step!

Shifting Elements

double[] myList = {4, 5, 6, 7, 8, 9}; double temp = myList[0]; // Retain the first element // Shift elements left for (int i = 1; i < myList.length; i++) { myList[i - 1] = myList[i]; } // Move the first element to fill in the last position myList[myList.length - 1] = temp;

Selection sort: from idea to sol

for (int i = 0; i < list.length; i++) { select the smallest element in list[i..listSize-1]; swap the smallest with list[i], if necessary; // list[i] is in its correct position. // The next iteration apply on list[i+1..listSize-1] }

Returning an Array from a Method

public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = {1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1);

Passing Arrays to Methods

public static void printArray(int[] array) { for (int i = 0; i < array.length; i++) { System.out.print(array[i] + " "); } } Invoke the method int[] list = {3, 1, 2, 6, 4, 2}; printArray(list); Invoke the method printArray(new int[]{3, 1, 2, 6, 4, 2});

Declaring array variables

datatype[] arrayRefVar; Example: double[] myList; datatype arrayRefVar[]; // This style is allowed, but not preferred Example: double myList[];

Linear search: From Idea to Solution

/** The method for finding a key in the list */ public static int linearSearch(int[] list, int key) { for (int i = 0; i < list.length; i++) if (key == list[i]) return i; return -1; }

Wrap it in a Method

/** The method for sorting the numbers */ public static void selectionSort(double[] list) { for (int i = 0; i < list.length; i++) { // Find the minimum in the list[i..list.length-1] double currentMin = list[i]; int currentMinIndex = i; for (int j = i + 1; j < list.length; j++) { if (currentMin > list[j]) { currentMin = list[j]; currentMinIndex = j; } } // Swap list[i] with list[currentMinIndex] if necessary; if (currentMinIndex != i) { list[currentMinIndex] = list[i]; list[i] = currentMin; } } }

Binary search: from idea to solution

/** Use binary search to find the key in the list */ public static int binarySearch(int[] list, int key) { int low = 0; int high = list.length - 1; while (high >= low) { int mid = (low + high) / 2; if (key < list[mid]) high = mid - 1; else if (key == list[mid]) return mid; else low = mid + 1; } return -1 - low; }

Pass Arguments to Invoke the Main Method

?

Using Indexed Variables

After an array is created, an indexed variable can be used in the same way as a regular variable. For example, the following code adds the value in myList[0] and myList[1] to myList[2]. myList[2] = myList[0] + myList[1];

Array Initializers

Declaring, creating, initializing in one step: double[] myList = {1.9, 2.9, 3.4, 3.5}; This shorthand syntax must be in one statement.

Call Stack

EX: public class Test { public static void main(String[] args) { int x = 1; // x represents an int value int[] y = new int[10]; // y represents an array of int values m(x, y); // Invoke m with arguments x and y System.out.println("x is " + x); System.out.println("y[0] is " + y[0]); } public static void m(int number, int[] numbers) { number = 1001; // Assign a new value to number numbers[0] = 5555; // Assign a new value to numbers[0] } } When invoking m(x, y), the values of x and y are passed to number and numbers. Since y contains the reference value to the array, numbers now contains the same reference value to the same array.

Binary Search

For binary search to work, the elements in the array must already be ordered. Without loss of generality, assume that the array is in ascending order. e.g., 2 4 7 10 11 45 50 59 60 66 69 70 79 The binary search first compares the key with the element in the middle of the array. Consider the 3 cases: ✦ If the key is less than the middle element, you only need to search the key in the first half of the array. ✦ If the key is equal to the middle element, the search ends with a match. ✦ If the key is greater than the middle element, you only need to search the key in the second half of the array The binarySearch method returns the index of the element in the list that matches the search key if it is contained in the list. Otherwise, it returns -insertion point - 1. The insertion point is the point at which the key would be inserted into the list.

Processing Command-Line Parameters

In the main method, get the arguments from args[0], args[1], ..., args[n], which corresponds to arg0, arg1, ..., argn in the command line.

Enhanced for Loop (for-each loop)

JDK 1.5 introduced a new for loop that enables you to traverse the complete array sequentially without using an index variable. For example, the following code displays all elements in the array myList: for (double value: myList) System.out.println(value); In general, the syntax is for (elementType value: arrayRefVar) { // Process the value } You still have to use an index variable if you wish to traverse the array in a different order or change the elements in the array.

Pass By Value

Java uses pass by value to pass arguments to a method. There are important differences between passing a value of variables of primitive data types and passing arrays. ✦ For a parameter of a primitive type value, the actual value is passed. Changing the value of the local parameter inside the method does not affect the value of the variable outside the method. ✦ For a parameter of an array type, the value of the parameter contains a reference to an array; this reference is passed to the method. Any changes to the array that occur inside the method body will affect the original array that was passed as the argument.

Passing Arrays as Arguments

Objective: Demonstrate differences of passing primitive data type variables and array variables.

Copying Arrays

Often, in a program, you need to duplicate an array or a part of an array. In such cases you could attempt to use the assignment statement (=), as follows: list2 = list1; But, this doesn't coy the contents of the array, but instead merely copies the reference value. Using a loop: int[] sourceArray = {2, 3, 1, 5, 10}; int[] targetArray = new int[sourceArray.length]; for (int i = 0; i < sourceArrays.length; i++) targetArray[i] = sourceArray[i];

length of an array

Once an array is created, its size is fixed. It cannot be changed. You can find its size using: arrayRefVar.length For example, myList.length returns 10

declaring and creating in one step

datatype[] arrayRefVar = new datatype[arraySize]; Example: double[] myList = new double[10]; datatype arrayRefVar[] = new datatype[arraySize]; Ex: double myList[] = new double[10];

Searching Arrays

Searching is the process of looking for a specific element in an array; for example, discovering whether a certain score is included in a list of scores. Searching is a common task in computer programming. There are many algorithms and data structures devoted to searching. In this section, two commonly used approaches are discussed, linear search and binary search.

Processing Arrays

See the examples in the text (Section 7.2). 1. (Initializing arrays with input values) 2. (Initializing arrays with random values) 3. (Printing arrays) 4. (Summing all elements) 5. (Finding the largest element) 6. (Finding the smallest index of the largest element) 7. (Random shuffling) 8. (Shifting elements)

Selection Sort

Selection sort finds the smallest number in the list and places it first. It then finds the smallest number remaining and places it second, and so on until the list contains only a single number.

The Arrays.binarySearch Method

Since binary search is frequently used in programming, Java provides several overloaded binarySearch methods for searching a key in an array of int, double, char, short, long, and float in the java.util.Arrays class. For example, the following code searches the keys in an array of numbers and an array of characters. int[] list = {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79}; System.out.println("Index is " + java.util.Arrays.binarySearch(list, 11)); char[] chars = {'a', 'c', 'g', 'x', 'y', 'z'}; System.out.println("Index is " + java.util.Arrays.binarySearch(chars, 't')); For the binarySearch method to work, the array must be pre-sorted in increasing order.

The Arrays.sort Method

Since sorting is frequently used in programming, Java provides several overloaded sort methods for sorting an array of int, double, char, short, long, and float in the java.util.Arrays class. For example, the following code sorts an array of numbers and an array of characters. double[] numbers = {6.0, 4.4, 1.9, 2.9, 3.4, 3.5}; java.util.Arrays.sort(numbers); char[] chars = {'a', 'A', '4', 'F', 'D', 'P'}; java.util.Arrays.sort(chars); Java 8 now provides Arrays.parallelSort(list) that utilizes the multicore for fast sorting.

Sorting Arrays

Sorting, like searching, is also a common task in computer programming. Many different algorithms have been developed for sorting. This section introduces a simple, intuitive sorting algorithms: selection sort.

The Arrays.toString(list) Method

The Arrays.toString(list) method can be used to return a string representation for the list.

Heap

The JVM stores the array in an area of memory, called heap, which is used for dynamic memory allocation where blocks of memory are allocated and freed in an arbitrary order.

indexed variables

The array elements are accessed through the index. The array indices are 0-based, i.e., it starts from 0 to arrayRefVar.length-1. In the example in Figure 6.1, myList holds ten double values and the indices are from 0 to 9. Each element in the array is represented using the following syntax, known as an indexed variable: arrayRefVar[index];

Linear Search

The linear search approach compares the key element, key, sequentially with each element in the array list. The method continues to do so until the key matches an element in the list or the list is exhausted without a match being found. If a match is made, the linear search returns the index of the element in the array that matches the key. If no match is found, the search returns -1.

Anonymous Array

The statement printArray(new int[]{3, 1, 2, 6, 4, 2}); creates an array using the following syntax: new dataType[]{literal0, literal1, ..., literalk}; There is no explicit reference variable for the array. Such array is called an anonymous array.

Default Values

When an array is created, its elements are assigned the default value of 0 for the numeric primitive data types, '\u0000' for char types, and false for boolean types.

Main Method Is Just a Regular Method

You can call a regular method by passing actual parameters. Can you pass arguments to main? Of course, yes. For example, the main method in class B is invoked by a method in A, as shown below: public class A { public static void main(String[] args) { String[] strings = {"New York", "Boston", "Atlanta"}; B.main(strings); } } class B { public static void main(String[] args) { for (int i = 0; i < args.length; i++) System.out.println(args[i]); } }

Variable-Length Arguments

You can pass a variable number of arguments of the same type to a method.

array

a data structure that represents a collection of the same types of data. collection of variables of the same type size is fixed

creating arrays

arrayRefVar = new datatype[arraySize]; Example: myList = new double[10]; myList[0] references the first element in the array. myList[9] references the last element in the array

The arraycopy Utility

arraycopy(sourceArray, src_pos, targetArray, tar_pos, length); Example: System.arraycopy(sourceArray, 0, targetArray, 0, sourceArray.length);

Command-Line Parameters

class TestMain { public static void main(String[] args) { ... } } java TestMain arg0 arg1 arg2 ... argn

Random shuffling

for (int i = 0; i < myList.length - 1; i++) { // Generate an index j randomly int j = (int)(Math.random() * myList.length); // Swap myList[i] with myList[j] double temp = myList[i]; myList[i] = myList[j]; myList[j] = temp; }

Printing arrays

for (int i = 0; i < myList.length; i++) { System.out.print(myList[i] + " "); }

Initializing arrays with random values

for (int i = 0; i < myList.length; i++) { myList[i] = Math.random() * 100; }

index

integer value used to specify the position of an element in the array. starts at 0

Initializing arrays with input values

java.util.Scanner input = new java.util.Scanner(System.in); System.out.print("Enter " + myList.length + " values: "); for (int i = 0; i < myList.length; i++) myList[i] = input.nextDouble();