import java io import java util public class

import java. io. *; import java. util. *; public class echo. File { public static void main(String[] args) throws File. Not. Found. Exception { Scanner console. In = new Scanner(System. in); System. out. print("Input file: "); String in. Filename = console. In. next. Line(); File in. File = new File(in. Filename); Scanner input. File = new Scanner(in. File); System. out. print("Output file: "); String out. Filename = console. In. next. Line(); File out. File = new File(out. Filename); Print. Stream output. File = new Print. Stream(out. File); String my. String; while ( input. File. has. Next. Line() ) { my. String = input. File. next. Line(); output. File. println(">" + my. String); } System. out. println("Done"); }} Copyright 2006 by Pearson Education 1

Input file: Sue. txt Output file: out. Sue. txt Done Sue. txt Sue, la

import java. io. *; Combining some import java. util. *; statements public class echo. File 2 { public static void main(String[] args) throws File. Not. Found. Exception { Scanner console. In = new Scanner(System. in); System. out. print("Input file: "); String in. Filename = console. In. next. Line(); Scanner input. File = new Scanner(new File(in. Filename)); System. out. print("Output file: "); String out. Filename = console. In. next. Line(); Print. Stream output. File = new Print. Stream(new File(out. Filename)); while ( input. File. has. Next. Line() ) output. File. println(">" + input. File. next. Line()); System. out. println("Finished copying file " + in. Filename + " to “ + out. Filename); }} Copyright 2006 by Pearson Education 3

import java. io. *; Combining even more statements. import java. util. *; Notice: No String public class Echo. File 3 { variables. public static void main(String[] args) throws File. Not. Found. Exception { Scanner console. In = new Scanner(System. in); System. out. print("Input file: "); Scanner input. File = new Scanner(new File(console. In. next. Line())); System. out. print("Output file: "); Print. Stream output. File = new Print. Stream(new File(console. In. next. Line())); while ( input. File. has. Next. Line() ) output. File. println(">" + input. File. next. Line()); System. out. println("Done"); } Copyright 2006 by Pearson Education 4

Building Java Programs Chapter 7: Arrays Copyright 2006 by Pearson Education 5

Chapter outline n array basics n n n array traversal algorithms n n declaring and initializing an array getting and setting values of elements of an array limitations of arrays for counting and tallying printing an array's elements searching an array reversing an array advanced array usage n n n arrays as parameters to methods String and Graphics methods that use arrays the Arrays class command-line arguments shifting elements in an array Copyright 2006 by Pearson Education 6

Array basics reading: 7. 1 Copyright 2006 by Pearson Education 7

A problem we can't solve (yet) n Consider the following program (input underlined): How many days' temperatures? 7 Day 1's high temp: 45 Day 2's high temp: 44 Day 3's high temp: 39 Day 4's high temp: 48 Day 5's high temp: 37 Day 6's high temp: 46 Day 7's high temp: 53 Average temp = 44. 6 4 days were above average. n We need the temperatures to compute the average, and again to tell how many were above average. Copyright 2006 by Pearson Education 8

Why the problem is tough n We appear to need each input value twice: n n n We could read each value into a variable. . . n n once to compute the average a second time to count how many were above average However, we don't know how many variables to declare. We don't know how many days' weather will be typed until the program is running. We need a way to declare many variables in one step. Copyright 2006 by Pearson Education 9

Activity: Find the difference between each element of the following array and the average value in the array Let’s do it ourselves. List of numbers 2 6 8 2 9 1 8 2 6 6 Average ____ Copyright 2006 by Pearson Education Difference from average 10

Activity: Find the difference between each element of the following array and the average value in the array Let’s do it ourselves. List of numbers 2 6 8 2 9 1 8 2 6 6 Average __5__ Copyright 2006 by Pearson Education Difference from average -3 1 3 -3 4 -4 3 -3 1 1 11

Arrays n array: A variable that stores many values of the same type. n n n element: One value in an array. index: A 0 -based integer used to access an element from an array. We usually draw an array as a row or column of boxes. n Example: an array of ten integers index 0 1 value 12 49 element 0 Copyright 2006 by Pearson Education 2 3 4 5 6 -2 26 5 17 -6 element 4 7 8 84 72 9 3 element 9 12

Array declaration n Declaring/initializing an array: <type> [] <name> = new <type> [ <length> ]; n n n The length of the array is specified between [ ] brackets. Example: int[] numbers = new int[10]; index 0 1 2 3 4 5 6 7 8 9 value 0 0 0 0 0 The array's length can be any expression. n Example: int len = console. next. Int; int[] data = new int[len]; Copyright 2006 by Pearson Education 13

Array auto-initialization n When arrays are initially constructed, every element is automatically initialized to a "zero-equivalent" value. n n n int: double: boolean: char: String or object: 0 0. 0 false '' null int[] mine = new int[5] index 0 1 2 3 value 0 0 (the "null character") (null means "no object") 4 An array of integers 0 double[] yours = new double[4]; index 0 1 2 3 value 0. 0 Copyright 2006 by Pearson Education An array of real numbers 14

Accessing array elements n Assigning a value to an array element: <array name> [ <index> ] = <value> ; n Example: int[] numbers = new int[10]; numbers[0] = 27; numbers[3] = -6; index 0 value 27 1 2 3 4 5 6 7 8 9 0 0 -6 0 0 0 Copyright 2006 by Pearson Education 15

Accessing array elements n Using an array element's value in an expression: <array name> [ <index> ] n Example: System. out. println(numbers[0]); if (numbers[3] < 0) { System. out. println("Element 3 is negative. "); } index 0 1 2 3 4 5 6 7 8 9 value 27 0 0 -6 0 0 0 Output: Element 3 is negative. Copyright 2006 by Pearson Education 16

Out-of-bounds indexes n The indexes that are legal to access in an array are those in the range of 0 to the array's length - 1. n n Reading or writing any index outside this range will throw an Array. Index. Out. Of. Bounds. Exception. Example: int[] data = new int[10]; System. out. println(data[0]); System. out. println(data[-1]); System. out. println(data[9]); System. out. println(data[10]); // // okay exception index 0 1 2 3 4 5 6 7 8 9 value 0 0 0 0 0 Copyright 2006 by Pearson Education 17

Arrays of other types n Arrays can contain other types, such as double. n n Example: double[] results = new double[6]; results[2] = 3. 4; results[5] = -0. 5; index 0 1 2 3 value 0. 0 3. 4 0. 0 4 5 0. 0 0. 5 Example: boolean[] tests = new boolean[6]; tests[3] = true; index 0 1 2 3 4 5 value false true false Copyright 2006 by Pearson Education 18

Activity: What are the final contents of the array? int[] numbers = new int[8]; numbers[1] = 4; numbers[4] = 99; numbers[7] = 2; int x = numbers[1]; numbers[x] = 44; numbers[7]] = 11; 0 1 numbers Copyright 2006 by Pearson Education 2 3 4 5 6 7

Activity: What are the final contents of the array? int[] numbers = new int[8]; numbers[1] = 4; numbers[4] = 99; numbers[7] = 2; int x = numbers[1]; numbers[x] = 44; numbers[7]] = 11; 0 numbers 0 1 4 Copyright 2006 by Pearson Education 2 11 0 3 4 44 0 5 6 0 7 2

Arrays and for loops n Arrays are very commonly used with for loops that pass over each element and process it in some way: n n Example (print each element of an array): for (int i = 0; i < 8; i++) { System. out. print(numbers[i] + " "); } System. out. println(); // end the line of output Output (when used on array from previous slide): 0 4 11 0 44 0 0 2 Copyright 2006 by Pearson Education 22

Assigning array values in a for loop n Sometimes we assign each array element a value in a for loop. n Example: for (int i = 0; i < 8; i++) { numbers[i] = 2 * i; } n index 0 1 2 3 4 value 0 2 4 6 8 5 6 7 10 12 14 What values would be stored into the array after this code? for (int i = 0; i < 8; i++) { numbers[i] = i * i; } value 0 Copyright 2006 by Pearson Education 1 4 9 16 25 36 49 23

Example: assigning each array element a value in a for loop. for (int

The. length field n An array's length field stores its number of elements. n n n It does not use parentheses like a String's. length(). Example (using array from previous slide): n n General syntax: <array name>. length for (int i = 0; i < numbers. length; i++) { System. out. print(numbers[i] + " "); } Output: 0 1 4 9 16 25 36 49 What expression refers to the last element of the array? The middle element? Copyright 2006 by Pearson Education 25

Weather question n Use an array to solve the weather problem: How many days' temperatures? 7 Day 1's high temp: 45 Day 2's high temp: 44 Day 3's high temp: 39 Day 4's high temp: 48 Day 5's high temp: 37 Day 6's high temp: 46 Day 7's high temp: 53 Average temp = 44. 6 4 days were above average. Copyright 2006 by Pearson Education 26

Weather solution: public static void main(String[] args) { Scanner console = new Scanner(System. in); System. out. print("How many days' temperatures? "); int days = console. next. Int(); int[] temperatures = new int[days]; // to store temps int sum = 0; for (int i = 0; i < days; i++) { // read & store each System. out. print("Day " + (i+1) + "'s high temp: "); temperatures[i] = console. next. Int(); sum += temperatures[i]; } double average = (double) sum / days; } int count = 0; // count days for (int i = 0; i < days; i++) { if (temperatures[i] > average) count++; } } System. out. printf("Average temp = System. out. println(count + " days Copyright 2006 by Pearson Education above average { %. 1 fn", average); above average");

For-Each Loop n a. k. a. Enhanced For Loop This was our for loop for printing an array: for (int i = 0; i < numbers. length; i++) { System. out. print(numbers[i] + " "); } Java has a simplified for loop that can be used when examining each element in an array: for (int num : numbers) { System. out. print(num + " "); } We read this loop as: "For each int num in numbers from element 0 through element length()-1. " Copyright 2006 by Pearson Education

Initializing Arrays n n Often we need to put preset values into an array. Java provide a way to quickly initialize an array: <type>[] <name> = {<value>, . . . <value>}; n Example: int[] days. In. Month = {31, 28, 31, 30, 31}; n n n This syntax is useful when you know in advance what the array's contents will be. You don't explicitly specify the array's size. The Java compiler figures out the size by looking at the number of values written between { and }. String[] days. Names = {"Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"}; Copyright 2006 by Pearson Education

Activity: n What are the final values stored in the array? int[] a =

Result n {2, 7, 8, 14, 28, 35, 44}; Copyright 2006 by Pearson Education

Arrays for counting and tallying reading: 7. 1 Copyright 2006 by Pearson Education 32

A multi-counter problem n Problem: Examine a large integer and count the number of occurrences of every digit from 0 through 9. n n We need to examine each digit of the large integer and count how many times we've seen that digit. n n Example: The number 229231007 contains: two 0 s, one 1, three 2 s, one 7, and one 9. This will require counters for each of the values 0 --9. We could declare 10 counter variables for this. . . n n A better solution is to use an array of size 10. The element at index i will store the counter for digit value i. Copyright 2006 by Pearson Education 33

Creating an array of tallies n The following code builds an array of digit counters: int num = 229231007; int[] counts = new int[10]; while (num > 0) { int digit = num % 10; counts[digit]++; num = num / 10; } index 0 1 2 3 4 5 6 7 8 9 value 2 1 3 1 0 0 0 1 Copyright 2006 by Pearson Education 34

Why does this work? Each item in an array is just that type of element. What did he say? int[] my. Array = new int[10]; Each item in my. Array is just an integer. You can do anything with each item that you can with an integer. my. Array[4]++; // Add 1 to item 4 in array counters for ( int i = 0; i < my. Array[7]; i++) { …. } Copyright 2006 by Pearson Education 35

Arrays as parameters n An array can be passed as a parameter. n Example: public static double average(int[] numbers) { n Method declaration syntax: n Call syntax: n Example: . . <method name>(<type>[] <name>) <method name>(<array name>); int[] scores = {13, 17, 12, 15, 11}; double avg = average(scores); Copyright 2006 by Pearson Education

Example: public static void main(String[] args) { int testdata[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; System. out. println("Average= "+average(testdata)); } // calculate the average of an array public static int average(int a[]) { int sum = 0; for (int val : a) { sum += val; } return sum / a. length; } Output: Average= 5 Copyright 2006 by Pearson Education

Activity: n Write a method to find and return the maximum value in an

Arrays are Objects and are stored by reference int[] a = new int[5]; a[2] = 22; a[4] = 44; The variable a holds a reference to an array. The array of 5 integers is stored elsewhere in memory. Just like with Objects, if you assign an array variable, you are just making a second reference to the same array. Copyright 2006 by Pearson Education Memory n a [0] [1] int[5] [2] [3] [4] 0 0 22 0 44

Arrays are Objects and are stored by reference int[] b = a; n Memory The variable b now holds a reference to array a. The array of 5 integers is stored elsewhere in memory. Just like with Objects, if you assign an array variable, you are just making a second reference to the same array. Copyright 2006 by Pearson Education a b [0] [1] int[5] [2] [3] [4] 0 0 22 0 44

Modifying array parameters n Because arrays are passed by reference, we can make methods that rearrange or change an array. // reverse the contents of an array // by swapping pairs of items // starting at the outside until reach the middle public static void reverse(int a[]) { int bot = 0; int top = a. length - 1; while (bot < top) { int temp = a[bot]; // swap a[bot] = a[top]; // pair of a[top] = temp; // items bot++; top--; } } Copyright 2006 by Pearson Education

0 1 2 3 4 5 6 7 Array a 3 9 temp = 3 12 a[0] = 11 4 a[7] = 3 16 0 9 11 top = 7 bot = 0 Two car garage analogy: How do you exchange the positions of 2 cars in a 2 car garage? Do you back one out and drive it into the other spot? NO You park one car in a temporary spot, move the other car into the now available spot and move the temp car to the newlyavailable spot. while (bot < top) { int temp = a[bot]; a[bot] = a[top]; a[top] = temp; bot++; top--; Copyright 2006 by Pearson Education // swap // pair of // items 42

0 1 2 3 4 5 6 7 Array a 3 9 12 4 16 0 9 11 Array a 11 9 12 4 16 0 8 3 Array a 11 8 12 4 16 0 9 3 Array a 11 8 0 4 16 12 9 3 top = 7 bot = 0 top = 6 bot = 1 top = 5 bot = 2 top = 4 bot = 3 Copyright 2006 by Pearson Education Array a 11 Stops because 8 bot is now 0 greater than or equal to top. 16 4 Question? 12 Under what 9 circumstance 3 would top = bot when finished? top = 3 bot = 4 43