Introducing arrays. Week 4: arrays and strings. Creating arrays. Declaring arrays. Initialising arrays. Declaring and creating in one step

Transcription

1 School of Computer Science, University of Birmingham Java Lecture notes. M. D. Ryan. September Introducing arrays Week 4: arrays and strings Arrays: a data structure used to store multiple data, all of the same type. Strings: a special case; an array of characters. The array is a data structure that stores a collection of the same types of data. Java treats these arrays as objects. An array of 10 elements of type double: myarr[0] myarr [1] myarr [2] myarr [3] myarr [4] myarr [5] myarr [6] myarr [7] myarr [8] myarr [9] double[] myarr = new double[10] Declaring arrays datatype[] arrayname; Example: String[] product; int[] price; Creating arrays arrayname = new datatype[arraysize]; Example: mylist = new double[10]; Declaring and creating in one step type[] arrayname = new type[size]; double[] price = new double[10]; Initialising arrays Using a loop: for (int i = 0; i < mylist.length; i++) mylist[i] = -1; note how to find length Declaring, creating, initializing in one step: double[] mylist = 1.9, 2.9, 3.4, 3.5; 1

2 How the cells are numbered The first cell is numbered 0, and the last one is numbered length-1. A for-loop to process each element of the array will typically look like for (int i = 0; i < arr.length; i++) // process arr[i]... Example: array of temperatures Let s have an array int[] temp; such that temp[0],..., temp[6] store the temperatures for Monday,..., Sunday. We ask the user to enter the temperatures... and print out a graph of the temperatures... along with information about which was the hottest day and the coolest day. Temperatures: output Please enter the temperature for Monday: 18 Please enter the temperature for Tuesday: 22 Please enter the temperature for Wednesday: 23 Please enter the temperature for Thursday: 19 Please enter the temperature for Friday: 18 Please enter the temperature for Saturday: 16 Please enter the temperature for Sunday: 17 Monday ****************** Tuesday ********************** Wednesday *********************** Thursday ******************* Friday ****************** Saturday **************** Sunday ***************** Hottest day was Wednesday, at 23 Coolest day was Saturday, at 16 import java.io.*; public class Temp public static void main( String[] args ) throws IOException String[] day = "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"; int[] temp = new int[day.length]; BufferedReader keyboard = new BufferedReader( new InputStreamReader( System.in ) ); for (int d=0; d<day.length; d++) System.out.print( "Please enter the temperature for "+day[d]+": "); String input = keyboard.readline(); temp[d] = Integer.parseInt(input); for (int d=0; d<day.length; d++) System.out.print(day[d]); for (int i=0; i<10-day[d].length(); i++) System.out.print(' '); for (int i=0; i<temp[d]; i++) System.out.print('*'); System.out.println(); System.out.println(); // now lets calculate max and min int max=0, min=0; for (int d=0; d<day.length; d++) if (temp[d]>temp[max]) max=d; if (temp[d]<temp[min]) min=d; System.out.println("Hottest day was "+day[max]+ ", at "+temp[max] ); System.out.println("Coolest day was "+day[min]+ ", at "+temp[min] ); Exceptions Exceptions are errors detected by the computer when the program is run. If you try to access temp[12], the program will throw an ArrayIndexOutOfBoundsException. If an error is detected when we read from a BufferedReader, for example when the program inputs something from the keyboard or a file, the program will throw an IOException. 2

3 Array to store frequency of letters What is the most common letter in /usr/dict/words? Basic idea: freq[i] stores how many occurrences of the i+1th letter we have seen. do ch = next-character-in-file; if (ch >= 'a') && (ch <= 'z') freq[ch-'a']++; while (not-end-of-file); for (i=0; i<26; i++) System.out.println(freq[i]); import java.io.*; // analyse the frequencies of letters in a file public class Frequency public static void main (String[] args) throws FileNotFoundException,IOException BufferedReader myfile = new BufferedReader(new FileReader("/usr/dict/words")); int[] freq = new int[26]; int ch; do ch = myfile.read(); if (ch>='a' && ch<='z') freq[ch-'a']++; while (ch!= -1); // print out frequencies for (int i=0; i<26; i++) System.out.println("Freq of " + (char)(i+'a') + " = " + freq[i]); Freq of a = Freq of b = 3691 Freq of c = 7762 Freq of d = 5555 Freq of e = Freq of f = 2495 Freq of g = 3901 Freq of h = 4929 Freq of i = Freq of j = 288 Freq of k = 1764 Freq of l = 9825 Freq of m = 5381 Freq of n = Freq of o = Freq of p = 5238 Freq of q = 366 Freq of r = Freq of s = 9761 Freq of t = Freq of u = 6445 Freq of v = 1811 Freq of w = 1796 Freq of x = 616 Freq of y = 3596 Freq of z = 410 Searching an array Set up a sorted array of random integers. Search for a particular integer requested by the user; let s say it s 571. Report The first position of 571 is 28, or 571 is not present Linear search Linear search: just look through the array until you find the number, or you reach the end. int j; for (j=0; j<numbers.length; j++) if (x == numbers[j]) break; if (j<numbers.length) // we broke out System.out.println("The first position of " +numbers[j]+" is " +j); else // we terminated the loop normally System.out.println(x+" not present"; Binary search Linear search doesn t exploit the fact that the array is sorted. Binary chop is more efficient. Suppose we are looking for

4 // binary search int from = 0; int to = numbers.length; // this is one more than // the highest valid index int mid; while (from < to-1) // stop when from == to-1 mid = (from+to)/2; if (x < numbers[mid]) to=mid; else from=mid; if (numbers[from] == x) System.out.println("The first position of "+x+" is "+from); else System.out.println(x+" not present"); Multi-dimensional arrays The arrays we have seen so far are linear: we think of the values they store as arranged in a straight line. Arrays can also store sets of values whose natural arrangement is two dimensional. Example: the pixels on the screen. screen[0][0] screen[12][0] Initialising the screen int[][] screen = new int [NO_COLS][NO_ROWS]; screen[6][16] int[][] screen = new int [21][17] screen[20][16] screen[6][16] // the 17th cell of the 7th column screen[6] // the 7th column screen.length // the number of columns, i.e. 21 screen[6].length // the number of rows (in the 7th column), i.e. 17 for (int col=0; col<no_cols; col++) for (int row=0; row<no_rows; row++) screen[col][row] = 0; Show on a picture the order in which the screen cells are initialised. Elementary graphics 4

5 The open circle: radius 7, centre (11,9) The solid circle: radius r, centre (x0,y0) for (angle=0; angle<360; angle++) radians = Math.PI*angle/180; row = (int)math.round(11 + 7*Math.cos(radians)); col = (int)math.round( 9 + 7*Math.sin(radians)); screen[row][col] = RED; for (x=-r; x<=r; x++) h = (int)math.round(math.sqrt(r*r-x*x)); for (y=-h; y<=h; y++) screen[x0+x][y0+y] = PURPLE; Strings We have seen: String greeting = "Hello World!"; String str = "The area of circle " + rad+" is " + rad*rad*math.pi; The.length() method returns the length of a string. There is a.charat(int i) method which returns the character at the ith position. Like in an array, the positions are numbered from 0 to str.length()-1 Some manipulations of strings Doubling the string: Mark MarkMark Advancing each character by 1, wrapping z/z to a/a. Zebedee Afcffeff Reversing the string: Stuart trauts Doubling the string Advancing each character by 1 (naïve) str = str + str; String str2 = ""; for (int i=0; i<str.length(); i++) char c = str.charat(i); str2 += (char)( (int)c+1 ); str += str; Example: "Az_qw4" "B`rx5" 5

6 Dealing only with letters, and wrapping Reversing a string String str2 = ""; for (int i=0; i<str.length(); i++) char c = str.charat(i); if ( (c >= 'a' && c < 'z') (c >= 'A' && c < 'Z') ) str2 += (char)( (int)c+1 ); else if (c == 'z') str2 += 'a'; else if (c == 'Z') str2 += 'A'; else str2 += c; Example: "Az_qw4" "Ba_rx4" String str2 = ""; for (j=str.length()-1; j>=0; j--) // note: counting down str2 += str.charat(j); System.out.println(str2); More string methods Comparing strings: use s1.equals(s2), not s1==s2. Method to break a string into an array of characters. equals String Comparisons String s1 = "Welcome"; String s2 = "welcome"; if (s1.equals(s2))... if (s1.equalsignorecase(s2))... Substrings String s1 = "Welcome to Java"; String s2 = s1.substring(0,10) + "HTML"; index of first character to be returned index one beyond the last character to be returned Summary of some methods for Strings s1.length() // the length of s1 s1.charat(4) // the 5th character of s1 s1.equals(s2) // whether s1 and s2 are equal s1.equalsignorecase(s2) s1.substring(2,10) // the substring of s1 which starts at the 3rd character and ends at the 10th character s1.substring(4) // the substring of s1 which starts at the 5th character and continues to the end s1.indexof(s2) // the index in s1 of the first character of a matching substring s2 s1.tolowercase() // s1 in lower case 6

7 Getting strings from the command-line If the user types java ProgName foo bar then the strings foo and bar are available within the program. The variable args which we saw in public static void main(string[] args) is an array of the strings typed in the command line. Thus, in our example, args.length is 2 and args[0] is "foo", args[1] is "bar". Example: de-commenting Let's write a program which removes comments of the form // this is a comment from a text file. This is a small part of what the Java compiler has to do. (We won't worry about the /*...*/ form of comment.) import java.io.*; // remove //-style comments from a file public class DeComment public static void main (String[] args) throws FileNotFoundException,IOException if (args.length!= 1) // wrong number of args System.out.println("Usage: java DeComment <filename>"); System.exit(1); String filename = args[0]; BufferedReader myfile = new BufferedReader(new FileReader(filename)); // DO THE REAL WORK (see next slide) // here is the real work String line = myfile.readline(); while(line!= null) String newline=""; // now we search for // in line for (int i=0; i<line.length(); i++) if (line.charat(i) == '/' && i+1 < line.length() && line.charat(i+1) == '/') break; // found a comment newline += line.charat(i); System.out.println(newline); line = myfile.readline()) 7