Timing ListOperations

Transcription

1 Timing ListOperations Michael Brockway November 13, 2017 These slides are to give you a quick start with timing operations in Java and with making sensible use of the command-line.

2 Java on a command-line Download ListOpsTimings.zip and unzip it into a folder ListOpsTimings/. Start a terminal session and cd into this folder. A listing might look like this. You do not need a BlueJ project (or any other project)...

3 Java on a command-line: compiling Build all the classes like this with javac: Now you have classes...

4 Java on a command-line Here is a small procedural java program to generate some random strings... import java.util.random; public class RndStrings { private static char[] alphabet = " qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM".toCharArray(); private static int ctr = 0; private static Random rnd = new Random(); private static String rndstg(int ln) { char[] chars = new char[ln]; if (ctr % 256 == 0) rnd.setseed(ctr); for (int i=0; i<ln; i++) chars[i] = alphabet[rnd.nextint(alphabet.length)]; return String.format("%04x - %s", ctr++, new String(chars));

5 Java on a command-line public static void main(string[] args) { if (args.length < 3) { System.out.println("Usage: java RndStrings n m p"); System.out.println(" to make n random strings, m <= length < m+p" return; int num = Integer.parseInt(args[0]), min = Integer.parseInt(args[1]), max = Integer.parseInt(args[2]); for (int k = 0; k < num; k++) System.out.println(rndStg(min+rnd.nextInt(max))); We can run this at the command-line with the java command:

6 Java on a command-line: running a program Here I have made 6 strings of random length 25 to 40 characters. Actually, I wanted to save my strings in a file, to use with some timing experiments. So I should redirect the output to a file:

7 Java on a command-line: redirecting output The first of these command makes 100 strings of random length 25 and under 60, and saves them to file strings.txt. The second command makes 100 more strings, of length 60 and under 70, and appends them to file strings.txt.

8 Java on a command-line: summary $cd to the folder containing your java sources. compile: $javac RndStrings.java run (without command-line arguments):. $java RndStrings run (with command-line arguments): $java RndStrings redirect output: $java RndStrings > strings.txt redirect output, appending: $java RndStrings >> strings.txt It is also possible to redirect input to a program from a file:...$java ProgClass < file. This is illustrated in Unix but works in the same way in Windows provided the PATH environment variable contains the file PATH to the java JDK installation. You may need to change the drive letter as well as cd to the working folder.

9 Timing operations in a Java program /** * TimedListOps.java * Data are in a List<E> (either an ArrayList<E> or a LinkedList<E>) * and random fetches are timed. */ import java.util.*; public class TimedListOps<E> { private List<E> data = null; public TimedListOps(List<E> lst) { data = lst; public int dlen() { return data.size(); public E fetch(int n) { E e = null; long tm = 0l, st = System.nanoTime(); e = data.get(n); tm = System.nanoTime() - st; System.out.printf("Fetched **%s** in %d nanosec\n", e, tm); return e;

10 Timing operations in a Java program public void insert(int n, E e) { long tm = 0l, st = System.nanoTime(); data.add(n, e); tm = System.nanoTime() - st; System.out.printf( "Inserted **%s** at pos %d in %d nanosec\n", e, n, tm); public E delete(int n) { E e = null; long tm = 0l, st = System.nanoTime(); e = data.remove(n); tm = System.nanoTime() - st; System.out.printf( "Deleted **%s** from pos %d in %d nanosec\n", e, n, tm); return e;

11 Timing operations in a Java program This class, TimedListOps, has a constructor that takes a list of objects of some type E (eg, String, or some other class); a method to fetch an object from the list at a particular position (random access); a method to insert an object into the list at a particular position; and a method to delete an object from the list at a particular position. Each of these is timed: System.nanoTime(); returns a time in nanoseconds from a high-resolution timer (actually accurate to about a microsecond) st = System.nanoTime(); records in long integer st a start time tm = System.nanoTime() - st; records in tm the time elapsed since st.

12 Timing operations in a Java program import java.util.*; import java.io.*; public class StringDrvr { private static List<String> loaddata(char mode, String path) throws IOException { List<String> data = null; if (mode == a ) { System.out.println("ArrayList<String>"); data = new ArrayList<String>(); else if (mode == l ) { System.out.println("LinkedList<String>"); data = new LinkedList<String>(); else { System.out.println("List mode unrecognised"); System.exit(0); Scanner input = new Scanner(new BufferedReader(new FileReader(path))); while (input.hasnextline()) data.add(input.nextline()); System.out.printf("%d items\n", data.size()); input.close(); return data;

13 Timing operations in a Java program public static void main(string[] args) throws IOException { if (args.length < 2) { System.err.println("Usage: java StringDrvr {a l <file path>"); return; TimedListOps<String> app = new TimedListOps<String>(loadData(args[0].charAt(0), args[1])); for (int i = 0; i < 10; i++) { app.fetch(app.dlen()-100+5*i); app.fetch(100-5*i); for (int i = 0; i < 10; i++) { app.insert(100-5*i, "******* insert *******"); for (int i = 0; i < 10; i++) { app.delete(100-5*i);

14 Timing operations in a Java program StringDrvr class is another procedural program. Method loaddata(...) takes a mode, a for ArrayList, l for LinkedList, and a path to a file of strings, constructs a List of the required type, and populates it with strings read from the file. The main method makes a new TimedListOps instance using the List just constructed and uses it to time some fetches, inserts and deletes. Here, I have run the application twice with the file strings.txt I made earlier - once with ArrayList and once with LinkedList. Each run generated lots of output which is redirected to two log files.