1.1 Basic Programming Model 1.2 Data Abstrac9on

Transcription

1 1.1 Basic Programming Model 1.2 Data Abstrac9on This presentation is adapted from slides accompanying Algorithms 4/e by Robert Sedgwick and Kevin Wayne and from Introduction to Programming in Java: An Interdisciplinary Approach by Robert Sedgwick and Kevin Wayne

2 1.5 Input and Output 2.2 Libraries and Clients 3.1 Using Data Types 1.1 Basic Programming Model 1.2 Data Abstrac9on This presentation is adapted from slides accompanying Algorithms 4/e by Robert Sedgwick and Kevin Wayne and from Introduction to Programming in Java: An Interdisciplinary Approach by Robert Sedgwick and Kevin Wayne

3 Libraries Library. A module whose methods are primarily intended for use by many other programs. Client. Program that calls a library. API. Contract between client and implementa9on. Implementa9on. Program that implements the methods in an API. 3

4 Using a Library public class RandomPoints { public static void main(string args[]) { int N = Integer.parseInt(args[0]); for (int i = 0; i < N; i++) { command line input double x = StdRandom.gaussian(0.5, 0.2); double y = StdRandom.gaussian(0.5, 0.2); StdDraw.point(x, y); textbook libraries compile run % javac RandomPoints.java % java RandomPoints

5 Input and Output Input devices. Keyboard Mouse Hard drive Network Digital camera Microphone Output devices. Display Speakers Hard drive Network Printer MP3 Player Our approach. Define Java libraries of func9ons for input and output. Use opera9ng system (OS) to connect Java programs to: file system, each other, keyboard, mouse, display, speakers. 5

6 Terminal Terminal. Applica9on where you can type commands to control the opera9ng system. Mac OS X Microsoft Windows 6

7 Command- Line Input and Standard Output Command- line input. Read an integer N as command- line argument. Standard output. Flexible OS abstrac9on for output. In Java, output from System.out.println() goes to standard output. By default, standard output is sent to Terminal. public class RandomSeq { public static void main(string[] args) { int N = Integer.parseInt(args[0]); for (int i = 0; i < N; i++) { System.out.println(Math.random()); % java RandomSeq

8 Bird's Eye View 8

9 Command- Line Input vs. Standard Input Command- line input. Use command- line input to read in a few user values. Not prac9cal for many user inputs. Input entered before program begins execu9on. Standard input. Flexible OS abstrac9on for input. By default, standard input is received from Terminal window. Input entered while program is execu9ng. 9

10 Standard Input and Output Standard input. StdIn is library for reading text input. Standard output. StdOut is library for wri9ng text output. libraries developed for IntroCS/Algs4 10

11 Standard Drawing Standard drawing. StdDraw is library for producing graphical output. libraries developed for IntroCS/Algs4 11

12 Standard Draw Standard drawing. We provide library StdDraw to plot graphics. To use. Download StdDraw.java and put in working directory. public class Triangle { public static void main(string[] args) { double t = Math.sqrt(3.0) / 2.0; StdDraw.line(0.0, 0.0, 1.0, 0.0); StdDraw.line(1.0, 0.0, 0.5, t); StdDraw.line(0.5, t, 0.0, 0.0); StdDraw.point(0.5, t/3.0); (½, ½ 3) % java Triangle (0, 0) (1, 0) 12

13 Data Visualiza9on Plot filter. Read in a sequence of (x, y) coordinates from standard input, and plot using standard drawing. public class PlotFilter { public static void main(string[] args) { double xmin = StdIn.readDouble(); double ymin = StdIn.readDouble(); double xmax = StdIn.readDouble(); double ymax = StdIn.readDouble(); StdDraw.setXscale(xmin, xmax); StdDraw.setYscale(ymin, ymax); rescale coordinate system while (!StdIn.isEmpty()) { double x = StdIn.readDouble(); double y = StdIn.readDouble(); StdDraw.point(x, y); read in points, and plot them 13

14 Data Visualiza9on % more < USA.txt bounding box coordinates of 13,509 US cities % java PlotFilter < USA.txt 14

15 Digital Audio in Java Standard audio. Library for playing digital audio. library developed for IntroCS/Algs4 15

16 Bird's Eye View (Revisited) 16

17 Non- Standard Input Standard input. Read from terminal window. Goal. Read from several different input streams. or use OS to redirect from one file In data type. Read text from stdin, a file, a web site, or network. Ex: Are two text files iden9cal? public class Diff { public static void main(string[] args) { In in0 = new In(args[0]); In in1 = new In(args[1]); String s = in0.readall(); String t = in1.readall(); StdOut.println(s.equals(t)); read from one file read from another file 17

18 Screen Scraping Goal. Find current stock price of Google. <tr> <td class="yfnc_tablehead1" width="48%"> Last Trade: </td> <td class="yfnc_tabledata1"> <big> <b>576.50</b> </big> </td> </tr> <tr> <td class="yfnc_tablehead1" width="48%"> Trade Time: </td> <td class="yfnc_tabledata1"> 11:45AM ET </td> </tr> NYSE symbol 18

19 Screen Scraping Goal. Find current stock price of Google. s.indexof(t, i): index of first occurrence of pa_ern t in string s, star9ng at offset i. Read raw html from Find first string delimited by <b> and </b> a`er Last Trade. public class StockQuote { public static void main(string[] args) { String name = " In in = new In(name + args[0]); String input = in.readall(); int start = input.indexof("last Trade:", 0); int from = input.indexof("<b>", start); int to = input.indexof("</b>", from); String price = input.substring(from + 3, to); StdOut.println(price); % java StockQuote goog

20 Day Trader Try this. Find updated version of StockQuote.java online: h_p://introcs.cs.princeton.edu/java/31datatype/stockquote.java Run it from the command line to check some stock prices, eg: java-algs4 StockQuote aapl The New Yorker, September 6, 1999 Modify it to prompt and get input from user using StdIn and StdOut (instead of command line args[0] and System.out.println()) Put in a loop so it keeps asking for more stock prices use StdIn.isEmpty() to test for end of input test the program: indicate the end of input using Ctrl- C 20

21 Day Trader (Con9nued) If you have been using an IDE, run the program again from command line The New Yorker, September 6, 1999 Run from command line, but this 9me redirect Standard Input to come from a text file: Create a small text file with some stock 9ckers in it, save it as 9ckers.txt Run StockQuote with input from this file: java-algs4 StockQuote <tickers.txt You can also redirect Standard output: java-algs4 StockQuote >tickers.out (creates file 9ckers.out with output from program you s9ll need to type in your input, but the prompts and other output is going to 9ckers.out). or both! java-algs4 StockQuote <tickers.txt >tickers.out java-algs4 StockQuote >tickers.out <tickers.txt (either order works) 21

22 Day Trader (Con9nued) The New Yorker, September 6, 1999 Add bells and whistles at own risk Plot price in real- 9me. No9fy user if price dips below a certain price. Embed logic to determine when to buy and sell. Automa9cally send buy and sell orders to trading firm. Warning. Please, please use at your own financial risk. 22

23 Sta9s9cs

24 Standard Sta9s9cs Ex. Library to compute sta9s9cs on an array of real numbers. library developed for IntroCS/Algs4 mean sample variance 24

25 Standard Sta9s9cs Ex. Library to compute sta9s9cs on an array of real numbers. public class StdStats { public static double max(double[] a) { double max = Double.NEGATIVE_INFINITY; for (int i = 0; i < a.length; i++) if (a[i] > max) max = a[i]; return max; public static double mean(double[] a) { double sum = 0.0; for (int i = 0; i < a.length; i++) sum = sum + a[i]; return sum / a.length; public static double stddev(double[] a) // see text 25

26 Day Trader (Con9nued back for some stats!) The New Yorker, September 6, 1999 Store the prices in an array, as they are being accessed. (if necessary, use java.u9l.arrays.copyof() to trim the array to size a`er all the values are there) Use StdStats.mean(prices) and StdStats.stddev(prices) to obtain the mean and standard devia9on of some stocks try some of the other StdStats methods. 26

27 Random Numbers The generation of random numbers is far too important to leave to chance. Anyone who considers arithmetical methods of producing random digits is, of course, in a state of sin. Jon von Neumann (left), ENIAC (right)

28 Random Numbers

29 Standard Random Standard random. Our library to generate pseudo- random numbers. library developed for IntroCS/Algs4 29

30 Standard Random public class StdRandom { // between a and b public static double uniform(double a, double b) { return a + Math.random() * (b-a); // between 0 and N-1 public static int uniform(int N) { return (int) (Math.random() * N); // true with probability p public static boolean bernoulli(double p) { return Math.random() < p; // gaussian with mean = 0, stddev = 1 public static double gaussian() /* see Exercise */ // gaussian with given mean and stddev public static double gaussian(double mean, double stddev) { return mean + (stddev * gaussian());... 30

31 Earlier example: Using a Library public class RandomPoints { public static void main(string args[]) { int N = Integer.parseInt(args[0]); for (int i = 0; i < N; i++) { command line input double x = StdRandom.gaussian(0.5, 0.2); double y = StdRandom.gaussian(0.5, 0.2); StdDraw.point(x, y); textbook libraries compile run % javac RandomPoints.java % java RandomPoints

32 Unit Tes9ng Unit test. Include main() to test each library. public class StdRandom {... public static void main(string[] args) { int N = Integer.parseInt(args[0]); for (int i = 0; i < N; i++) { StdOut.printf(" %2d ", uniform(100)); StdOut.printf("%8.5f ", uniform(10.0, 99.0)); StdOut.printf("%5b ", bernoulli(.5)); StdOut.printf("%7.5f ", gaussian(9.0,.2)); StdOut.println(); % java StdRandom true false true true false