teaching machines

CS 145 Lecture 32 – File Reading and Writing

November 17, 2014 by Chris Johnson. Filed under cs145, fall 2014, lectures.

Agenda

• what ?s
• computer anatomy
• files
  • interoperability
  • persistence
• PrintWriter
• buffered I/O
• extracting out the ID3 tag from an MP3
• random access

TODO

• The next Mercy Fortnight will go into effect starting December 6. It has the following stipulations:
  • You must complete and submit homework 7 before December 6. It covers the creation of custom types using objects, and our discussion this week and your book should give you sufficient background to complete it. If you choose not to participate in Mercy Fortnight, homework 7 must be turned in before December 13.
  • Any previous homeworks may be resubmitted starting December 6 up to the final exam on December 17. That’s not quite a fortnight, but it’s close.
  • You must inform me through email that you’ve resubmitted a homework.
  • At most one resubmission may be made per day.

Code

RandomPhobia.java

package lecture1117;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Random;
import java.util.Scanner;

public class RandomPhobia {
  public static void main(String[] args) throws FileNotFoundException {
    // read in list of phobias
    
    // reconnaissance
    Scanner in = new Scanner(new File("/Users/johnch/Desktop/phobias.txt"));
    int nLines = 0;
    while (in.hasNextLine()) {
      in.nextLine();
      ++nLines;
    }
    in.close();
    
    // actual loading
    String[] phobias = new String[nLines];
    in = new Scanner(new File("/Users/johnch/Desktop/phobias.txt"));
    for (int i = 0; i < nLines; ++i) {
      phobias[i] = in.nextLine();
    }
    in.close();
    
    // pick one at random
    Random g = new Random();
    int iRandom = g.nextInt(phobias.length);
    System.out.println(phobias[iRandom]);
  }
}

Xor.java

package lecture1117;

import java.io.FileNotFoundException;
import java.io.PrintWriter;

public class Xor {
  public static void main(String[] args) throws FileNotFoundException {
    int width = 256;
    int height = 256;
    PrintWriter out = new PrintWriter("/Users/johnch/Desktop/xor.ppm");
    
    out.println("P3");
    out.println(width + " " + height);
    out.println(255);
    
    for (int r = 0; r < height; ++r) {
      for (int c = 0; c < width; ++c) {
        out.printf("%d %d %d%n", (r ^ c) % 256, r % 256, c % 256);
      }
    }
    
    out.close();
  }
}

ID3.java

package lecture1117;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;

public class ID3 {
  public static void main(String[] args) throws IOException {
    RandomAccessFile raf = new RandomAccessFile(new File("/Users/johnch/Desktop/mystery.mp3"), "r");
    raf.seek(raf.length() - 125);
  
    byte[] stuff = new byte[30];
    
    raf.read(stuff);
    System.out.println(new String(stuff));
    
    raf.read(stuff);
    System.out.println(new String(stuff));
   
    raf.read(stuff);
    System.out.println(new String(stuff));
    
    raf.close();
  }
}

Haiku