University of Washington, CSE 142

Lab 5: File Processing

Except where otherwise noted, the contents of this document are Copyright 2012 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges, Whitaker Brand and Hélène Martin

Basic lab instructions

• Talk to your classmates for help. You can even work on the lab with a partner if you like.
• You may want to bring your textbook to future labs to look up syntax and examples.
• Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
• Complete as many problems as you can within the allotted time. You don't need to keep working on these exercises after you leave the lab.
• Feel free to complete problems in any order.
• Before you leave today, make sure to check in with one of the TAs in the lab to get credit for your work.

Today's lab

Goals for today:

• get more practice using Strings
• use Scanner and File objects to read input data from files
• understand the way a Scanner breaks input into tokens
• Where you see this icon, you can click it to check the problem in Practice-It!

Exercise : Syntax errors

• The following Java program has 11 errors. Work with your neighbor to find them all. Can you explain why each of them is an error and describe how Java reacts to them?

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

  public class StringOops { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("Type your name: "); String name = console.nextString(); process(name); } public static void process(string "name") { if (name == Whitaker) { System.out.println("You must be really awesome."); } replace("a", "e"); toUppercase(name); name.substring(0, 3); System.out.println(name + " has " + name.length + " letters"); } }

• Copy and paste the code into jGrasp and see if you can fix the errors.

Exercise - answer

1. line 5: nextString should be next
2. line 9: string should be String
3. line 9: name should not be in quotes
4. line 10: Whitaker should be in quotes
5. line 10: cannot compare strings with ==; must use .equals
6. line 13: cannot call replace without specifying a string object (name)
7. line 14: toUppercase should be toUpperCase
8. line 14: name. should come before toUpperCase, not passed as a parameter to it
9. line 14: must say name = to store the result of toUpperCase
10. line 15: must say name = to store the result of substring
11. line 16: must use parentheses () when calling length

Exercise - Corrected version

public class StringOops {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        String name = console.next();
        process(name);
    }

    public static void process(String "name") {
        if (name.equals("Whitaker")) {
            System.out.println("You must be really awesome.");
        }
        name = name.replace("a", "e");
        name = name.toUpperCase();
        name = name.substring(0, 3);
        System.out.println(name + " has " + name.length() + " letters");
    }
}

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery3(int x) {
    int y = 0;
    while (x % 2 == 0) {
        y++;
        x = x / 2;
    }
    System.out.println(x + " " + y);
}

19 0

21 1

3 4

5 3

1 6

Exercise : longestName

Write a method named longestName that reads names typed by the user and prints the longest name (the name that contains the most characters) in the format shown below. Your method should accept a console Scanner and an integer n as parameters and should then prompt for n names.

A sample execution of the call longestName(console, 4) might look like the following:

name #1? roy
name #2? DANE
name #3? sTeFaNiE
name #4? Erik
Stefanie's name is longest

Try to solve this problem in Practice-It: click on the check-mark above!

File Scanner usage

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import java.io.*;	// for File
import java.util.*;	// for Scanner

public class FileScannerDemo {
   public static void main(String[] args) throws FileNotFoundException {
      File f = new File("test.txt");
      Scanner input = new Scanner(f);	
   }
}	

• The File class is in the java.io package
• To read from a file, construct a Scanner object on that file's File object
• Any method that constructs a Scanner object on a File needs to acknowledge the possibility of a FileNotFoundException
• The throws clause is like a legal waiver: "I know bad things will happen if the file is missing but I accept that"

Exercise : File Scanner declaration syntax

Which of the following choices is the correct syntax for declaring a Scanner to read the file example.txt in the current directory ?

1. Scanner input = new Scanner("C:\example.txt");
2. Scanner input = new Scanner(new File("example.txt"));
3. Scanner input = new File("example.txt");
4. File input = new Scanner("/example.txt");
5. Scanner input = new Scanner("C:/example.txt");

Scanner methods

Exercise -A: Tokenizing

How many tokens are in the following String? 3

welcome...to the matrix.

Exercise -B: More tokenizing

How many tokens are in the following String? 9

in fourteen-hundred 92
columbus sailed the ocean blue :)

Exercise -A: Scanner practice

The next couple problems are about a file called readme.txt that has the following contents:

6.7  This file has
several input
LINES!

   10 20

What would be the output from the following code, as it would appear on the console?

Scanner input = new Scanner(new File("readme.txt"));
System.out.println(input.next());  // 6.7
System.out.println(input.next());  // This
System.out.println(input.next());  // file

Exercise -B: Scanner practice

Input file: readme.txt

6.7  This file has
several input
LINES!

   10 20

What would be the output for the following code? If there would be an error, write error .

Scanner input = new Scanner(new File("readme.txt"));
System.out.println(input.nextDouble());  // 6.7
System.out.println(input.nextDouble());  // error

Exercise -C: Scanner practice

Input file: readme.txt

6.7  This file has
several input
LINES!

   10 20

What would be the output for the following code? If there would be an error, write error .

Scanner input = new Scanner(new File("readme.txt"));
while (!input.hasNextInt()) {
    input.next();
}
System.out.println(input.nextInt());  // 10

Exercise : Words

• Download the following files Words.java and wordinput.txt to your machine and open them with jGrasp.
• The program is supposed to read the input file and count how many words it contains. Finish the program so that it runs properly. Don't forget to add import statements and a throws clause to the code.

Exercise - answer

import java.io.*;     // for File
import java.util.*;   // for Scanner

public class Words {
    public static void main(String[] args) throws FileNotFoundException {
        int wordCount = 0;
        Scanner input = new Scanner(new File("wordinput.txt"));
        
        // your code goes here ...
        while (input.hasNext()) {
            String word = input.next();
            wordCount++;
        }
        
        System.out.println("Total words = " + wordCount);
    }
}

Exercise : runningSum

Write a static method called runningSum that accepts as a parameter a Scanner holding a sequence of real numbers and that outputs the running sum of the numbers followed by the maximum running sum. For example if the Scanner contains the following data:

3.25 4.5 -8.25 7.25 3.5 4.25 -6.5 5.25

Your method should produce the following output:

running sum = 3.25 7.75 -0.5 6.75 10.25 14.5 8.0 13.25
max sum = 14.5

Click on the check-mark above to try out your solution in Practice-it!

Exercise : frequentFlier

Write a method frequentFlier that accepts a Scanner for an input file of ticket type / mileage pairs and reports how many frequent-flier miles the person earned.

• 1 frequent flyer mile is earned for each mile traveled in coach.
• 2 frequent flyer miles are earned for each mile traveled in first class.
• 0 frequent flyer miles are earned on a discounted flight.

For example, given the input below, your method should return 15600 (2*5000 + 1500 + 100 + 2*2000).

firstclass  5000 coach   1500  coach
100 firstclass 2000  discount 300

Exercise : flipLines

Write a method named flipLines that accepts a Scanner for an input file and writes to the console the same file's contents with each pair of lines reversed in order. For example, if the file contains:

Twas brillig and the slithy toves
did gyre and gimble in the wabe.
All mimsey were the borogroves,
and the mome raths outgrabe.

The End

your method should produce the following output:

did gyre and gimble in the wabe.
Twas brillig and the slithy toves
and the mome raths outgrabe.
All mimsey were the borogroves,
The End

Exercise : Debug ZipCode Case Study

In this exercise we will practice the jGRASP debugger using the Case Study example from the end of Chapter 6. To download this example, follow these steps:

1. Go to the class web page and click the "Textbook" link.
2. Find the section labeled "Code Files" and click the "code files" link.
3. This will bring you to a directory listing that includes an entry for each chapter. Click the link for "ch06".
4. You want to download and save the files ZipLookup.java and zipcode.txt. Right-click the file names and choose the option to save the link in whatever folder you have been using for lab work. Make sure to save them in the same folder.
5. Compile and run ZipLookup.java in jGRASP. You might try using your own ZIP code and a relatively small radius like 0.5 miles. The program takes a while to run because it has to search a large data file.

continued on the next slide...

Exercise - jGRASP Debugger

• The debugger can be particularly helpful to understand a program that processes a large data file. In this exercise are going to debug the ZipLookup program.
• This program makes two passes through the data file. First it looks for the target zip code in the method named find. Then it shows all matches in the method named showMatches. We want to debug showMatches.
• The data file has a total of 43,191 zip codes in it, so it is not practical to have it display all of the data. That's why it's important to have good debugging skills to be able to selectively stop the program at certain points to see what is going on.
• Ask a TA for help if you have trouble setting or clearing break points or otherwise completing this exercise.

continued on the next slide...

Exercise - jGRASP Debugger

• We'll debug the program as it searches for all zip codes with 0.3 miles of the White House, at zip code 20500. Run the program with those input values:

  What zip code are you interested in? 20500
  And what proximity (in miles)? 0.3
  
  20500: Washington, DC
  zip codes within 0.3 miles:
      20045 Washington, DC, 0.26 miles
      20500 Washington, DC, 0.00 miles
      20501 Washington, DC, 0.27 miles
      20502 Washington, DC, 0.27 miles
  

• Now let's see what is true just before the while loop in the showMatches method executes. Set a break point on the while loop itself. Then debug to find lat1 and long1 (latitude and longitude of the White House ZIP code).

  lat1		38.894781
  long1		-77.036122

continued on the next slide...

Exercise - jGRASP Debugger

Clear your previous break point and set a new break point inside on the printf inside the if. Then hit the resume button that looks like a play button and fill in the table below with the values for zip, lat2, and long2.

Exercise : allDigitsOdd

Write a method named allDigitsOdd that returns whether every digit of a positive integer is odd. Your method should return true if the number consists entirely of odd digits and false if any of its digits are even. 0, 2, 4, 6, and 8 are even digits, and 1, 3, 5, 7, 9 are odd digits.

For example, allDigitsOdd(135319) returns true but allDigitsOdd(9145293) returns false.

Hint: You can pull apart a number into its digits using / 10 and % 10.

Exercise : sameDashes

Write a method sameDashes that takes two strings as parameters and that returns whether or not they have dashes in the same places (returning true if they do and returning false otherwise). For example, below are four pairs of strings of equal length that have the same pattern of dashes. Notice that the last pair has no dashes at all.

string 1:    "hi--there-you."    "-15-389"    "criminal-plan"    "abc"
string 2:    "12--(134)-7539"    "-xy-zzy"    "(206)555-1384"    "9.8"

Solve this problem in Practice-It by clicking on the check-mark above.

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!