University of Washington, CSE 142 (190)

Lab 6: File Processing

Except where otherwise noted, the contents of this document are Copyright 2010 Stuart Reges and Marty Stepp.

lab document created by Whitaker Brand and Marty Stepp

Today's lab

Goals for today:

• use Scanner and File objects to read input data from files
• understand the way a Scanner breaks input into tokens
• explore token-based and line-based input
• Practice downloading files from the class web page and setting break points with the jGRASP debugger to find out what is going on inside a program
• Where you see this icon, you can click it to check the problem in Practice-It!

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");

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 :)

Recall: Scanner Methods

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.nextLine());  // 6.7  This file has
System.out.println(input.nextLine());  // several input
System.out.println(input.nextLine());  // LINES!

Exercise -B: Scanner practice

Input file: readme.txt

6.7  This file has
several input
LINES!

   10 20

What would be the output if the code was changed to the following?

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 -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"));
System.out.println(input.nextDouble());  // 6.7
System.out.println(input.nextDouble());  // error

Exercise -D: 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 : Getting textbook code files

We want to once again practice downloading textbook files to your computer by downloading and storing the case study from the end of chapter 6. To do so, follow these steps:

1. Go to the class web page.
2. Click on the "textbook" tab.
3. Find the section labeled "Code Files" and click on the "code files" link.
4. This will bring you to a directory listing that includes an entry for each chapter. Click on the link for "ch6".
5. You want to download and save the files ZipLookup.java and zipcode.txt. Right-click on 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.
6. Compile and run ZipLookup.java in jGRASP. You might try using your own zip code with a relatively small radius like 0.5 miles to see what results you get. The program takes a little while to run because it has to search through a large data file twice.

Exercise : jGRASP Debugger

When you write a program that processes a large data file, the debugger can be particularly helpful to allow you to see what's going on. We are going to debug the ZipLookup program that you downloaded from the class web page.

This program makes two passes through the data file. First it looks for the target zip code in the method called find. Then it shows all matches in the method called 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. This will be a helpful skill when you work on the programming assignment.

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 are going to debug the program as it searches for all zip codes with 0.3 miles of the White House, which has 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 : 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 : 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 : countWords errors

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// Counts the total lines and words in the given input scanner.
public static void countWords(Scanner input) {
    Scanner input = new Scanner(new File("example.txt"));
    int lineCount = 0;
    int wordCount = 0;
    
    while (input.nextLine()) {
        String line = input.line();       // read one line
        lineCount++;
        while (line.next()) {             // count tokens in line
            String word = line.hasNext;
            wordCount++;
        }
    }
}

The above attempted solution to Practice-It problem "countWords" has 5 errors. Open Practice-It from the link above, copy/paste this code into it, and fix the errors. Complete the code so that it passes the test cases.

Exercise - answer

1. line 3: should not declare another Scanner for the file
2. line 7: nextLine should be hasNextLine
3. line 8: line should be nextLine
4. line 10: need a second line Scanner to read the tokens of each line
5. line 11: hasNext should be next()
6. line 14: need to add 3 println statements to print line/word stats

Exercise - solution

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// Counts the total lines and words in the given input scanner.
public static void countWords(Scanner input) {
    Scanner input = new Scanner(new File("example.txt"));
    int lineCount = 0;
    int wordCount = 0;
    
    while (input.hasNextLine()) {
        String line = input.nextLine();   // read one line
        lineCount++;
        Scanner lineScan = new Scanner(line);
        while (lineScan.hasNext()) {      // count tokens in line
            String word = line.next();
            wordCount++;
        }
    }
    
    System.out.println("Total lines = " + lineCount);
    System.out.println("Total words = " + wordCount);
    System.out.printf("Average words per line = %.3f\n", (double) wordCount / lineCount);
}

Exercise : coinFlip

Write a method named coinFlip that accepts a Scanner for an input file of coin flips that are heads (H) or tails (T). Consider each line to be a separate set of coin flips and output the number and percentage of heads in that line. If it is more than 50%, print "You win!". Consider the following file:

H T H H T
T t    t  T h  H
      h

For the input above, your method should produce the following output:

3 heads (60.0%)
You win!

2 heads (33.3%)

1 heads (100.0%)
You win!

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 Chapter 6 and try to solve them!