University of Washington, CSE 142

Lab 6: File Processing

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

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

Basic lab instructions

• Mouse over highlighted words if you're not sure what they mean!
• Talk to your classmates for help.
• 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 much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
• Feel free to complete problems in any order.
• Make sure you've signed in on the sign-in sheet before you leave!

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
• get more practice using Strings
• explore token-based and line-based input
• Where you see this icon, you can click it to check the problem in Practice-It!

Recap: String Equality

Recall that although we use == to compare the values of primitive types, such as ints, doubles, and chars. However, == has a different meaning for Strings: it evaluates to true when one String is set to equal another (ex. String a = b). To see whether two Strings have the same value, we instead use .equals().

Enter true or false for the following:

String a = "hello";
String h = "h";
String b = h + "ello";

false

String a = "hello";
String h = "h";
String b = h + "ello";

true

String a = "world";
String b = a;

true

String a = "world";
String b = a;

true

Files

So far, we've used Scanners to read user input. Now, we're going to look at using Scanners to read over files!

A File object allows you to interact with actual files on your computer. Make sure to put any file you want to read with a Scanner in the same folder as the program that wants to use it!

    // necessary to use Files
    import java.io.*;

    // makes a File object, holding the same info as the file named "actualNameOfFile"
    File fileVariableName = new File("actualNameOfFile");

    // a Scanner that reads over the File!
    Scanner fileScanner = new Scanner(fileVariableName);

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

Recap: Scanner method examples

    4.2 abc 4

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

New methods: Scanner method examples

    4.2 abc 4

Exercise : .hasNext() caution

.hasNext() methods only check the type of the next token. .next() methods consume tokens, changing what token the Scanner is looking at next! For each of the following, enter the output if the loop terminates, or write infinite if the loop loops forever!

Consider the following as part of input.txt

Jello world :)

Scanner input = new Scanner(new File("input.txt"));
while (input.hasNext()) {
   String nextWord = input.next();
   System.out.print(nextWord + " ");
}

Jello world :) 

Scanner input = new Scanner(new File("input.txt"));
while (input.hasNext()) {
   System.out.println("hi");
}

infinite

Exercise : flipLines

Write a method named flipLines that accepts a Scanner holding an input file and writes to the console the same file's contents with each pair of lines reversed in order. If the input file has an odd number of lines, the last line should be printed in its original position! 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

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

File Exceptions

When you work with Files, the Java compiler gets concerned that you might be trying to access a File that doesn't exist. So any method that uses Files must declare that it might throw a FileNotFoundException: this basically tells Java that if the desired "actualNameOfFile" cannot be found, it's okay to crash.

Every method that directly or indirectly calls a method that works with Files also needs to declare that it might throw a FileNotFoundException.

• Directly: method A() calls methodB(), and methodB() uses a File. Then A() and B() must declare that they might throw a FileNotFoundException.
• Indirectly: method A() calls method B(), and method B() calls method C(). Method C() uses a File. Then A() and B() and C() must all declare that they might throw a FileNotFoundException.

  public static void main(String[] args) throws FileNotFoundException {
                           ...
  }

Exercise : Words

• Download the following files Words.java and wordinput.txt to your machine (and into the same folder!) 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 declaration that the method might throw a FileNotFoundException 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);
    }
}

.next() vs .nextLine()

• .next() reads and returns the next token as a String. Tokens are separated by white space.
• .nextLine() reads and returns the next line as a String. A line is all the characters up to the next \n (line break).
• In this class, a single Scanner should only ever read its input using either .next() (mixing with .nextDouble() and .nextInt() and .nextBoolean() is fine), or using .nextLine().

String Scanners

Consider the following problem:

We already know how to read full lines or individual words from a Scanner. Now we need a new strategy: we need to read lines from the input, and then we need to somehow read word-by-word through each line.

We can do this by creating a second Scanner! So far, we've used Scanners to read user input and to read Files. Scanners can also read over Strings!

    Scanner fileScannerName = new Scanner(new File("fileName"))
    while (fileScannerName.hasNextLine()) {
       String line = fileScannerName.nextLine(); // reads the next line from the input file
       Scanner lineScanner = new Scanner(line);
       while (lineScanner.hasNext()) {
          String word = lineScanner.next(); // reads the next word from the input line
          ...
       }
    }

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!

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 = lineScan.hasNext;
            wordCount++;
        }
    }
}

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

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 = lineScan.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 : 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 chapter 6 supplements for the textbook.
2. 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.
3. 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.

continued on the next slide...

Exercise - jGRASP Debugger

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

38.894781

-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 : printDuplicates

Write a method printDuplicates that accepts a Scanner for an input file. Examine each line for consecutive occurrences of the same token on the same line and print each duplicated token along how many times it appears consecutively. For example the file:

hello how how are you you you you
I I I am Jack's Jack's smirking smirking smirking smirking smirking revenge
one fish two fish red fish blue fish
   bow  wow wow yippee yippee   yo yippee   yippee yay  yay yay

leads to the following console output:

how*2 you*4
I*3 Jack's*2 smirking*5

wow*2 yippee*2 yippee*2 yay*3

Exercise : mostCommonNames

Write a method mostCommonNames that accepts a Scanner for an input file with names on each line separated by spaces. Some names appear multiple times, if they do, they are listed consecutively. For example:

Benson Eric   Eric  Marty Kim  Kim Kim   Jenny  Nancy Nancy  Nancy  Paul  Paul
Stuart Stuart Stuart Ethan Alyssa Alyssa Helene Jessica Jessica Jessica Jessica
Jared  Alisa Yuki   Catriona  Cody   Coral   Trent Kevin  Ben Stefanie Kenneth

For each line, print the most commonly occurring name. If there's a tie, use the first name that had that many occurrences.

Most common: Kim
Most common: Jessica
Most common: Jared

Also return the total number of unique names in the whole file (e.g. 23 for the above input).

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

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!