Building Java Programs

Lab: File Processing

Except where otherwise noted, the contents of this document are Copyright 2019 Stuart Reges and Marty Stepp.

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

Lab goals

Goals for this problem set:

Scanner methods

Method name Description
next() reads and returns the next token as a String
nextLine() reads and returns as a String all the characters up to the next new line (\n)
nextInt() reads and returns the next token as an int, if possible
nextDouble() reads and returns the next token as double, if possible
hasNext() returns true if there is still a token in the Scanner
hasNextLine() returns true if there is still at least one line left to be read in the Scanner
hasNextInt() returns true if the next token can be read as an int
hasNextDouble() returns true if the next token can be read as an double

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 ?

Exercise -A: Tokenizing

How many tokens are in the following String? 3

welcome...to the matrix.

What are the tokens that the String breaks up into?

Exercise -B: More tokenizing

How many tokens are in the following String? 9

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

What are the tokens that the String breaks up into?

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

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 : 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

continued on the next slide...

Exercise - jGRASP Debugger

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.

zip lat2 long2
20045 38.896599 -77.0319
20500 38.894781 -77.036122
20501 38.89872 -77.036198
20502 38.89872 -77.036198

Exercise : runningSum practice-it

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 practice-it

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 practice-it

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
// 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 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

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
// 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 : coinFlip practice-it

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 : printDuplicates practice-it

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 practice-it

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 in a row. 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 practice-it

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.

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 : stripHtmlTags practice-it

Write a method stripHtmlTags that accepts a Scanner for an input file containing an HTML web page, then reads that file and prints the file's text with all HTML tags removed. A tag is any text between the character < and >. For example, consider the following text:

<html>
   <head>
   <title>My web page</title>
   </head>  
</html>

If a file containing these lines were passed to your program, your program should output the following:

	
	
My web page