University of Washington, CSE 142

Lab 5: Random, Boolean, 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:

• use Random to generate random numbers
• get practice with the boolean type
• 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!

Random methods

To use these methods, you need a variable of type Random in scope:

Random randy = new Random();
int aRandomNumber = randy.nextInt(10);  // 0-9

Exercise : Random expressions

Fill in the boxes to produce expressions that will generate random numbers in the provided ranges. Assume that the following Random variable has been declared:

Random rand = new Random();

Exercise : makeGuesses

Write a method named makeGuesses that will output random numbers between 1 and 50 inclusive until it outputs one of at least 48. Output each guess and the total number of guesses made. Below is a sample execution:

guess = 43
guess = 47
guess = 45
guess = 27
guess = 49
total guesses = 5

Try solving this problem in Practice-It! from the link above.

Exercise : flip

Write a method flip that takes a Random object as a parameter and that prints information about a coin-flipping simulation.

Your method should use the Random object to produce a sequence of simulated coin flips, printing whether each flip comes up "heads" or "tails". Each outcome should be equally likely. Your method should stop flipping when you see three heads in a row.

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

The boolean type

The boolean type represents logical values of true or false. Combine boolean expressions with logical operators && (and), || (or), and ! (not).

Example:

boolean test1 = 7 < 10;            // true
boolean test2 = (1 == 2);          // false
if ((test1 || test2) && 2 + 2 != 5) {
    System.out.print("hello");     // output: hello
}

String methods with boolean results

String name = "Professor Smith";
if (name.startsWith("Prof")) {
    System.out.println("When are your office hours?");
}

Exercise : Boolean Expressions

Write the result of each expression as either true or false, given the following variables.

int x = 12;
int y = 7;
int z = 28;
String s = "mid term";

true

false

true

true

false

true

true

false

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 : 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!

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 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 : stripHtmlTags

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

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!