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CSE 142

Lecture Notes

Week 6

boolean

  • boolean: A logical type whose value is either true or false.
  • You can create boolean variables, pass booleans as parameters, and return boolean values from methods

DeMorgan's Laws

Rules used to negate or reverse boolean expressions

Original Expression Negated Expression Alternative
a && b `!a
`a b`

Midterm Review Problems

  1. Write a static method called seven that takes a Random object as a parameter and that uses the random object to generate up to 10 numbers between 1 and 30 inclusive, printing them and stopping if the "lucky numver" 7 comes up and returning whether or not that happened.

    public static boolean seven(Random r) {
    for (int i = 0; i < 10; i++) {
        int next = r.nextInt(30) + 1;
        System.out.print(next + " ");
        
        if (next == 7) {
            System.out.println("You got a lucky number 7!");
            return true;
        }
    }
    return false;
}

  2. Write a static method called digitSum that takes an integer n as a parameter and that returns the sum of the digits of n. You may assume n is not negative.

    public static int digitSum(int n) {
    int sum = 0;  
    while (n > 0) {
        sum = sum + n % 10;
        n = n / 10;
    }
    return sum;
}

  3. Write a static method called roll7 that simulates the rolling of two dice until their sum is equal to 7. The method should print each roll and its sum and show a count of how many rolls it took to get to 7, as in:

    3 + 5 = 8
2 + 1 = 3
1 + 4 = 5
3 + 4 = 7
sum of 7 after 4 rolls

    You must exactly reproduce the format of this sample execution.

    public static void roll7() {
    Random r = new Random();
    int sum = 0;
    int rolls = 0;   
    while (sum != 7) {
        int roll1 = r.nextInt(6) + 1;
        int roll2 = r.nextInt(6) + 1;
        sum = roll1 + roll2;
        System.out.printf("%d + %d = %d\n", roll1, roll2, sum);
        rolls++;
    }
    System.out.printf("sum of 7 after %d rolls\n", rolls);
}

  4. Write a static method called stutter that takes a String as a parameter and that returns a new String where each letter of the original String has been replaced with two of that letter. For example, stutter("hello") should return hheelllloo.

    public static String stutter(String s) {
    String output = "";
    for (int i = 0; i < s.length(); i++) {
        char letter = s.charAt(i);
        output = output + letter + letter;
    }
    return output;
}

Fencepost problems

  • Spaces at the end are okay, you do not need to solve the fencepost problem for spaces

Random Numbers

  • When deciding on the formula to use with nextInt, first ask yourself how many nummbers do I need, then add the offset to that
  • e.g., random numbers between 1 and 30, so that is 30 numbers, offset by 1, so our forumla is r.nextInt(num of numbers) + offset, or in code r.nextInt(30) + 1

File Processing

  • To make a .txt file in jGRASP: File > New > Plain Text
  • To use the File class you need to have an import statement, import java.io.*;
  • File f = new File("numbers.txt"); creates a new File object
  • You can connect your file to a Scanner object with Scanner input = new Scanner(f);
  • absoulte path: complete path to a file, can use that path anywhere on your system and it will locate the file
  • relative path: path to the file from the current directly, if you used this path in a different directory it would not go to your file
  • Scanner objects can only go forwards, they cannot read information backwards; if you need to read a file twice then you would need to create two Scanner objects
  • Because of some exception rules in Java you need to add throws FileNotFoundException to any method that constructs a Scanner from a File object

File Example

import java.io.*; // to use the File calss
import java.util.*; // to use the Scanner class

public class FileExample {
    public static void main(String[] args) throws FileNotFoundException {
        Scanner input = new Scanner(new File("numbers.txt"));
        double sum = 0.0;
        while (input.hasNextDouble()) {
            double n = input.nextDouble();
            System.out.println("n = " + n);
            sum+=n;
        }
        System.out.println("sum = " + sum);
    }
}

Common File Methods

Method name Description
f.canRead() returns whether the file f is able to be read
f.delete() removes the file f from the disk
f.exists() returns true if the file f exists, otherwise returns false
f.getName() returns the name of file f
f.length() returns the number of bytes in the file f
f.renameTo(name) changes the name of file f to name

Common Errors

  • InputMismatchException: When you try and read a token of the wrong type
  • NoSuchElementException: When you try and read a token that does not exist

Testing for valid input with Scanner

Assuming, a Scanner object has already been created named input

Method name Description
input.hasNext() returns true if there are more tokens of input to read
input.hasNextInt() returns true if there is a next token and it can be read as an int
input.hasNextDouble() returns true if there is a next token and it can be read as an double

Reading HTML from a webpage

    import java.net.*; // to read from URL
    import java.util.*; // to be able to use Scanner
    
    public class WebExample {
        public static void main(String[] args) throws FileNotFoundException {
            Scanner input = new Scanner(new URL("http://www.cs.washington.edu/").openStream());
        }
    }