Building Java Programs

Lab: Boolean logic

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

Lab goals

Goals for this problem set:

• use boolean expressions and variables to represent logical true/false expressions

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

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 : hasMidpoint

Write a method hasMidpoint that accepts three integers as parameters, and returns true if one of the numbers is the midpoint of the other two and returns false otherwise.

For example, the call hasMidpoint(3, 7, 5) would return true because one of the parameters (5) is the midpoint of the other two (3 and 7).

Exercise : before

Write a method before that takes as parameters two month/day combinations and that returns whether or not the first date comes before the second date (true if the first month/day comes before the second month/day, false if it does not). The method will take four integers as parameters that represent the two month/day combinations.

The first integer in each pair represents the month and will be a value between 1 and 12 (1 for January, 2 for February, etc, up to 12 for December). The second integer in each pair represents the day of the month (a value between 1 and 31). One date is considered to come before another if it comes earlier in the year.

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"

Exercise : "Boolean Zen"

This attempted solution to Self-Check 5.15 (isVowel) has several problems:

// Returns whether the given string represents a vowel:
// a, e, i, o, or u, case insensitively.
public static boolean isVowel(String s) {
    if (s == "a") {
        return true;
    } else if (s == "e") {
        return true;
    } else if (s == "i") {
        return true;
    } else if (s == "o") {
        return true;
    } else if (s == "u") {
        return true;
    } else {
        return false;
    }
}
  

Exercise - things to fix

Fix the following aspects of the code:

• It has a bug related to how strings are compared.
• It isn't case sensitive; it fails for uppercase vowels.
• It has too many unnecessary if/else statements.
• (advanced) It does not use "Boolean Zen" as described in textbook section 5.3.

Exercise - answer

public static boolean isVowel(String s) {
    s = s.toLowerCase();
    if (s.equals("a") || s.equals("e") || s.equals("i")
            || s.equals("o") || s.equals("u")) {
        return true;
    } else {
        return false;
    }
}
  

The above can be improved. "Boolean Zen" version:

public static boolean isVowel(String s) {
    s = s.toLowerCase();
    return s.equals("a") || s.equals("e") || s.equals("i")
           || s.equals("o") || s.equals("u");
}
  

Exercise : assertions

Identify whether each assertion is always/never/sometimes true at each point.

public static void mystery(int x, int y) {
    int z = 0;

    // Point A
    while (x != y) {
        // Point B
        z++;

        if (x > y) {
            // Point C
            x = x / 10;
        } else {
            // Point D
            y = y / 10;
        }
    }

    // Point E
    System.out.println(x + " " + y + " " + z);
}