Building Java Programs

Lab: Midterm Practice

 

 

Lab goals

Goals for this problem set:

Exercise 1: lastDigit

Exercise 2: firstDigit

Exercise 3: season

Exercise 4: 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

 

Exercise 5: 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 6: hopscotch

Write a method named hopscotch that accepts an integer parameter for a number of "hops" and prints a hopscotch board of that many hops.

For example, the call hopscotch(3); would produce the following output:

   1
2     3
   4
5     6
   7
8     9
   10

 

Exercise 7: 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 8: 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 9: 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"
To be considered a match, the strings must have exactly the same number of dashes in exactly the same positions. The Strings might be of different length.

 

Exercise 10: 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.

 

Exercise 11: "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;
    }
}
  

Open Practice-It from the link above, copy/paste this code into it, then see the next slide.

Exercise - things to fix

Fix the following aspects of the code:

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 12: Expressions

For each expression in the left-hand column, indicate its value in the right-hand column. Be sure to list a constant of appropriate type (e.g., 7.0 rather than 7 for a double, Strings in quotes).

4 * (2 + 4) - 3 * 5
9
54 % 10 + 8 * 3 % 9
10
3 * 2 + 4 + "+" + 2 + 3 * 4
"10+212"
2.3 * 3 + 19 / 5 / 2 + 6.0 / 5
9.1
108 / 20 * 3 / 4 / 2.0 + 1.0 / 2
2.0

Exercise 13: Parameter Mystery

Exercise 14: if/else mystery

Consider the following Java code.

public static void ifElseMystery(int a, int b) {
    if (a < b) {
        a = a * 2;
    }
    if (a > b) {
        a = a - 10;
    } else {
        b++;
    }
    System.out.println(a + " " + b);
}

Fill in the boxes with the output produced by each of the method calls.

ifElseMystery(10, 3);
0 3
ifElseMystery(6, 6);
6 7
ifElseMystery(3, 4);
-4 4
ifElseMystery(4, 20);
8 21

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery4(int n) {
    int x = 1;
    int y = 2;
    while (y < n) {
        if (n % y == 0) {
            n = n / y;
            x++;
        } else {
            y++;
        }
    }
    System.out.println(x + " " + n);
}
mystery4(2);
1 2
mystery4(5);
1 5
mystery4(24);
4 3
mystery4(28);
3 7