# University of Washington, CSE 142 (190)

## Lab 5.5: Random numbers, boolean logic, and midterm practice

Except where otherwise noted, the contents of this document are Copyright 2010 Stuart Reges and Marty Stepp.

lab document created by Whitaker Brand and Marty Stepp

# Today's lab

Goals for today:

• use `Random` objects to produce random numbers
• use `boolean` expressions and variables to represent logical true/false expressions
• examine logical assertions that can be made about a running program
• practice problems similar to what will be on the midterm exam
• Where you see this icon, you can click it to check the problem in Practice-It!

# Recall: `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
```
Method name Returns...
`nextInt()` a random integer
`nextInt(max)` a random integer between 0 (inclusive) and max (exclusive)
`nextDouble()` a random real number between 0.0 and 1.0
`nextBoolean()` a random `boolean` value: `true` or `false`

# 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();
```
 Example: a random integer from 1 to 5 inclusive: ```rand.nextInt(5) + 1 ``` a random integer from 0 to 3 inclusive: `rand.nextInt(4)` a random integer from 5 to 10 inclusive: ```rand.nextInt(6) + 5 ``` a random integer from -4 to 4 inclusive: ```rand.nextInt(9) - 4 ``` a random even integer from 16 to 28 inclusive: (Hint: To get only even numbers, scale up.) ```rand.nextInt(7) * 2 + 16 ```

# Exercise : Boolean Expressions

Write the result of each expression as either `true` or `false`, given the following variables. Recall the logical operators: `&&` (and), `||` (or), `!` (not).

```int x = 12;
int y = 7;
int z = 28;
String s = "mid term";
```
 `x < 14` `true` `!(x % 2 < 1)` `false` `x < y || x < z` `true` `z / x < x / y * x` `true` `s.length() == y` `false` `s.toUpperCase().equals("MID TERM")` `true` `!s.equals("mid term") || x * y != z` `true` `s.substring(z / x).length() > y` `false`

# 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;
}
}
```

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:

• 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.

```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 : 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 : assertions

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

`x > y` `z == 0` `x == y`
A
B
C
D
E
```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);
}
```

# 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 : 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
```

Try solving this problem in Practice-It: click on the check-mark above!

# 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).

Try to solve this problem in Practice-It: click on the check-mark above!

# Exercise : longestName

Write a method named `longestName` that reads names typed by the user and prints the longest name (the name that contains the most characters) in the format shown below. Your method should accept a console `Scanner` and an integer n as parameters and should then prompt for n names.

A sample execution of the call `longestName(console, 4)` might look like the following:

```name #1? roy
name #2? DANE
name #3? sTeFaNiE
name #4? Erik
Stefanie's name is longest
```

Try to solve this problem in Practice-It: click 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 Sample Midterm Exams and try to solve them!