lecture-notes.md

CSE 142

Lecture Notes

Week 5

Fencepost problems

• Sometimes illustrated as |-|-|-|-| with the | representing a fence post and the - representing the wire of the fence
• The idea is that you need 1 more post than you do wire sections; you begin with a post and end with a post
• Also sometimes called a "loop-and-a-half"
• Common solutions usually have the loop run one less times than needed and then handle the last post outside of the loop

Examples

// handles the first post outside the loop
// prints a comma separated list of numbers from 1 up to max
public static void printNumbers(int max) {
    System.out.print(1);
    for(int i = 2; i <= max; i++) {
        System.out.print(", " + i );
    }
    System.out.println();
}

// handles the last post ouside the loop
// prints a comma separated list of numbers from 1 up to max
public static void printNumbers(int max) {
    for(int i = 1; i <= max - 1; i++) {
        System.out.print(i + ", ");
    }
    System.out.println(max);
}

while loop

• Repeatedly executes its body as long as a logical test is true
• Note: The for loop is a specialized form of the while loop
• Use while when it is unknown how many times the loop will repeat (meaning you don't know right now, even if you could put in some time and effort to figure out the exact amount)
• Use for when it is known how many times the loop will repeat

Structure

while (test) {
    statement(s);
}

Sentinel values

• A value that signals the end of user input
• sentinel loop: A loop that repeats until a sentinel value is seen

Example

// The redundancy here is valid because we are also solving a fencepost problem
// Alternatively, you could move the repeated lines into another method
public static void sum() {
    int sum = 0;
    System.out.print("Enter an integer (-1 to quit): ");
    int number = console.nextInt();
    while (number != -1) {
        sum = sum + number;
        
        System.out.print("Enter an integer (-1 to quit): ");
        number = console.nextInt();
    }
}

Boolean

• boolean is a type
• boolean variables can hold either true or false

String

• Strings in Java are objects
• Strings can contain the same characters but not be equal because in Java they are stored as different objects (even though they have the same characters)
• Because of this, you should use .equals() when comparing Strings and not ==

  // word1 and word2 are different objects
  String word1 = "hello";
  String word2 = "hello";
  
  // do not use!
  if (word1 == word2) {
     ...
  }
  
  // use this instead!
  if (word1.equals(word2)) {
     ...
  }
  
  // you could also have use this; it does the same as the one directly above
  if (word2.equals(word1)) {
     ...
  }

String test methods

Note: These notes are repeated from week4 beacause they were discussed in more detail this week.

Methods	Description
str1.equals(str2)	tests whether str1 contains the same characters as str2
str1.equalsIgnoreCase(str2)	tests whether str1 contains the same characters as str2, ignoring case
str1.startsWith(str2)	tesets whether str1 starts with the characters in str2
str1.endsWith(str2)	tests whether str1 ends with the characters in str2
str1.contains(str2)	tests whether str2 is found inside of str1

Random class

• A Random object generates pseudo-random numbers
• pseudo-random means simulated randomness, but not truly random
• In order to use the Random class you will need to import the util package: import java.util.*;

Random methods

Method Name	Description
nextInt()	returns a random integer
nextInt(max)	returns a random integer in the range [0, max) (i.e., 0 to max - 1 inclusive)
nextDouble()	returns a random real number in the range [0.0, 1.0)

Structure

To get a number in an inclusive range of min to max

nextInt(max - min + 1) + min

Example code

Random rand = new Random();
// randomNumber1 will store a random number in the range 0 – 9
int randomNumber1 = rand.nextInt(10);
// randomNumber2 will store a random number in the range 1 - 20
int randomNumber2 = rand.nextInt(20) + 1;
// randomNumber3 will store the first 5 even numbers (0, 2, 4, 6, 8)
int randomNumber3 = rand.nextInt(5) * 2;

Logical assertions

• Assertion: A statement that is either true or false
• Tips
  • Right after a variable is initialized, its value is known
  • At the start of a loop's body, the loop's test must be true
  • After a loop, the loop's test must be false
  • Inside a loop's body, the loop's test may become false
  • Reading from a Scanner, reading from a Random object, or parameter values are unknown and usually result in a "Sometimes" assertion
  • If you are unsure, guess "Sometimes"