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Unit 4 Lesson • 33 min read

Description

Lesson for APCSA Unit 4

  • 4.4: Nested Iteration
  • 4.5: Informal Code Analysis
  • HACKS
  • Iteration

    According to CollegeBoard, Iteration is a way to simplify code that would otherwise be repeated many times in succession. Using loops, we can finally implement complex algorithms and solutions to common problems that we weren’t able to before.

    Iteration is repeating sequences to simplify code of advanced algorithms

    Iteration accounts for 17.5%-22.5% of the APCSA AP Exam

    4.1: WHILE LOOPS

    Learning Objective: Represent Iterative Processes using a while loop

    Question: What is a loop and what are some real life examples of this (Setting a Song to repeat on your music player)

    You know the i variable that you use for while/for loops? It actually has a name, loop control variable

    int i = 0; // initialize loop control variable
    while (i < 10)  // checks the loop control variable
    {
        System.out.println("Doing some code");
        i++;  // update the loop control variable
    }
    
    Doing some code
    Doing some code
    Doing some code
    Doing some code
    Doing some code
    Doing some code
    Doing some code
    Doing some code
    Doing some code
    Doing some code
    
    // Popcorn Hack: Simplify the code segment below
    int i = 0;
    System.out.println(i);
    i++;
    System.out.println(i);
    i++;
    System.out.println(i);
    i++;
    System.out.println(i);
    i++;
    System.out.println(i);
    i++;
    
    /* Hint:
    // while (condition) {
    
    }
    for (condition) {
    
    }
    */
    
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    int i = 0;
    while (i < 5) { System.out.println(i);
    i++;
    }
    
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    Infinite loop

    An infinite loop is when a while loop always evaluates to true. avoid this when you can because it’s probably not good for your computer. if this happens by accident, I recommend copying all code in the block and deleting the block. After you delete the code block, close and reopen the tab that the code block was in.

    What’s wrong with this code block?

    while (true)
    {
        System.out.print("CONTROL ");
    }
    // DO NOT RUN THE CODE
    

    Do While loop

    What will this code block output?

    // Quite shrimple
    int i = 0;
    do 
    {
        System.out.print("Quite shrimple. ");
        i++;
    }
    while (i < -5);
    
    Quite shrimple. 
    

    In a do while loop, it will run the “do” once before it reaches the “while”, and at that point it will start to act like a while loop.

    For loop

    this is the standard structure of a for loop

    for (initialization; Boolean expression; update)
    {
        System.out.println("Doing some code");
    }
    

    Initialization will run at the start of the loop, boolean expression will get checked with every loop, and update runs after every loop.

    How many times will this code print “Doing some code?”

    for (int num = 1; num <= 5; num++)
    {
        System.out.println("Doing some code");
    }
    
    Doing some code
    Doing some code
    Doing some code
    Doing some code
    Doing some code
    

    In this code, it creates the variable num at the start of the loop, it checks if num is less than or equal to 5 after each loop, and it adds 1 to num after each loop.

    Enhanced for loop

    this is essentially a javascript for loop, as it will iterate through a list and run code in the loop to each variable inside the list

    int[] list = {1, 4, 6, 2};
    for (int j : list)
    {
        System.out.print(j);
        System.out.print(" ");
    }
    
    1 4 6 2 
    

    Break and Continue

    In java there are breaks, but there are also continues.

    Break

    Breaks, as you likely already know, end a loop. They tend to be used with an if statement

    How many times will this code print “Big guy?”

    int i = 0; 
    while (i < 10) 
    {
        System.out.println("Big guy");
        i++;  
        if (i == 5) {
            break;
        }
    }
    
    Big guy
    Big guy
    Big guy
    Big guy
    Big guy
    

    Continue

    Continue will skip code for an iteration, but will still keep the loop running

    int i = 0; 
    while (i < 10) 
    {
        if (i == 5) {
            i++; // don't forget this, it creates an error similar to an infinite loop
            System.out.println("");
            continue;
        }
        System.out.println(i);
        i++;  
    }
    
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    4.3: Developing Algorithms Using Strings

    Learning Objective: For algorithms in the context of a particular specification that involves String objects:

    Methods in Java that help to MANIPULATE STRINGS

    • String.substring - Retrieves a particular portion of a String
    • String.equals - Compares the content of two strings
    • String.length - Returns the length of a String
    public class Compare {
        public static void main(String[] args) {
            String string1 = "Coding is cool!";
            String string2 = "Coding is coding!";
    
            int minLength = Math.min(string1.length(), string2.length());
    
            for (int i = 0; i < minLength; i++) {
                String subString1 = string1.substring(0, i + 1);
                String subString2 = string2.substring(0, i + 1);
    
                if (subString1.equals(subString2)) {
                    System.out.println("Common Prefix: " + subString2);
                }
            }
        }
    }
    Compare.main(null)
    
    Common Prefix: C
    Common Prefix: Co
    Common Prefix: Cod
    Common Prefix: Codi
    Common Prefix: Codin
    Common Prefix: Coding
    Common Prefix: Coding 
    Common Prefix: Coding i
    Common Prefix: Coding is
    Common Prefix: Coding is 
    Common Prefix: Coding is c
    Common Prefix: Coding is co
    

    Where are the 3 methods in the above Java Cell and how do they contribute to the program’s functionality?

    String word = "supercalifragilisticexpialidocious";
    int count = 0;
    
    for (int i = 0; i < word.length(); i++) {
        char letter = word.charAt(i);
        if (letter == 'a' || letter == 'e' || letter == 'i' || letter == 'o' || letter == 'u') {
            count++;
        }
    }
    
    System.out.println("The Number of vowels in \"" + word + "\" is " + count);
    
    
    The Number of vowels in "supercalifragilisticexpialidocious" is 16
    

    What does word.length() do and how do we use it above?

    What Boolean Operator is used?

    public class Main {
        public static void main(String[] args) {
            String word = "Scooby Doo";
            String sub = "Doo";
            boolean found = false;
    
            for (int i = 0; i <= word.length() - sub.length(); i++) {
                String portion = word.substring(i, i + sub.length());
                if (portion.equals(sub)) {
                    found = true;
                }
            }
            if (found) {
                System.out.println("We found the Smaller String!");
            } else {
                System.out.println("We did not find the Smaller String! \t Ruh Roh!");
            }
        }
    }
    Main.main(null)
    
    We found the Smaller String!
    

    String concatenation

    String concatenation is when you want to add to strings together

    String original = "String";
    String reversed = "";
    for (int i = 0; i < original.length(); i++)
    {
        String single = original.substring(i,i+1);
        reversed = single + reversed;
    }
    System.out.println("Original String: " + original);
    System.out.println("Reversed String: " + reversed);
    
    
    Original String: String
    Reversed String: gnirtS
    

    4.4: Nested Iteration

    Learning Objective: Represent nested iteration processes

    Essential Knowledge:

    • Nested iteration is when an iteration statement appears inside the body of another iteration statement
    • The inner loop must complete all of its iterations before the outer loop can continue.

    Before uncommenting the code, guess what the output will look like:

    1 2 1 2 1 2 1 2

    public class NestedLoops{
    
        public static void main(String[] args){
    
            for (int outer = 1; outer < 5; outer++){
    
                for (int inner = 1; inner < 3; inner++){
                    
                    System.out.print(inner + " ");
                }
    
                System.out.println();
    
            } 
    
        }
    }
    
    NestedLoops.main(null)
    
    1 2 
    1 2 
    1 2 
    1 2 
    

    What will the output of the code above be if we switch the loop headers (the stuff inside of the for loop)?

    1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

    After making a prediction actually switch the loop headers for yourself. What do you notice about the output compared to the output before the change?

    It goes til 4 rather than just stopping at 2

    4.5: Informal Code Analysis

    Essential Knowledge:

    • A statement exectution count indicates the number of times a statement is executed by the program
    for (int outer = 0; outer < 3; outer++){
        for (int inner = 0; inner < 4; inner++){
            // statement #1
        }
    }
    

    In the code above, how many times will the inner loop execute when outer = 0? 15

    In the code above, how many times will the inner loop execute when outer = 1? 10

    In the code above, how many times will the inner loop execute when outer = 2? 5

    In the code above, how many times will the inner loop execute in total? 12

    for (int outer = 5; outer > 0; outer--){
        for (int inner = 0; inner < outer; inner++){
            // statement #1
        }
    }
    

    In the code above, how many times will the inner loop execute when outer = 5? 5

    In the code above, how many times will the inner loop execute when outer = 4? 4

    In the code above, how many times will the inner loop execute when outer = 3? 3

    In the code above, how many times will the inner loop execute in total? 15

    int k = 0;
    while (k < 5){
        int x = (int)(Math.random()*6) + 1;
        while (x != 6){
            //statement #1
            x = (int)(Math.random()*6) + 1;
        }
        k++;
    }
    

    In the code above, how many times will the statement #1 execute? it’ll be random every time

    for (int k = 0; k < 135; k++){
        if (k % 5 == 0){ // Statement #1
            System.out.print(k); // Statement #2
        }
    }
    
    05101520253035404550556065707580859095100105110115120125130
    

    In the code above, how many times will the statement #1 execute? 135

    In the code above, how many times will the statement #2 execute? 27

    Rewrite the code above to be more effecient based on execution count.

    HACKS

    These hacks will be due on Monday (October 16th) before class

    Hacks

    • Finish the popcorn hacks (0.2)
    • Rewrite the for loop (0.25)
    • Complete the Ceaser Cipher Code (0.45)
    for (int k = 0; k < 40; k++){
        if (k % 4 == 0){
            System.out.println(k); 
        }
    }
    
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    Rewrite the code above 3 different times

    • Your code should be more efficient based execution count
    • Your code should use 3 different types of loops that you learned above (Hint: You may need to use a list)
    // Using a while loop
    for (int k = 0; k < 40; k++){
        if (k % 4 == 0){
            System.out.println(k); 
        }
    }
    
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    // Using a for loop with an array of integers
    int[] numbers = new int[40];
    for (int i = 0; i < 40; i++) {
        numbers[i] = i;
    }
    
    for (int num : numbers) {
        if (num % 4 == 0) {
            System.out.println(num);
        }
    }
    
    
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    // Using a do while loop
    int k = 0;
    do {
        if (k % 4 == 0) {
            System.out.println(k);
        }
        k++;
    } while (k < 40);
    
    
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    Ceaser Cipher Hacks

    Try to write a cipher program that shifts each letter in a message 3 letters forward. Use any of the methods you learned today. Use it to decode the 3 messages we’ve given you!

    public class CaesarCipher {
    
        public static void main(String[] args) {
            String[] letters = {
                "a", "b", "c", "d", "e", "f", "g", "h", "i", "j",
                "k", "l", "m", "n", "o", "p", "q", "r", "s", "t",
                "u", "v", "w", "x", "y", "z"
            };
    
            String message1 = "Kfzb gly!";
            String message2 = "zlab zlab zlab";
            String message3 = "prmbozxifcoxdfifpqfzbumfxifalzflrp";
    
            String decryptedMessage1 = decryptMessage(message1, letters);
            String decryptedMessage2 = decryptMessage(message2, letters);
            String decryptedMessage3 = decryptMessage(message3, letters);
    
            System.out.println("Decrypted Message 1: " + decryptedMessage1);
            System.out.println("Decrypted Message 2: " + decryptedMessage2);
            System.out.println("Decrypted Message 3: " + decryptedMessage3);
        }
    
        public static String decryptMessage(String message, String[] alphabet) {
            StringBuilder decryptedMessage = new StringBuilder();
    
            for (int i = 0; i < message.length(); i++) {
                char currentChar = message.charAt(i);
                if (Character.isLetter(currentChar)) {
                    char lowercaseChar = Character.toLowerCase(currentChar);
                    int index = lowercaseChar - 'a';
                    int decryptedIndex = (index + 3 + 26) % 26; 
                    char decryptedChar = alphabet[decryptedIndex].charAt(0);
                    if (Character.isUpperCase(currentChar)) {
                        decryptedChar = Character.toUpperCase(decryptedChar);
                    }
                    decryptedMessage.append(decryptedChar);
                } else {
                    decryptedMessage.append(currentChar);
                }
            }
            return decryptedMessage.toString();
        }
    }
    
    CaesarCipher.main(null)
    
    
    Decrypted Message 1: Nice job!
    Decrypted Message 2: code code code
    Decrypted Message 3: supercalifragilisticexpialidocious