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)

A loop is a code sequence that repeats until a condition is met, like a dishwasher running cycles until the dishes are clean.

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
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

int i = 0;
while (i < 5) { System.out.println(i);
i++;
}

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?

it is an infinite loop, so there will be no stopping of the loop.

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

Do While loop

What will this code block output?

It will output “Quite Shrimple.”

// 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");
}

|   for (initialization; Boolean expression; update)

not a statement

|   for (initialization; Boolean expression; update)

';' expected

|   for (initialization; Boolean expression; update)

not a statement

|   for (initialization; Boolean expression; update)

')' expected

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?”

5 times.

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?”

5 times

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++;  
}

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 - Comparees 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

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

Math.min(int a, int b): Retrieves the smaller of two int values, used here to get the shorter string’s length to avoid IndexOutOfBoundsException.

String.substring(int beginIndex, int endIndex): Creates a substring of the original string, used in the loop to compare parts of string1 and string2.

String.equals(Object anObject): Compares two strings for equality, used here to check if the substrings from string1 and string2 are identical.

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?

It finds the number of characters in the String.

What Boolean Operator is used?

Or operator is used, represented by

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:

(write your guess here)

1 2

public class NestedLoops{

    public static void main(String[] args){

        for (int inner = 1; inner < 3; inner++){ 
        
            for (int outer = 1; outer < 5; outer++){
                
                 System.out.print(inner + " ");
             }

             System.out.println();

         } 

    }
}

NestedLoops.main(null)

1 1 1 1 
2 2 2 2 

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

1 1 1 1

2 2 2 2

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 matches my prediction.

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? __4

In the code above, how many times will the inner loop execute when outer = 1? __4

In the code above, how many times will the inner loop execute when outer = 2? __4

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? __5*6/2 = 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? __We cannot predict this (it will be a rnadom number of times due to Math.random)

for (int k = 0; k < 135; k+=5){
    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. Now Statment 1 runs 27 times instead of 135.

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+= 4){
    if (k % 4 == 0){
        System.out.println(k); 
    }
}

int k = 0;
while (k < 40) {
    System.out.println(k);
    k += 4;
}

for (int i = 0; i <= 3; i++) {
    for (int j = 0; j <= 9; j++) {
        int k = i * 10 + j;
        if (k % 4 == 0) {
            System.out.println(k);
        }
    }
}

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)

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!

import java.util.List;
import java.util.Arrays;

public class CaesarDecoder {
    public static int getCharIndex(String[] alphabet, char searchChar) {
        String charAsString = String.valueOf(searchChar);
        for (int pos = 0; pos < alphabet.length; pos++) {
            if (alphabet[pos].equals(charAsString)) {
                return pos;
            }
        }
        return -1; // Character not found
    }
    
    public static String decodeMessage(String cipherText, String[] alphabet) {
        StringBuilder decodedText = new StringBuilder();
        cipherText = cipherText.toLowerCase();

        List<String> alphabetList = Arrays.asList(alphabet);
        for (char currentChar : cipherText.toCharArray()) {
            String characterAsString = String.valueOf(currentChar);
            if (!alphabetList.contains(characterAsString)) {
                decodedText.append(characterAsString);
            } else {
                int charPosition = getCharIndex(alphabet, currentChar);
                decodedText.append(alphabet[(charPosition + 3) % alphabet.length]);
            }
        }

        return decodedText.toString();
    }

    public static void main(String[] args) {
        String[] alphabet = { "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 secretMessage1 = "Kfzb gly!";
        String secretMessage2 = "zlab zlab zlab";
        String secretMessage3 = "prmbozxifcoxdfifpqfzbumfxifalzflrp";

        String revealedMessage1 = decodeMessage(secretMessage1, alphabet);
        String revealedMessage2 = decodeMessage(secretMessage2, alphabet);
        String revealedMessage3 = decodeMessage(secretMessage3, alphabet);

        System.out.println(revealedMessage1);
        System.out.println(revealedMessage2);
        System.out.println(revealedMessage3);
    }
}

CaesarDecoder.main(null);

nice job!
code code code
supercalifragilisticexpialidocious

Unit 4 Lesson • 34 min read

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