U4 | Iteration

4.1 while Loops

A while loop is a fundamental control structure in programming used for repeated execution of a block of code as long as a condition is true.
The loop starts by evaluating the condition. If the condition is true, the code inside the loop is executed.
After each iteration, the condition is re-evaluated, and if it’s still true, the loop continues. If the condition is false initially, the loop code is never executed.
While loops are used when you don’t know in advance how many times the loop needs to execute.
There’s a risk of infinite loops if the condition never becomes false, so be cautious. You can use variables and complex expressions as loop conditions.
It’s essential to update the loop control variable within the loop to prevent infinite loops.
While loops are typically used for tasks such as iterating over collections or waiting for a specific condition to be met.
You can always break out of a while loop prematurely using the break statement.

Example of While Loops

public class PyramidPattern {
    public static void main(String[] args) {
        int height = 5;
        int row = 1;

        while (row <= height) {
            int spaces = height - row;
            int stars = 2 * row - 1;

            // Print spaces
            int spaceCount = spaces;
            while (spaceCount > 0) {
                System.out.print(" ");
                spaceCount--;
            }

            // Print stars
            int starCount = stars;
            while (starCount > 0) {
                System.out.print("*");
                starCount--;
            }

            System.out.println(); // Move to the next line for the next row
            row++;
        }
    }
}

4.2 for Loops

Iterative statement that checks for condition
Repeatedly execute a a block of code as long as the condition is met
Condition specifies amount of times

for Loops vs. while Loops

while Loops: use when number of iterations is unknown
for Loops: use when number of iterations is known

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

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

Three parts in for loop header: variable initialization, Boolean (conditional) expression, and increment/decrement statement

Question: Which part is which?

variable initialization (int i=0): sets variable before loop starts
Boolean (conditional) expression (i < 5): defines condition for loop to run, in this case, the loop continues as long as i is less than 5, so loops 5 times i 05
increment/decrement statement (i++): increases variable each time code block in the loop is executed, in this case it increases by 1
variable can be used in the code block for other various reasons besides specifying how many times the loop will repeat
Boolean (conditional) expression and increment/decrement statement together determine how many times the loop will repeat

Popcorn Hacks: Calculate and print the sum of all even numbers from 1 to a given positive integer ‘n’ (user input n)

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int i = 1;  // Start from 1
        int sum = 0;
        Scanner scanner = new Scanner(System.in);
        
        System.out.print("Enter a positive integer (n): ");
        int n = scanner.nextInt();

        while (i <= n) {
            if (i % 2 == 0) {
                sum += i;
            }
            i++;
        }

        System.out.println("The sum of even numbers from 1 to " + n + " is: " + sum);
    }
}

Main.main(null)

Enter a positive integer (n): The sum of even numbers from 1 to 20 is: 110

4.3 Developing Algorithms Using Strings

LEARNING OBJECTIVES: For algorithms in the context of a particular specification that involves String objects:

identify standard algorithms
modify standard algorithms
develop an algorithm

Java has many methods that are helpful when working with strings:

String .substring –> retrieves portion of a string
String .equals –> compares two strings
String .length –> returns length of a string
for Loop –> iterating through characters of a string

Finding a substring within a string

We can use the “window” method:

A “window” is created the same length as the substring. We then iterate through the main string in sections and compare to the substring

For example:

I T E R A T E

with substring “ERA”

public class StringFinder {
    public static void main(String[] args) {
        String word = "iterate";
        String sub = "era";
        boolean found = false; // will be set to true once substring is found

        for (int i = 0; i < word.length(); i++) { //iterating forwards: starting at first index (0) and going to the length of the word.. let's try word.length
            String portion = word.substring(i, i + sub.length());
            if (portion.equals(sub)) // make sure you use .equals!!
                found = true;
        }

        if (found)
            System.out.println("substring is found within string!");
        else
            System.out.println("substring is NOT within string");
    }

    }

    StringFinder.main(null);

POPCORN HACK: Run the code.. what happened? How can we fix it?

The loop will keep looking for 2 more letters after the last letter and so to fix this issue, we subtract the substring length from the upper limit of letters so it stops early

Another issue:

I T E R A T E

What if our substring was the word “RATE”? Note that RATE is at the end of the whole string

It would end much earlier since we are now looking at 4 letters instead of 3 and so it wouldn’t find the substring we are looking for so the very end of the bounds need to be included

HACKS

Create a algorithm similar to the one above. Except this time, use iteration to count the number of vowels within the main string.

HINT: Use the boolean expressions we have learned in previous lessons. Which would you use when comparing your “window” with multiple options of substrings?

public class VowelCounter {
    public static void main(String[] args) {
        String word = "iterate";
        int vowelCount = 0;

        for (int i = 0; i < word.length(); i++) {
            char currentChar = word.charAt(i);
            boolean isVowel = isVowel(currentChar);

            if (isVowel) {
                vowelCount++;
            }
        }

        System.out.println("Number of vowels in the string: " + vowelCount);
    }

    // Helper method to check if a character is a vowel
    public static boolean isVowel(char c) {
        c = Character.toLowerCase(c); // Convert to lowercase to handle both upper and lower case vowels

        // Check if the character is one of the vowels (a, e, i, o, u)
        return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u';
    }
}

VowelCounter.main(null)

Number of vowels in the string: 4

4.4 Nested Iteration

nested iteration occurs when we have a loop inside of another loop, similar to nested conditional statements in unit 3

When you have one loop inside another, the inner loop has to finish all its rounds before the outer loop moves to the next round. If the inner loop has a “stop” command, it only stops for that round of the outer loop. The next time the outer loop starts a new round, the inner loop starts over.

If you have two nested loops without stops, and the outer one runs n times while the inner one runs m times each time the outer one goes around, the inner loop will run m times n times, which is m * n times in total. This rule also applies if you have more than two nested loops. To find the total number of times the innermost loop runs, just multiply how many times each loop runs per round.

public class NestedLoopsDemo {
    public static void main(String[] args) {
        int n = 3; //numb of times the outside loop runs
        int m = 2; //numb of times the inside loop runs

        //the nested loops
        for (int i = 1; i <= n; i++) {
            System.out.println("Outer loop iteration: " + i);
            for (int j = 1; j <= m; j++) {
                System.out.println("Inner loop iteration: " + j);
            }
        }
    }
}
NestedLoopsDemo.main(null)

Break Statement

break statement is used to exit a loop prematurely, typically when a certain condition is met. In the case of nested loops, it can be used to break out of the innermost loop.

public class BreakExample {
    public static void main(String[] args) {
        for (int i = 1; i <= 3; i++) {
            System.out.println("Outer loop iteration " + i);

            for (int j = 1; j <= 3; j++) {
                System.out.println("Inner loop iteration " + j);

                if (i == 2 && j == 2) {
                    System.out.println("Breaking inner loop");
                    break; //break out of the inside loop when i is 2 and j is 2.
                }
            }
        }
    }
}
BreakExample.main(null)

Popcorn HACK

When the targetNumber is found, you can print a message and use the break statement to exit the loop. When it’s not found, you can print a message indicating that the number was not found.

public class BreakHack {
    public static void main(String[] args) {
        int targetNumber = 42; //numb we want
        int[] numbers = {10, 20, 30, 40, 50, 60, 70}; //numb array

        for (int number : numbers) {
            if (number == targetNumber) {
                System.out.println("Target Number is found!");
                break;
            }
            else {
                System.out.println("Target Number is not found :(");
            }
        }
    }
}
BreakHack.main(null)

Target Number is not found :(
Target Number is not found :(
Target Number is not found :(
Target Number is not found :(
Target Number is not found :(
Target Number is not found :(
Target Number is not found :(

Continue Statement

continue statement is used to skip the current iteration of a loop and move to the next iteration. In the case of nested loops, it applies to the innermost loop.

public class ContinueExample {
    public static void main(String[] args) {
        for (int i = 1; i <= 3; i++) {
            System.out.println("Outer loop iteration " + i);

            for (int j = 1; j <= 3; j++) {
                if (i == 2 && j == 2) {
                    System.out.println("Skipping inner loop iteration " + j);
                    continue; //skip the iteration when i is 2 and j is 2.
                }
                System.out.println("Inner loop iteration " + j);
            }
        }
    }
}
ContinueExample.main(null)

Patterns and Shapes

import java.util.Scanner;

public class InteractivePyramid {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter the symbol you want to use: ");
        char symbol = scanner.next().charAt(0);

        System.out.print("Enter the number of rows for the pyramid: ");
        int numRows = scanner.nextInt();

        for (int i = 1; i <= numRows; i++) {
            //print space before the symbol
            for (int j = 1; j <= numRows - i; j++) {
                System.out.print(" ");
            }

            //print
            for (int k = 1; k <= 2 * i - 1; k++) {
                System.out.print(symbol);
            }

            System.out.println(); //next line
        }
        scanner.close();
    }
}
InteractivePyramid.main(null)

Enter the symbol you want to use: 

Enter the number of rows for the pyramid:    *
  ***
 *****
*******

Hacks

Modify pyramid code:

Create different patterns (other then pyramid) by modifying nested loop structure

Questions

What is a nested iteration, continue statement, and break statement (in your own words)?
Create a simple example of a continue statement or break statement

import java.util.Scanner;

public class InteractiveBall {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter the symbol you want to use: ");
        char symbol = scanner.next().charAt(0);

        System.out.print("Enter the radius of the ball: ");
        int radius = scanner.nextInt();

        for (int i = -radius; i <= radius; i++) {
            for (int j = -radius; j <= radius; j++) {
                // Calculate the distance from the center
                double distance = Math.sqrt(i * i + j * j);

                // Check if the point is within the given radius
                if (distance <= radius) {
                    System.out.print(symbol);
                } else {
                    System.out.print(" ");
                }
            }
            System.out.println(); // Next row
        }
        
        scanner.close();
    }
}
InteractiveBall.main(null)

Enter the symbol you want to use: Enter the radius of the ball:      *     
  *******  
 ********* 
 ********* 
 ********* 
***********
 ********* 
 ********* 
 ********* 
  *******  
     *     

I tried to make a ball one lol but this is the best i can do

Questions:

What is a nested iteration, continue statement, and break statement (in your own words)?

Nested iterations is where you use one loop within another loop in order to often times deal with an array or other structures that need to be processed

Countinue Statement is when it stops the cuurent iteration and instantly goes onto the next iteration without exiting the loop

Break statement is when it stops a loop prematurely if certain conditions aren’t met

Create a simple example of a continue statement or break statement

Example of Continue:

public class ContinueExample {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            if (i == 3) {
                System.out.println("Skipping iteration " + i);
                continue; // Skip the rest of the loop and move to the next iteration
            }
            System.out.println("Iteration " + i);
        }
    }
}
ContinueExample.main(null)

Iteration 1
Iteration 2
Skipping iteration 3
Iteration 4
Iteration 5

4.5 Informal Code Analysis

Learning objective: Compute statement execution counts & informal run-time comparison of iterative statements

Essential Knowledge: A statement execution count indicates the number of times a statement is executed by the program

What IS informal code analysis?

Answer: Computing the informal runtime based on the number of times the statement is executed

// CODE EXAMPLE #1 (for loop)
public class InformalCodeAnalysis {
    public static void main(String[] args) {
        int count = 0;
        for (int k = 0; k < 30; k++)
        {
            if (k % 3 == 0) // statement 1
            {
                count++; // statement 2
            }
        }
    }
}

How many times will statement 1 execute?

Answer: 30 times

How many times will statement 2 execute?

Answer: 10 times because only 10 numbers are divisible by 3

// CODE EXAMPLE #2 (for loop)
public class InformalCodeAnalysis {
    public static void main(String[] args) {
        int count = 0;
        for (int k = 4; k < 30; k+=3)
        {
            count++; // statement 3
        }
    }
}

How many times will statement 3 execute?

Answer: 9 times

// Rewrite the code segment below to have a faster run-time based on statement execution counts
for (int k = 0; k < 135; k+= 5)
{
    System.out.println(k);
}

// CODE EXAMPLE #3 (while loop)

int num = (int)(Math.random() * 10);
while (num % 2 != 0)
{
    num = (int)(Math.random() * 10) // statement 4
}

What is the min/max number of times statement 4 will execute?

Answer: The minimum is 0 if the first number generated is divisible by 2, the max is impossible to define because there is a very slight chance only the number one is generated an infinite amount of times

// CODE EXAMPLE #4 (nested loop)

for (int outer = 0; outer < 3; outer++)
{
    for (int inner = 0; inner < 4; inner++)
    {
        // statement #5
    }
}

How many times will statement #5 execute?

Answer: 3*4 = 12 so 12 times because it iterates all 3 times and through all 4 numbers

// CODE EXAMPLE #5 (nested loop)

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

How many times will statement #6 execute?

Answer: We can’t define this exactly because it can happen either multiple times or no times

4.5 Hacks

#1 How many times will statement #1 and statement #2 execute in the code segments below?

for (int k = 0; k < 1000; k++)
{
    // statement #1
}

It will execute 1000 times

for (int k = 6; k < 50; k++)
{
    // statement #2
}

It will execute 44 times

#2 How many times will statement #3 execute for the code segment below?

int k = 1;
while (k <=7)
{
    for (int z = 0; z < 4; z++)
    {
        // statement #3
    }
    k++;
}

4*7 = 28 so 28 times

#3 Create 3 seperate code segments that execute a statement 10 times using:

(a) a for loop

(b) a while loop

for (int k = 0; k < 10; k++)
{
    System.out.println("The statement has been executed " + (k+1) + " times");
}

The statement has been executed 1 times
The statement has been executed 2 times
The statement has been executed 3 times
The statement has been executed 4 times
The statement has been executed 5 times
The statement has been executed 6 times
The statement has been executed 7 times
The statement has been executed 8 times
The statement has been executed 9 times
The statement has been executed 10 times

int k = 0;
while (k < 10)
{   
    k++;
    System.out.println("The statement has been executed " + k + " times");
}

The statement has been executed 1 times
The statement has been executed 2 times
The statement has been executed 3 times
The statement has been executed 4 times
The statement has been executed 5 times
The statement has been executed 6 times
The statement has been executed 7 times
The statement has been executed 8 times
The statement has been executed 9 times
The statement has been executed 10 times

int j = 0;
for (int k = 0; k < 2; k++){
    int l = 0;
    while (l < 5){
    j++;
    System.out.println("The statement has been executed " + j + " times");
    l++;
    }
}

The statement has been executed 1 times
The statement has been executed 2 times
The statement has been executed 3 times
The statement has been executed 4 times
The statement has been executed 5 times
The statement has been executed 6 times
The statement has been executed 7 times
The statement has been executed 8 times
The statement has been executed 9 times
The statement has been executed 10 times

U4 | Iteration • 49 min read

Description

U4 | Iteration

4.1 while Loops

Example of While Loops

4.2 for Loops

for Loops vs. while Loops

4.3 Developing Algorithms Using Strings

Finding a substring within a string

I T E R A T E

POPCORN HACK: Run the code.. what happened? How can we fix it?

Another issue:

I T E R A T E

HACKS

4.4 Nested Iteration

Break Statement

Popcorn HACK

Continue Statement

Patterns and Shapes

Hacks

Questions:

Example of Continue:

4.5 Informal Code Analysis

What IS informal code analysis?

4.5 Hacks