Unit 4 - Iteration and Loops

For Loops

For loops repeat a command or commands.
Each for loop declares a counter variable to monitor the loop's progression, usable inside the loop.
The header of the for loop has three parts:
- Initialization: Declares and initializes the counter variable.
- Test Expression: Checks if the loop should continue. If the test expression evaluates to false the loop terminates and the program continues to the code after the loop.
- Update: Modify the variable at the end of each loop cycle.

Example:

for (int i = 0; i < 4; i++) {
    System.out.println(i);
}

Tracing the Loop:

int i = 0: Declares and initializes i to 0.
i < 4: Checks if i is less than 4. If true, the loop continues.
System.out.println(i): Prints the value of i.
i++: Increments i by 1 at the end of each loop.
The loop continues until i is no longer less than 4.

While Loops and Do While Loops

While Loops: Repeat commands while a Boolean expression remains true.
```
while (true) {
    // This loop will run forever
}
```
```
int i = 4;
while (i > 0) {
    // This loop will also run forever because i is never decremented
}
```
To make the above loop terminate, inside the body of the loop decrement the value of i.

int i = 4;
while (i > 0) {
    System.out.println(i);
    i--;
}

A while loop might not execute even once if the condition is initially false.
Do While Loops: Checks the Boolean expression at the end of the loop so it always runs at least once.

Example using while loop:

int userGuess = 0;
int secretNumber = 23;

while (userGuess != secretNumber) {
    System.out.println("Guess the number I'm thinking of:");
    // Assume user input updates userGuess here
}

System.out.println("You got it right!");

Do While Loop Example:

int i = -2;
do {
    System.out.println(i);
    i--;
} while (i > 0);

Note: Do while loops have a semicolon at the end.

Reversing a String Using a For Loop and Substring

Goal: Reverse the letters in a string.

String original = "pupils";
String reverse = ""; // Empty string

for (int i = original.length() - 1; i >= 0; i--) {
    reverse = reverse + original.substring(i, i + 1);
}

System.out.println(reverse);

Explanation:

original.length() returns the number of characters in the string.
Strings are treated as arrays internally, so indexing starts at 0.
The index of the last character is length - 1.
substring(i, i + 1) extracts the character at index i.
substring is "inclusive exclusive", meaning it includes the start index but excludes the end index.
The loop iterates backward through the string and concatenates each character to the reverse string.

Nested Loops

Definition: A loop inside another loop.
Outer loop runs once, the inner loop goes through a complete cycle.
Useful when working with two-dimensional arrays.

Example:

class NestedLoop {
    public static void main(String[] args) {
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 3; j++) {
                System.out.print("# ");
            }
        }
    }
}

Explanation:

The outer loop runs twice (i = 0 and i = 1).
The inner loop runs three times for each outer loop iteration (j = 0, j = 1, and j = 2).
Total inner loop executions: 2 * 3 = 6.
Any type of loop (for, while, do while) can be nested.
Nested loops can contain more complex algorithms, such as if statements.