Loops and Recursion

In this lesson we will go over loops and recursion, two fundamental ways to repeat instructions in Java.

---

For Loop

What it is: A counter‑controlled loop. You set a start value, a stop condition, and how the counter changes each time.

Pattern:

for (initialization; condition; update) {
    // repeated code
}

Example: Count 1 to 5

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

Example: Sum the first N numbers

int n = 10;
int sum = 0;
for (int i = 1; i <= n; i++) {
    sum += i;
}
System.out.println("Sum = " + sum); // 55

Mini‑exercise

• Print all multiples of 3 from 3 to 30.

[!WARNING] Common mistakes

• Off‑by‑one errors (i < n vs i <= n).

• Forgetting the update step and creating an infinite loop.

---

While Loop

What it is: A condition‑controlled loop. It keeps running while a condition is true.

Pattern:

while (condition) {
    // repeated code
}

Example: Guessing game skeleton

int secret = 7;
int guess = -1;
java.util.Scanner sc = new java.util.Scanner(System.in);
while (guess != secret) {
    System.out.print("Guess: ");
    guess = sc.nextInt();
}
System.out.println("Correct!");

Mini‑exercise

• Starting from 50, keep subtracting 7 and print the value while it remains non‑negative.

[!WARNING] Common mistakes

• Forgetting to change the variables used in the condition → infinite loop.

• Checking the wrong condition (e.g., == vs .equals for strings).

---

Enhanced For‑Each Loop

What it is: A simple way to loop over arrays and collections without using an index.

Pattern:

for (ElementType item : collectionOrArray) {
    // use item
}

Example: Array

String[] fruits = {"Apple", "Banana", "Cherry"};
for (String fruit : fruits) {
    System.out.println(fruit);
}

Example: ArrayList

java.util.List<Integer> nums = java.util.Arrays.asList(2, 4, 6, 8);
for (int n : nums) {
    System.out.println(n);
}

When to use

• You want to visit every item in order and you don’t need the index.

Mini‑exercise

• Given int[] a = {5, 1, 5, 2}; print each element on one line using a for‑each loop.

[!WARNING] Common mistakes

• Trying to modify the collection’s structure (add/remove) while iterating.

• Needing the index but using for‑each; in that case, prefer a classic for.

---

Recursion

Definition: Recursion is when a method calls itself to solve a smaller version of the same problem until a base condition is reached.

Key Concepts:

• Base Case: The stopping condition that ends the recursion.

• Recursive Case: The part where the method calls itself with a simpler/smaller input.

Example Recursive Factorial Function

int factorial(int n) {
	if (n == 0) { // base case
		return 1;
	} else {
		return n * factorial(n - 1); // recursive case
	}
}

How it works for factorial(3):

1. factorial(3)

2. → 3 * factorial(2)

3. → 3 * (2 * factorial(1))

4. → 3 * (2 * (1 * factorial(0)))

5. → 3 * (2 * (1 * (1)))

6. → 6

When to use recursion:

• Problems that can be broken down into smaller, similar subproblems.

• Examples: factorial, Fibonacci numbers, traversing trees, depth-first search.

Mini-exercise:

• Write a recursive function to calculate the sum of numbers from 1 to n.

---