What is recursion?

Recursion

• Recursion is when a method calls itself until some terminating condition is met, known as the base case.
• Recursion can be used in situations where the next result of an operation depends on the current result.
• Some good examples including the Fibonacci sequence or a factorial function
• Each step until the base case is reached is called a recursive step.
• All recursive functions can be written iteratively (without calling itself)

​

What if we don’t

have a base case?

Stack overflow

occurs. WHY?

Recursion Examples

Write a Java program that sums the numbers from 1 to 10:

Iterative method

?

Recursive method

?

Write a Java program that sums the numbers from 1 to 10:

Iterative method

Recursive method

?

public class SumNumbers {

public static void main(String[] args) {

int sum = 0;

for (int i = 1; i <= 10; i++) {

sum += i; // Add each number to sum

}

System.out.println("The sum of numbers from 1 to 10 is: " + sum);

}

}

​

Write a Java program that sums the numbers from 1 to 10:

Iterative method

public class SumNumbers {

public static void main(String[] args) {

int sum = 0;

for (int i = 1; i <= 10; i++) {

sum += i; // Add each number to sum

}

System.out.println("The sum of numbers from 1 to 10 is: " + sum);

}

}

​

Recursive method

int n = 10; // Define the upper limit

int result = sum(n);

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

}

// Recursive method to calculate the sum

public static int sum(int n) {

if (n <= 0) {

return 0; // Base case: if n is 0 or less, the sum is 0

} else {

return n + sum(n - 1); // Recursive case: sum of n and sum of numbers from 1 to n-1

}

}

}

• 13

Write a Java program that calculates the factorial of 5.

Iterative method

?

Recursive method

?

Write a Java program that calculates the factorial of 5.

Iterative method

Recursive method

public class FactorialCalculation {

public static void main(String[] args) {

int number = 5; // Number to calculate the factorial of

int factorial = 1;

for (int i = 1; i <= number; i++) {

factorial *= i; }

System.out.println("The factorial of " + number + " is: " + factorial);

}

}

​

?

int number = 5; // Number to calculate the factorial of

int factorial = 1;

for (int i = 1; i <= number; i++) {

factorial *= i; }

System.out.println("The factorial of " + number + " is: " + factorial);

}

​

​

Write a Java program that calculates the factorial of 5.

Iterative method

Recursive method

int number = 5; // Number to calculate the factorial of

int factorial = 1;

for (int i = 1; i <= number; i++) {

factorial *= i; }

System.out.println("The factorial of " + number + " is: " + factorial);

}

​

​

int number = 5; // Number to calculate the factorial of

int result = factorial(number);

System.out.println("The factorial of " + number + " is: " + result);

}

public static int factorial(int n) {

if (n == 0) {

return 1; // Base case: factorial of 0 is 1

} else {

return n * factorial(n - 1); // Recursive case: n * factorial of n-1

}

}

Course website

• Please review the resources and watch the videos provided in the course website.