How does recursion work? [closed]

Solution 1:

As been pointed out, recursion is when a function calls itself. Here illustrated using factorial, where factorial(5) mathematically evalutates to values 5 * 4 * 3 * 2 * 1.

public int factorial(int x) {
   if (x == 1) return 1;
   else return x * factorial (x - 1);
}

// Try routine
factorial(3);

Which evaluates as

 factorial(1) = 1                                        = 1
 factorial(2) = 2 * factoral(1)                          = 2 * 1 
 factorial(3) = 3 * (2 * factorial(2) * (factorial(1))   = 3 * 2 * 1
 ...

Solution 2:

Usually, it's a function that calculates one result itself, and calls itself to get the rest of the results.

For example, to get all positive numbers less or equal to 3, the function says, "One result is 3, and the rest of the results are all positive numbers less or equal to 2. Let's call myself with 2 and add that result to the one I calculated."

Of course, recursive functions need to be careful to have an "end condition" or they will never return a result. In the case of this example, the end condition would be when the function is called with 0, it should just return 0.

Solution 3:

Here's a simple example of recursive method: -

public int recur(int count) {
   if (count < 10) {
       return count + recur(count++);
   }
   return count;
}

System.out.println(recur(0));  // Invoke first time

Solution 4:

Recursion is a method which calls itself from within itself.

Example:

public void recur(int x)
    recur(10);
end