Recursion

[Will Myers]

I was wondering if anyone had a good simple explanation of how to work out what the result is from a recursive method? For example the following produces 24 as the result when you pass in 4... (1 * 2 * 3 * 4 = 24)

public static long factor(long arg){
		if(arg == 1 || arg == 0){
			return 1;
		}
		return arg * factor(arg-1);
		
	}

but how would you break it down if presented with this sort of question in an interview?

Another example would be

public static int recursion(int a, int b){
		if(a == 1) return 1;
		if(b == 1) return 1;
		
		return (a + b) * recursion(a-1, b-2);
	}

which will return 28 if you pass in 3 and 4 as the args but again how would you work this out manually? Is there a standard formula for breaking these down?

[fred rosenberger]

paper and pencil is what I would do. if you pass in 3 and 4, it would return

(3 + 4) * recursion(2,2)

that would return

(3+4) * ( (2+2)* (recursion(1,0) )

that would return

(3 + 4) * ( (2+2) * 1)

[Ulf Dittmer]

Either that, or just follow the link.

[Will Myers]

thanks Fred!

Nice work Ulf