Recursive Number Sequence

Question

I want to make a program to output this number sequence: (2),(5),(11),(23),...
where xⁱ = 2*x^i-1 + 1, and x⁰=2.

Here's my code:

    public static int num(int n){
        if(n <= 0)
            return 2;
        else
            return ((2 * 2)+1);

    }

I'm having trouble finding a way to output the numbers 11, 23 and onwards. Would it work if I set a counter variable and continuously loop around the second return statement?

check what you are returning, you should use n there so return ((2 * 2) +1); should be return ((2 * num(n-1))+1); — Anand Shah
– Anand Shah, Commented Mar 26, 2015 at 18:20

Mshnik · Accepted Answer · 2015-03-26 18:19:30Z

2

Well seeing as you want it to be recursive, let's make it recursive!

public static int num(int n){
    if(n <= 0)
        return 2;
    else
        return (2 * num(n-1))+1; //Recursive call here
}

With a quick runnable method to check it:

public static void main(String[] args){
    for(int i = 0; i < 10; i++){
        System.out.println("num(" + i + ")=" + num(i));
    }
}

Output:

num(0)=2
num(1)=5
num(2)=11
num(3)=23
num(4)=47
num(5)=95
num(6)=191
num(7)=383
num(8)=767
num(9)=1535

answered Mar 26, 2015 at 18:19

Mshnik

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