Is it possible to recursively print out a pattern affected by the parameter?-CodePudding

I've been curious about this as I was learning recursion.

I'm aware of recursive print methods like


public static void countdown(int n) {
    if (n == 0) {
        System.out.println("Blastoff!");
        return;
    } else {
        System.out.println(n);
        countdown(n - 1);
    }
}

And ones that create "pyramids" But what about a pattern that's not simplistic or easy to code? For example,

public static void pattern(int n){
code;
}

Output if pattern(2);
 
  
 

Output if pattern(3):

 
  
 
   
 
  
  

Output if pattern(4):

I've been at the drawing board for a decent amount of time trying to figure out how someone would code/approach this. I tried it with the modulo operator, but the issue with it is limitation due to n being any # greater than 0.

I have also tried having cases, but that does not work either due to n being some number.

CodePudding user response：

In Java, if you want to use recursion but the parameters in the method header don't really lend themselves to it, then it's standard to make a helper method whose parameters are more appropriate, and internally call that.

In python, or another language that supports default parameters, you'd just make k's default value fill itself in from n, and expect callers to not provide it.

I've implemented your case as a recursion (it has different paths for if it's increasing, decreasing, or in the middle - and in either direction, it iterates in a range of [2 .. 2n-2] with n as the midpoint). It's kind of clumsy and could certainly be optimized, but you can get the picture - this is an awkward case to handle with recursion, it would be more straightforward with a simple while loop.

public static void pattern(int n) {
    _pattern(n, n);
}

private static void _pattern(int n, int k) {
    // calculate number of characters to print
    int d = n - Math.abs(k - n);
    // base case - do nothing
    if (d < 2) {
        return;
    }
    // recursive flow
    // note that, for the initial run, _both_ if statements activate
    if (k <= n) {
        _pattern(n, k - 1);
        System.out.println(" ");
    }
    System.out.println(" ".repeat(d));
    if (k >= n) {
        System.out.println(" ");
        _pattern(n, k   1);
    }
}