Question

I am writing a recursive program which calculates the Ackermann function.

Here is the code:

public class Ackermann{

    public static long ackermann( long m,long n) {
        return
            (m==0)? n+1:
            (m>0 && n==0)? ackermann(m-1,1):
            (m>0 && n>0)? ackermann(m-1, ackermann(m,n-1));
    }


    public static void main(String[]args) {
        long m=4;
        long n=2;
        System.out.println(ackermann(m,n));
    }
}

But it shows me mistakes:

Ackermann.java:7: : expected
   (m>0 && n>0)? ackermann(m-1, ackermann(m,n-1));
                                                 ^
Ackermann.java:7: ';' expected
   (m>0 && n>0)? ackermann(m-1, ackermann(m,n-1));
                                                  ^
Ackermann.java:18: illegal start of expression
public static void main(String[]args){
^
Ackermann.java:18: ';' expected
public static void main(String[]args){
      ^
Ackermann.java:18: illegal start of expression
public static void main(String[]args){
              ^
Ackermann.java:18: ';' expected
public static void main(String[]args){
                       ^
Ackermann.java:18: ';' expected
public static void main(String[]args){
                                    ^
Ackermann.java:26: reached end of file while parsing
}
 ^
8 errors

How can this be fixed?

Answer 1

Simply use

public static long ackermann(long m, long n) {
  return (m==0)?
           n+1:
           (m>0 && n==0)?
             ackermann(m-1,1):
             ackermann(m-1, ackermann(m,n-1)); 
}

Your last ternary operator is useless, and wasn’t even complete (missing the else part).