Finding Max of an array using Recursion

Question

So it's pretty simple to find the max of an array using a for loop or a while loop, but I wanted to try it out with recursion. For some reason, the substring doesn't work - it says "cannot find symbol". Why is this? My strategy is continue subdividing and comparing the two sides until there is only one left which should be the max....am I doing it right? Thanks

 public static int max(int[] array) {
 if (array.length == 1) {
 return array[0];
 } else {
 int mid = (array.length) / 2;
 int leftmax = max(array.substring(0, mid));
 int rightmax = max(array.substring(mid, array.length));
 if (leftmax > rightmax) {
 return leftmax;
 } else {
 return rightmax;
 }
 }
}

• Daniel S.: what programming language is this? Java? JavaScript?
• user2591612: Why recursion? You might blow your stack.
• Freedom: I need to practice my recursion :o
• libik: If you wait few minutes, I will add answer with explained recursion which works. I hope you will learn from it, right now, you are not using recursion at all :).
• clcto: @libik why do you say he is not using recursion? he is calling max() from within max(). That is recursion.

Answer 1

You are going to want to use Arrays.copyOfRange. Substring isn't going to work on an array.

int[] firstHalf = Arrays.copyOfRange(original, 0, original.length/2);
int[] secondHalf = Arrays.copyOfRange(original, original.length/2, original.length);

I can't comment on your algorithm.

Answer 2

Since array is of type int[] not String, you cannot use substring(). Instead, keep track of which indexes you are searching through. Copying the array each iteration is a waste of both space and time.

int max( int[] array ){ return max( array, 0, array.length - 1 ); }
int max( int[] array, int low, int high )
{
 if (low == high) {
 return array[low];
 } 
 else {
 int mid = (high + low) / 2;
 int leftmax = max(array, low, mid );
 int rightmax = max(array, mid, high );
 if (leftmax > rightmax) {
 return leftmax;
 } 
 else {
 return rightmax;
 }
 }
}