Finding the minimum of an array using recursion?

Question

Ok, so I've been trying to wrap my head around recursion in Java and I can accomplish easy tasks such as sum, reversing etc. but I have been struggling to do this exercise:

I'm trying to find the minimum number in an array using recursion but keep getting an answer of 0.0.

My understanding for recursion is that there I need to increment one element and then provide a base case that will end the recursion. I think I'm messing up when I have to return a value and when is best to call the recursion method.

This is what I have so far:

public static double findMin(double[] numbers, int startIndex, int endIndex) {

double min;
int currentIndex = startIndex++;

if (startIndex == endIndex)
    return numbers[startIndex];

else {
    min = numbers[startIndex];
    if (min > numbers[currentIndex]) {
        min = numbers[currentIndex];
        findMin(numbers, currentIndex, endIndex);
    }
            return min;
}       
} //findMin

score 6 · Answer 1 · answered Apr 08 '11 at 23:23

Here's a simplified version:

public static double min(double[] elements, int index) {

  if (index == elements.length - 1) {
    return elements[index];
  }

  double val = min(elements, index + 1);

  if (elements[index] < val)
    return elements[index];
  else
    return val;
}

score 5 · Answer 2 · answered Apr 08 '11 at 22:06

5

Hint: You're calling findMin recursively, but then not using its return value.

What's the relationship between (1) the min of the whole array, (2) the first element, and (3) the min of everything apart from the first element?

answered Apr 08 '11 at 22:06

Gareth McCaughan

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So, I should call the findMin method recursively and assign the return value to...? This is where I'm confused. Do I define the return value before calling the findMin method? – Vance Apr 08 '11 at 22:43

score 4 · Accepted Answer · answered Apr 08 '11 at 22:12

4

There are a variety of problems in this code including:

You don't use the result of the recursive findMin call.
startIndex will be the same for every call to findMin, because currentIndex is being set to the value of startIndex before startIndex is incremented.
If the number at index 1 in the array is <= the number at index 0, you just return that number without even making the recursive call.

answered Apr 08 '11 at 22:12

ColinD

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So, should the return statement occur before I call the recursive findMin method? Also, I'm not understanding how the startIndex is not changing. When I call the recursive findMin method, I'm using the parameters with currentIndex (instead of startIndex), which should then be passed as a new startIndex, correct? I think I'm starting to confuse myself :( – Vance Apr 08 '11 at 22:39
@user699302: The problem is the `currentIndex = startIndex++`. When `startIndex` is 0, `currentIndex` becomes 0 and `startIndex` becomes 1. When you call `findMin`, you pass it `currentIndex`, so in that call `startIndex` will once again be 0, leading to infinite recursion and a stack overflow. – ColinD Apr 09 '11 at 01:27

dfb · Answer 4 · 2011-04-08T22:16:23.073

1

A few observations in addition to the first answer:

int currentIndex = startIndex++; - you're going to miss your first element here. In general, you don't want to modify the input to your recursive function. Work off the input and generate new values when you're ready to call the function again - i.e. 'findMin(numbers, currentIndex+1, endIndex)'

edited Apr 08 '11 at 22:16

answered Apr 08 '11 at 22:10

dfb

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The default value of a `double` doesn't come in to play here, since `min` is always assigned to a value from the array before it's read. Really, it shouldn't be declared until the point where it's assigned though. – ColinD Apr 08 '11 at 22:14

Finding the minimum of an array using recursion?

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