Recursive max int in array

Question

I would like a review for this simple recursive algorithm which returns the largest int in an array of integers. I would like feedback on the transfer of state via the third parameter specifically. Please also note 2 things:

It must be recursive
This is not for production usage, but rather recursion practice

#include <stdio.h>
#include <stdlib.h>

int max_val(int arr[], int length, int curr_max)
{
    if(length <= 0)
        return curr_max;

    if(curr_max < *arr)
    {
        curr_max = *arr;
    }
    arr++;
    return max_val(arr,length-1,curr_max);
}

int main()
{
    int arr[] = {2,73,39,5,8,62,9};
    printf("Max: %d\n", max_val(arr,7,0));  
    return EXIT_SUCCESS;
}

Your code wrongfully prints "Max: 0" for arr[] = {-1, -2, -3} — Martin R
– Martin R, Commented Sep 25, 2017 at 5:39

200_success · Accepted Answer · 2017-09-25 05:57:06Z

Never omit "optional" braces, as you did for your first if statement. That's a bad habit that will eventually lead to a coding accident. If you feel compelled to omit braces, then at least put the whole statement on the same line.

You should avoid unnecessary mutation, as you did with arr++. This is especially important with an exercise in recursion: it is one of the principles of functional-programming.

The curr_max parameter is annoying and unnecessary. It also leads to a bug with negative array members, when a poorly selected initial value is chosen. (You should use INT_MIN from <limits.h> instead of 0.) Furthermore, your base case is questionable: is the maximum of a zero-length array well defined?

Suggested solution

#include <assert.h>

int max_val(int arr[], size_t length)
{
    assert(length > 0);
    if (length == 1) return arr[0];
    int max_tail = max_val(arr + 1, length - 1);
    return arr[0] > max_tail ? arr[0] : max_tail;
}

if (length == 1) return arr[0]; and Never omit "optional" braces do not play well together. — vnp
– vnp, Commented Sep 25, 2017 at 16:43
@vnp I also said that it’s acceptable if you put it all on the same line. — 200_success
– 200_success, Commented Sep 25, 2017 at 16:44

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