array sum recursion function doesn't work

Question

hello everyone I'm puzzled why this function returns 0 every time. I use arr[0] as index and define it size of array in main func.

int sum_array (int arr[]) {
    int result;
    arr[0] = arr[0]-1;
    if(arr[0]<=0){
        return 0;
    }
    result = ((sum_array(arr))+(arr[arr[0]]));
    return result;


}

If I use if(arr[0]<=1) instead of if(arr[0]<=0) it returns 5. I also don't get why it returns 5.

array = {0,1,1,2,3,3,4}

The line result=((sum_array(arr))+(arr[arr[0]])); exhibits undefined behaviour because the result is dependant on the evaulation order of + as the left side modifies arr[0] and the right side reads the same value. Your program is ill-formed. Therefore the compiler is under no obligation to output anything sensible. — bitmask
– bitmask, Commented Apr 26, 2020 at 23:43
... also, the line mentioned above is practically unreachable with your array input because the if statement will be true in the first call and any subsequent call as well. — bitmask
– bitmask, Commented Apr 26, 2020 at 23:50
of course, if you don't post the code in main.... or you don' t say what do you pass as the arrary parameter, it's impossible to guess what is even the purpose of your routine. Please, have a look at this page — Luis Colorado
– Luis Colorado, Commented Apr 29, 2020 at 13:37

user3629249 · Accepted Answer · 2020-04-27 15:05:40Z

1

the following proposed code:

uses recursion
performs the desired functionality

and now, the proposed code:

int sum_array (int arr[]) 
{
    if(arr[0]<=0)
    {
        return 0;
    }
    else
    {
        int index = arr[0];
        arr[0]--;
        return arr[ index ] + sum_array( arr );
    }
}

edited Apr 27, 2020 at 15:05

answered Apr 27, 2020 at 14:45

user3629249

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