C Max Numbers In An Array Algorithm

Question

So, I'm just working on C code, particularly a function which accepts 3 arguments: an array, the size of the array, and the number of max elements you want returned.

Here's my code:

int* findMaxElements(int base_array[],int size_of_base_array, int number_of_elements_to_find);

int main( void )
{

    printf("Find Max Values in an Array\n\n");

    // Set up array

    int kinch[6] = {1,2,3,4,5,6};

    // Pass to function and get a pointer to new array filled with only the max elements

    int *given = findMaxElements(kinch,6,3);

    for(int i = 0; i < 3; i++)
    {
        printf("\nMax Value = %d\n", *(given + i));
    }
    return 0;

}

int* findMaxElements(int base_array[],int size_of_base_array, int number_of_elements_to_find)
{

    // Set up all initial variables

    int i,k,c,position;
    int maximum = 0;



    int returnArray[100];

    /*Actual Algorythm */

    for(i = 0; i < number_of_elements_to_find; i++)
    {

        // Get the max value in the base array

        for(k = 0; k < size_of_base_array; k++)
        {
            if(base_array[k] > maximum)
            {
                maximum = base_array[k];
            }
        }

        // Find the position of the max value

        for(position = 0; position < size_of_base_array; position++)
        {

            if(base_array[position] == maximum)
            {
                break;
            }

        }

        // Delete the maximum value from the array and shift everything

        for(c = position - 1; c < size_of_base_array - 1; c++)
        {
            base_array[c] = base_array[c+1];
        }

        // Reduce the size of the array

        size_of_base_array -= 1;

        // Push max value into return array

        returnArray[i] = maximum;

        // Reset max value

        maximum = 0;
    }

    return returnArray;

}

I have a feeling somewhere in the function something goes wrong.

// Set up array

    int kinch[6] = {1,2,3,4,5,6};

    // Pass to function and get a pointer to new array filled with only the max elements

    int *given = findMaxElements(kinch,6,3);

    for(int i = 0; i < 3; i++)
    {
        printf("\nMax Value = %d\n", *(given + i));
    }

This should output the numbers 6, 5, and 4, because they are the three largest in the array, however the output I get is always 6, 6, and 6. What's wrong with it?

Answer 1

This may not be your only problem, but in the lines

for(c = position - 1; c < size_of_base_array - 1; c++)
    {
        base_array[c] = base_array[c+1];
    }

You copy the element at [c+1] (which is the maximum) to [c] - so you keep finding the max...

You should start the loop with c = position, not c = position - 1.

And add keyword static in front of the array you use to store the return values, so they remain valid (this is one way to address the issue that Jonathan Leffler identified).

Answer 2

One problem is that you are returning a pointer to a local variable, returnArray, in the function. You can't do that reliably — it leads to undefined behaviour.

There may well be other problems too, but that's enough to be a show-stopper on its own.

Answer 3

The whole approach to find the Kth largest element is not efficient and elegant. I will suggest you to modify your algorithm, although with above suggestions it will work fine, but it's not good way to solve this problem.

I will suggest you to look into below link to modify your algorithm http://www.geeksforgeeks.org/k-largestor-smallest-elements-in-an-array/