Sending an array to a function only sends the first element of the array

Question

I have an array of the following struct

typedef struct monom {
    int coefficient; 
    int power;
}MONOM;

I have an array of monoms and I want to send it to a merge-sort function, when I do it, using the following call,

mergeSort(&polynomial, logSize);

only the first monom of the array of monoms is sent to the merge-sort function.

When I debug the function I see the full array on the line before the call to mergeSort but when I continue into mergeSort, only the first element is sent.

These are my mergeSort and merge, I'm completely clueless:

void mergeSort(MONOM** polynomial, int size)
{
    MONOM** res;
    int i;

    if (size < 2)
        return;

    else
    {
        mergeSort(*polynomial, size/2); // merge first half of array
        mergeSort(*polynomial+(size/2),size-(size/2)); // merge second half of array

        // allocate result array
        res = (MONOM**)malloc(size*sizeof(MONOM*));

        // merge both sorted halfs of the array into 'res'
        merge(*polynomial,size/2,*polynomial+(size/2),size-(size/2),res);

        // copy 'res' to 'arr'
        for (i = 0; i < size; i++)
            polynomial[i] = res[i];

        // release unused memory
        free(res);
    }
}

void merge(MONOM** poly1, int n1, MONOM** poly2, int n2, MONOM** res)
{
    int i1 = 0, i2 = 0;
    int resIndex = 0;

    while (i1 < n1 && i2 < n2)
    {
        if (poly1[i1]->power < poly2[i2]->power)
        {
            res[resIndex] = poly2[i2];
            i2++;
        }

        else if (poly1[i1]->power > poly2[i2]->power)
        {
            res[resIndex] = poly1[i1];
            i1++;
        }

        else
        {
            res[resIndex]->power = poly1[i1]->power;
            res[resIndex]->coefficient = poly1[i1]->coefficient + poly2[i2]->coefficient;
            i1++;
            i2++;
        }

        resIndex++;
    }

    // fill 'res' array when one of the arrays is finished
    while (i1 < n1)
    {
        res[resIndex] = poly1[i1];
        i1++;
        resIndex++;
    }

    while (i2 < n2)
    {
        res[resIndex] = poly2[i2];
        i2++;
        resIndex++;
    }
}

The first argument of mergeSort() is of type MONOM **. You're calling it recursively as mergeSort(*polynomial, size/2) - i. e. as its first argument, you pass in a MONOM *. What do you expect? — user529758
– user529758, Commented Jul 18, 2013 at 17:30
By the way, don't roll your own sorting algorithm, qsort() from <stdlib.h> works well. — user529758
– user529758, Commented Jul 18, 2013 at 17:30
@H2CO3 it's not the usual merge-sort, it also combines elements with the same power so I don't think any lib function will work — Quaker
– Quaker, Commented Jul 18, 2013 at 17:42
You might want to look at this stackoverflow.com/questions/6645161/… — nj-ath
– nj-ath, Commented Jul 18, 2013 at 17:50

nj-ath · Accepted Answer · 2013-07-18 17:39:33Z

1

Try this,

mergeSort(polynomial, logSize); (Given polynomial is MONOM* or MONOM[] type)

and also

void mergeSort(MONOM* polynomial, int size)

answered Jul 18, 2013 at 17:39

nj-ath

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9 Comments

Quaker Over a year ago

but I need polynomial to change, I must call it with an & don't I?

nj-ath Over a year ago

Not for an array, for an array by default it will change(without using & operator). Using & operator refers to the first element of the array(hence your problem)

nj-ath Over a year ago

You have to make many changes for your program. Eg, In some places you have dereferenced the pointer twice

user529758 Over a year ago

@TheJoker No, the & operator refers to the entire array, and it yields a pointer-to-array type (of which the value can be the same as the pointer to the first element of the array, which the array decays into).

nj-ath Over a year ago

You are doing that by default. An array name always decays into pointer to the first element of the array.

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