sorting 2-d array based on the first column

Question

I have a temp2.dat file that looks like this:

0.060493 1 0.5 1
1.596961 0 0.1 2
0.87758  1 0.3 1.5
0.165453 1 0   3
0.07085  1 0.3 4
0.125379 1 0.2 3
0.454202 1 0.2 2
0.373227 1 0.3 1
0.131486 1 0.3 3
0.867477 0 0.5 4
0.122609 0 0.8 9

Now I want to write the function in C to sort these 4 columns in an increasing order but only based on the values of the first column. I tried modifying the following code but it failed:

struct data_t
{
    double x;
    int y;
    double z;
    double k;
};

int compare(const void *a, const void *b)
{
    const struct data_t *m = a;
    const struct data_t *n = b;
    if(m->x == n->x)
        return m->y > n->y;
        return m->z > n->z;
        return m->k > n->k;
    return m->x > n->x;
}

Could someone please help me do this? The expected result should look like this:

0.060493 1 0.5 1
0.07085  1 0.3 4
0.122609 0 0.8 9
0.125379 1 0.2 3
0.131486 1 0.3 3
................
0.87758  1 0.3 1.5
1.596961 0 0.1 2

Your compare function is bizarre. Please look up some examples of, presumably, qsort, and be wary of testing floating point values for equality. Please post the Minimal, Complete, and Verifiable example that shows the problem. — Weather Vane
– Weather Vane, Commented Mar 8, 2018 at 21:52
return m->k > n->k; and return m->x > n->x; are useless, they will never be reached, you can only return 1 object from a function. And I highly recommend adding some brackets { ... } to your if statement to make it clear with you want, or at least changing the indentation. What you have now is misleading. — yano
– yano, Commented Mar 8, 2018 at 21:54
@yano: thank you so much. I would note this point next time. — user177196
– user177196, Commented Mar 9, 2018 at 1:15

Barmak Shemirani · Accepted Answer · 2018-03-09 00:47:23Z

1

Try this compare function

int compare(const void *a, const void *b)
{
    const struct data_t *m = a;
    const struct data_t *n = b;

    if(m->x != n->x)
        return (m->x > n->x) - (m->x < n->x);

    if(m->y != n->y)
        return m->y - n->y;

    if(m->z != n->z)
        return (m->z > n->z) - (m->z < n->z);

    if(m->k != n->k)
        return (m->k > n->k) - (m->x < n->k);

    return 0;
}

This will compare the first column x. If x is the same in the two elements, it moves to the second column y. If the second column is the same, it moves to the third column and so on.

We want the difference between the two values. Example, m->y - n->y. The compare function should return an integer value 0, negative, or positive.

When comparing double values, we cannot use m->x - n->x, because the return value for compare is int. We use a comparison function instead.

Testing

struct data_t
{
    double x;
    int y;
    double z;
    double k;
};

int compare(const void *a, const void *b)
{
    const struct data_t *m = a;
    const struct data_t *n = b;

    if(m->x != n->x)
        return (m->x > n->x) ? 1 : -1;

    if(m->y != n->y)
        return m->y - n->y;

    if(m->z != n->z)
        return (m->z > n->z) ? 1 : -1;

    if(m->k != n->k)
        return (m->k > n->k) ? 1 : -1;

    return 0;
}

int main(void)
{
    struct data_t data[] = 
    { 
        { 0.060493, 3, 0.4, 7 },//1st column is the same
        { 0.060493, 2, 0.5, 8 },
        { 0.060493, 1, 0.6, 9 },

        { 0.060493, 3, 0.3, 4 },//1st & 2nd columns are the same
        { 0.060493, 3, 0.2, 5 },
        { 0.060493, 3, 0.1, 6 },

        { 0.060493, 1, 0.5, 3 },//1st & 2nd & 3rd columns are the same
        { 0.060493, 1, 0.5, 2 },
        { 0.060493, 1, 0.5, 1 },

        { 0.122609, 0, 0.8, 9 },
        { 0.125379, 1, 0.2, 3 },
        { 0.131486, 1, 0.3, 3 },
    };

    int count = sizeof(data) / sizeof(data[0]);
    qsort(data, count, sizeof(data[0]), compare);

    for(int i = 0; i < count; i++)
    {
        printf("%.6f %d %.1f %.0f\n",
            data[i].x, data[i].y, data[i].z, data[i].k);
    }

    return 0;
}

output:

0.060493 1 0.5 1
0.060493 1 0.5 2
0.060493 1 0.5 3
0.060493 1 0.6 9
0.060493 2 0.5 8
0.060493 3 0.1 6
0.060493 3 0.2 5
0.060493 3 0.3 4
0.060493 3 0.4 7
0.122609 0 0.8 9
0.125379 1 0.2 3
0.131486 1 0.3 3

edited Mar 9, 2018 at 0:47

answered Mar 8, 2018 at 22:37

Barmak Shemirani

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2 Comments

user177196 Over a year ago

Many thanks for your great help! If the first column is not the same, each time your compare() function would automatically shift the elements in the same row but in remaining columns, down 1 row?

Barmak Shemirani Over a year ago

I printed the output, it looks right. It's sorting the 1st column, then 2nd column etc.

sleighty · Accepted Answer · 2018-03-08 21:57:33Z

0

First off, in C if you want more than one statement inside any block, you have to surround it with { and }. So your last 3 return statements would never be reached (the indentation does not matter for the compiler, only for humans).

Second, when you call return, your code does not come back. So you could use nested ifs to compare the values in case they are the same in some of the columns. Something like:

int compare(const void *a, const void *b)
{
    const struct data_t *m = a;
    const struct data_t *n = b;
    if(m->x == n->x) {
        if (m->y == n->y) {
            if (m->z == n->z) {
                return m->k > n->k;
            }
            return m->z > n->z;
        }
        return m->y > n->y;
    }
    return m->x > n->x;
}

answered Mar 8, 2018 at 21:57

sleighty

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4 Comments

Patrick Roberts Over a year ago

I'm assuming the correct logic is to subtract, not to compare

sleighty Over a year ago

Ah, I missed the qsort tag. Just inferred what the compare function did from what OP wrote.

user177196 Over a year ago

@BrunoEly: Thank you so much! Let me try your code to see if it actually does what I want to. Did you try it with the given sample dataset above?

chux Over a year ago

Note: This compare() is sufficient to use with qsort() as that expects negative return values too.

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