Reading text file and sorting two arrays

You can declare a structure with x and y, so that the two values are tied together, x and y will keep their relation when array elements are swapped during sort. Example:

struct data_t
{
    double x;
    int y;
};

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->x > n->x;
}

int main(void) 
{
    struct data_t data[MAX];
    FILE* file = fopen("Temp.dat", "r");
    int count = 0;
    while(fscanf(file, "%lf %d", &data[count].x, &data[count].y) == 2)
    {
        count++;
        if (count == MAX)
            break;
    }

    qsort(data, count, sizeof(struct data_t), compare);
    for(int i = 0; i < count; i++)
        printf("%lf %d\n", data[i].x, data[i].y);
    return 0;
}

^{Use %lf for double values, or use %f for float , as shown in the other answers}

About the compare function, lets say you have these values in your data:

0.060493 1
0.060493 5
0.060493 2

In this case the x value from first element is same as x value from the other element. If you only test for m->x > n->x then sorting does not occur. So you want to compare m->y > n->y if x is the same.

qsort is a standard C function. It doesn't know your data type, we have to have to tell it about the data type. That's done inside the compare function which receives pointers a and b, we know these are pointers to elements in data array, so that's reason behind the conversion.

The data set can contain up to 200 items since we declared it as struct data_t data[MAX]; where MAX is 200.

double arr[MAX + 1][8]

Note that the first index of the array is rows from 0 to MAX. It's set up this way so it can be sorted later. Now we can read the file directly in to the array, and sort the array (no need for structure). Make sure you don't mix this up with the old solutions which had arr[8][MAX + 1]

int compare_2d_array(const void *pa, const void *pb)
{
    double a = *(double*)pa;
    double b = *(double*)pb;
    return a > b;
}

int main(void)
{
    //array with MAX + 1 rows, and 8 columns, initialized to zero
    double arr[MAX + 1][8] = { 0 };
    FILE* file = fopen("temp.dat", "r");
    int count = 0;
    while(fscanf(file, "%lf %lf", &arr[count][0], &arr[count][1]) == 2)
    {
        count++;
        if(count == MAX) break;
    }

    qsort(arr, count, sizeof(arr[0]), compare_2d_array);

    //arr[0] and arr[1] are ready, now set up the other columns:
    for(int i = 0; i < count; i++)
    {
        //make modifications to other columns
        arr[i][2] = i ? arr[i - 1][0] : 0;
        arr[i][3] = arr[i][0];
        arr[i + 1][4] = i + 1;

        printf("%.6lf %.0lf %.6lf %.6lf %.0lf\n", 
                    arr[i][0], arr[i][1], arr[i][2], arr[i][3], arr[i][4]);
    }
    return 0;
}