Bubble Sort in C with NAN, INFINITY AND -INFINITY

问题

I have been experimenting with the bubble sort code as I have recently started to gain my knowledge of C code. However I am unable to input NAN into the code for it to print out when building and running it. I am having the same problem with INFINITY AND -INFINITY. The code, however, works when I run the code and input NAN, INFINITY AND -INFINITY as one of the integers. Help would be appreciated, thanks.

/* Bubble sort code */

#include <stdio.h>
#include <math.h>

int main()
{
     float array[100], swap;
     int c, d, n;

printf("Enter number of elements\n");
scanf("%d", &n);

    printf("Enter %d integers\n", n);

    for (c = 0; c < n; c++)
        scanf("%f", &array[c]);

    for (c = 0; c < (n - 1); c++)
    {
        for (d = 0; d < n - c - 1; d++)
        {
            if (array[d] > array[d + 1]) /* For decreasing order use < */
            {
                swap = array[d];
                array[d] = array[d + 1];
                array[d + 1] = swap;
             }
        }
     }

    printf("Sorted list in ascending order:\n");

    for (c = 0; c < n; c++)
        printf("%f\n", array[c]);

    return 0;
}

回答1:

The floating point value NAN is unordered with respect to other values.

If a NAN value is an operand to the <, >, <=, >=, or == operators, the result will always evaluate to false. Also, if NAN is an operand of the != operator, the result will always be true. It follows from this that NAN != NAN is true and NAN == NAN is false.

Because of this, you won't get any meaningful results attempting to sort a list of floating point numbers that contains NAN. You need to check for this value using the isnan function and either ignore it or ask the user to enter a different number.

The values -inf and inf however are ordered. You can sort a list containing these values.

Using your existing code, we can see that inf and -inf are handled properly:

Enter number of elements
5
Enter 5 integers
3.5
infinity
2.9
9
-infinity
Sorted list in ascending order:
-inf
2.900000
3.500000
9.000000
inf

But NAN is not:

Enter number of elements
6
Enter 6 integers
8.4
7.5
nan
6.7
3.5
4.4
Sorted list in ascending order:
7.500000
8.400000
nan
3.500000
4.400000
6.700000

来源：https://stackoverflow.com/questions/42578497/bubble-sort-in-c-with-nan-infinity-and-infinity

标签

nan

infinity