Problems with casting and sqrt() in C

Question

This program finds the distance between two points on a plane capable of floating points. But what I can't figure out (which might be due to my ignorance) is the problem with computing the dist variable. It is set to a Pythagorean equation that returns what should be the distance. All other variables in the system are correct.

What I am mostly having a problem though is an obvious bug with the sqrt() in it's use in this example. For example, if I were to make xdist = 1 with everything else set to 0, the distance should be 1. However, what is printed as dist is exactly 1072693248. I only found one question related to this number. But what was mentioned in the answers was that only the wrong use of %d in printf caused that issue, but I see (to what I know) there isn't a problem with printf and formatting anywhere in the program. What is the solution to get the correct value?

double distance(xdist, ydist)
{
    return sqrtf(pow(xdist, 2.0) + pow(ydist, 2.0));
}

int main()
{
    double x1, y1, x2, y2;

    double xdist, ydist, dist;

    int i;

    for (i = 1; i <= 2; i++)
    {
            printf("%d: Enter an x and y coordinate:\n", i);
            switch (i)
            {
                case 1:
                    scanf("%lf", &x1);
                    scanf("%lf", &y1);
                    continue;
                case 2:
                    scanf("%lf", &x2);
                    scanf("%lf", &y2);
                    continue;
            }
    }

    xdist = fabs(x2 - x1);
    ydist = fabs(y2 - y1);

    dist = distance(xdist, ydist);

    printf("\nxdist = %.2f, ydist = %.2f\n", xdist, ydist);
    printf("The distance between coordinates \n(%.2f, %.2f) to (%.2f, %.2f) \nis %.2f.", x1, y1, x2, y2, dist);

    return 0;
}

`double distance(xdist, ydist)` ... call me ignorant about C programming, but don't you need to declare the types of `xdist` and `ydist`? — Tim Biegeleisen, Dec 15 '16 at 12:32
Also, you don't need to take the absolute value of the difference between the x and y ordinates, as squaring loses the sign anyway (the codomain of y=x^2 where x is real rather than complex, is y >= 0). — Evil Dog Pie, Dec 15 '16 at 12:37
1) makes more sense to use `sqrt()` than `sqrtf()` in `double distance(xdist, ydist) { return sqrtf(pow(xdist, 2.0) + pow(ydist, 2.0));}` 2) Simplify `double distance(double xdist, double ydist) { return hypot(xdist, ydist); }` — chux - Reinstate Monica, Dec 15 '16 at 16:02
I used a lot of the suggestions with this question and it's combinations but I still can't find it to work. I think it might have something to do with my compiler. I tried it at school today and it worked flawlessly (the only thing changed was the distance() arguments set to their intended data type). — Tristen Woodruff, Dec 15 '16 at 21:48

score 4 · Answer 1 · answered Dec 15 '16 at 12:32

4

Type of arguments are not defined in distance function so it will be default int. you have to define their type double

 double distance(double xdist, double ydist)

answered Dec 15 '16 at 12:32

e.jahandar

1,715
12
30

score 0 · Answer 2 · answered Dec 15 '16 at 21:56

I found my solution. Problems with the scanf() caused some weird printing of floats I think. I had switched "%lf" to "%f" and then to "%lf" again. But within that I don't know what fixed it exactly. The final code is:

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

double distance(double xdist, double ydist)
{
    return hypot(xdist, ydist);
}

int main()
{
    double x1, y1, x2, y2;

    double xdist, ydist, dist;

    int i;

    for (i = 1; i <= 2; i++)
    {
            printf("%d: Enter an x and y coordinate:\n", i);
            switch (i)
            {
                case 1:
                    scanf("%lf", &x1);
                    scanf("%lf", &y1);
                    continue;
                case 2:
                    scanf("%lf", &x2);
                    scanf("%lf", &y2);
                    continue;
            }
    }

    xdist = x2 - x1;
    ydist = y2 - y1;

    dist = distance(xdist, ydist);

    printf("\nxdist = %.2f, ydist = %.2f\n", xdist, ydist);
    printf("The distance between coordinates \n(%.2f, %.2f) to (%.2f, %.2f) \nis %.2f.", x1, y1, x2, y2, dist);

    return 0;
}

But if I were to guess, what fixed this was simplified use of hypot(), removal of fabs() to find the ydist and xdist, and the distance() function taking double parameters.

Problems with casting and sqrt() in C

2 Answers2