I'm supposed to implement a function that finds the middle number from three numbers given as input. All inputs are positive integers. I seem to not be treating a special case which is causing my program to fail some tests, but with a VERY small margin. I mean 1.46 standard deviation for 8bit pixel values, so the cases I'm missing ought to be very specific and rather few in between.

As I'm understanding this problem, there's three possibilities for the numbers:

1. They are all equal, so return either
2. Two of them are equal, thus return the smaller one (larger one doesn't work either)
3. They're all different, thus find the middle value

There either is a fourth case I'm not seeing or I've missed something for the above three. I can't use libraries for sorting.

int middle(int a, int b, int c)
{
    if (a == b && b == c)
    {
        return c;
    }

    if ((a < b && b < c) || (c < b && b < a))
    {
        return b;
    }

    if ((b < a && a < c) || (c < a && a < c))
    {
        return a;
    }

    if ((a < c && c < b) || (b < c && c < a))
    {
        return c;
    }

    if (a == b || b == c || a == c)
    {
        if (a == b && a < c)
        {
            return c;
        }
        else
        {
            return a;
        }

        if (b == c && b < a)
        {
            return a;
        }
        else
        {
            return b;
        }

        if (a == c && a < b)
        {
            return b;
        }
        else
        {
            return c;
        }

    }

return c; // code sometimes reaches this point; it shouldn't
}

CodePudding user response：

This case here:

if ((b < a && a < c) || (c < a && a < c))

is incorrect. It should be:

if ((b < a && a < c) || (c < a && a < b))

Further, in the

if (a == b || b == c || a == c)

Section, you have

if (...)
    return ...
else
    return ...

This will always return, and ignore the if statements following it.

That said, it would be a lot easier to sort the 3 values, then return b

if ( b < a ) {
    int t = a;
    a = b;
    b = t;
}
if ( c < b ) {
    int t = c;
    c = b;
    b = t;
}
if ( b < a ) {
    int t = a;
    a = b;
    b = t;
}
return b;

CodePudding user response：

In the interest of promoting simplicity when it is available, with only three values to compare, it is completely reasonable to use a single expression to determine and return the middle value. The following uses nested ternary operators to do this. It is tested with the inputs shown in the code:

int main(int argc, char *argv[])
{
//  int val[] = {7,12,1};
//  int val[] = {0,0,1};
    int val[] = {-1,-4,-2};
//  int val[] = {7,6,2};
//  int val[] = {100,99,103};
    
    printf ("middle value is: %d\n", return_middle(val[0], val[1], val[2]));
    return 0;
}

int return_middle(int a, int b, int c)
{
    return a>b ? ( c>a ? a : (b>c ? b : c)) : (c>b ? b : ( a>c ? a : c));
}