Why am I getting incorrect distances when calculating distances between points in a C vector?

Correct Utilization of Iterators in C Vector

Problem Statement:

When calculating distances between points stored in a vector, an incorrect result is obtained, potentially due to improper usage of iterators.

Code Snippet:

<code class="cpp">typedef struct point {
    float x;
    float y;
} point;

float distance(point *p1, point *p2)
{
    return sqrt((p1->x - p2->x)*(p1->x - p2->x) +
                (p1->y - p2->y)*(p1->y - p2->y));
}

int main()
{
    vector<point> po;
    point p1; p1.x = 0; p1.y = 0;
    point p2; p2.x = 1; p2.y = 1;
    po.push_back(p1);
    po.push_back(p2);

    vector<point>::iterator ii;
    vector<point>::iterator jj;
    for (ii = po.begin(); ii != po.end(); ii++)
    {
        for (jj = po.begin(); jj != po.end(); jj++)
        {
            cout << distance(ii, jj) << " ";
        }
    }
    return 0;
}</code>

Incorrect Output:

0
1
-1
0

Solution:

The issue arises from unknowingly calling the std::distance() function, which operates on iterators. To resolve this, prefix standard library types with std:: and change the distance() function to take references instead.

<code class="cpp">float distance(const point& p1, const point& p2)
{
    return sqrt((p1.x - p2.x)*(p1.x - p2.x) +
                (p1.y - p2.y)*(p1.y - p2.y));
}</code>

Then, call the function using dereferenced iterators:

<code class="cpp">distance(*ii, *jj);</code>

Additional Optimization:

Instead of a typedef, use struct point directly in C as it is no longer necessary to define it as a struct in typedef.

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