How to retrive pair of points stored in list

Question

I am working in QT Creator. I want to find slope of two points. For Two points A(x1,y1) , B(x2,y2) using the slope formula

m = (y2-y1)/(x2-x1).

Problem :

Points are stored in list p. I want to take two points at a time from this list and find the slope of two points till the end of list.

For Example if the list contains 5 points {a,b,c,d,e}. I want to find slope of 1. ab 2. bc 3. cd 4. de

Code:

    QList< QgsPoint > p;
    {
    /* some Process */ 

     p.push_front( path->vertex( e.inVertex() ).point() ); /* some points are added to list*/
    }

    QList< QgsPoint>::iterator it;   
    for ( it = p.begin(); it != p.end(); ++it )
    {
    mrbPath->addPoint( *it );
    }

Answer 1

You can do it without iterators. May be you'll find this code more clean:

for(int i = 0; i <= list.count() - 2; i++) {
  QgsPoint a = list[i], b = list[i + 1];
  //...
}

Answer 2

You could use a java-style iterator:

QListIterator<QgsPoint> i(p);
while(i.hasNext())
{
    if(!i.hasPrevious())
    {
        i.next();
        continue;
    }
    m = (i.peekNext().y() - i.peekPrevious().y()) / (i.peekNext().x()-i.peekPrevious().x());
    i.next();
}

Answer 3

@itwasntpete's comment is the right answer. But you can do it with one iterator if you want:

for( QList< QgsPoint >::iterator it = p.empty() ? p.begin() - 1 : p.begin(); it + 1 != p.end(); ++it )
{
    const float slope = ( it[1].y() - it->y() ) / ( it[1].x() - it->x() );
    //Do something with slope here
}