CGAL users discussion list

[Panagiotis Cheilaris <>]

On Sat 2014-01-18 at 23:25:42 +0100, Alessandro Attanasi wrote:
> Hi,
> I'm not understanding the behaviour of the graham_andrew_scan()
> what should I expect if I have the following points?
> 
>                q7
>                           q4
> q1     q2      q3
>                           q5
>                q6
> 
>  q1  (-1,0)
>  q2  (0,0)
>  q3  (1,0)
>  q4  (2,1)
>  q5  (2,-1)
>  q6  (1.5,-2)
>  q7  (1.5,2)
> 
> I filled a std::vector<Point_2> scan_test with this points in the order
> given by the numbers above, then I called
> 
> CGAL::graham_andrew_scan(scan_test.begin(),scan_test.end(),std::back_inserter(scan_result));

Hi Alessandro,

First of all, I think there is no graham_andrew_scan but only
ch_graham_andrew_scan. Please check this. As a general remark,
whenever you have a problem, you could send a minimal example
of code that compiles and when run gives your claimed output.

> being std::vector<Point_2> scan_result.
> 
> And as result I obtained
> 
> q1,q3,q4 into scan_result. Why?

I think ch_graham_andrew_scan requires the input vector of
points to be somehow sorted and your input is not.
I think you should sort the points with Less_xy_2.

What did you expect to get?

Best,
Panos