Some comments on the Geometry page.
I see now why I was unable to use the area function correctly. I first cut-and-pasted the definition
area=:[: +/ (-&{: -:@* +&{.)~ AtnpWhen I noticed the reference to another definition Atnp, I then pasted that into my session.
So, my attempt to use area gave me the following error:
area 1 1 _1 _1*-:%:2 0,0 2,2 0,:0 2 |domain error: Atnp | area 1 1 _1 _1*-:%:2 0,0 2,2 0,:0 2
Once I defined area again, having previously defined Atnp, it worked fine.
area=:[: +/ (-&{: -:@* +&{.)~ Atnp
area 1 1 _1 _1*-:%:2 0,0 2,2 0,:0 2
1I also defined my own version of this verb, using essentially the same method (Surveyor's Formula) but with an equivalent to Atnp incorporated directly into it.
Signed area that works on 2-column point array:
areapts=: [:([: -:[: +/(0{]) * [:(1&|.-_1&|.) 1{]) |:
areapts 2 0,2 4,0 4,:0 0 NB. 2x4 square
8