Error with netlogo gis extension vertex-lists-of - gis

Question

I want to extract points from line feature from shapefile.
I have search for solutions, it shows that most of people use gis:vertex-lists-of to do this.
For example,
However I got an error message below when I try to do that:
Extension exception: not a VectorFeature:
org.myworldgis.netlogo.VectorDataset#ca24265
error while observer running GIS:VERTEX-LISTS-OF
called by procedure DRAW-CITIES
called by Button 'draw-cities'
Code below:
to draw-cities
gis:set-drawing-color red
gis:draw cities-dataset 1
foreach gis:vertex-lists-of cities-dataset [
a ->
]
end
cities-dataset is a points collection shapefile from Netlogo library ""GIS general examples" model
I don't understand that even I put a points feature in the function, I just got an error.
Do I misuse the function? How can I corectly use it? Thank you.

Answer 1

The reason for the error is because your foreach function is missing the action steps a foreach function looks something like this:
foreach [1 2 3][a -> print a + 1]
If you wanted to do something like the code you linked to then it would look like this:
extensions [ gis ]
globals [ cities-dataset]
breed [ cities city ]
breed [nodes node]
cities-own [ name country population ]
to setup
set cities-dataset gis:load-dataset "C:/Program Files/NetLogo 6.2.2/app/models/Code Examples/Extensions Examples/gis/data/cities.shp"
draw-cities
end
to draw-cities
gis:set-drawing-color red
gis:draw cities-dataset 1
foreach gis:feature-list-of cities-dataset[
i ->
foreach gis:vertex-lists-of i[
j ->
let first-node-point nobody
let previous-node-point nobody
foreach j [
k ->
let location gis:location-of k
if not empty? location [
ifelse any? nodes with [
xcor = item 0 location and ycor = item 1 location
]
[]
[
create-nodes 1[
set xcor item 0 location
set ycor item 1 location
set size 0.23
set shape "circle"
set color 23
set hidden? false
]
]
;to create links
let node-here (nodes with [
xcor = item 0 location and ycor = item 1 location
])
ifelse previous-node-point = nobody
[set first-node-point node-here]
[let who-node 0
let who-prev 0
ask node-here
[create-link-with previous-node-point
set who-node who]
ask previous-node-point[
set who-prev who
]
set previous-node-point one-of node-here
]
]
]
]
]
end
I'm not entirely sure what you're looking to do, but feel free to ask for more help.