Identifying direct neighbours connected by network in QGIS

Question

I have a point layer of settlements and a LineString layer of a road network. The settlements have been snapped to the network so they are directly connected by it. Settlements are not always at the start or end of the line - they may be halfway along it etc. I would like to output a list of (or otherwise identify) which settlements are directly connected to each other by road.

So for example, in the above image we would identify that point A was connected to B and C, but not D. The desired result in full would be the table below.

Point	Connected to
A	B, C
B	A, C, D
C	A, B, D
D	B, C

Answer 1

You can use QGIS expressions and check for each point which other points are on the same line. For this to work, you have first to make sure that each network line connects only two points: see the screenshot where each separate line has another color and below for how to achieve that.

The label (2nd line) is based on the expression below and automatically generates the name of the neighboring points:

To prepare this, proceed as follows:

1. Create a new (empty) multipart line layer, copy and paste all lines from the network there.

2. Select and merge all features so that the whole network consists of one single multipart feature.

3. Split the lines with the points. See here how to do so.

Now you're ready to use the expression on the point layer (replace network in line 13 with the name of the layer created in step 3; and replace name at the end of line 19 and the 4th last line with the name of the attribute field that contains the name of the point):

array_to_string (
    array_distinct(
        array_filter (
            string_to_array(
                array_to_string (
                    array_foreach (
                        generate_series (1, num_geometries(collect($geometry))),
                        with_variable(
                            'iteration',
                            @element,
                            array_to_string (
                                array_foreach(
                                    overlay_intersects ('Split',$geometry),  -- change layer name here
                                    if (
                                        intersects (
                                            buffer (@element,0.1),
                                            geometry(get_feature_by_id(@layer,@iteration))
                                        ),
                                        attribute (get_feature_by_id (@layer,@iteration),'name'), -- change fieldname here
                                        'delete'
                                    )
                                )
                            )
                        )
                    )
                )
            ),
            @element <>'delete' and @element <> name  -- change fieldname here
        )
    )
)

Answer 2

If you are open to a Python script:

from itertools import product
from collections import defaultdict
from qgis.core import QgsProject, QgsFeatureRequest
point_layer = QgsProject.instance().mapLayersByName("point_layer")[0]
line_layer = QgsProject.instance().mapLayersByName("line_layer")[0]
dict_result = defaultdict(list)
for line in line_layer.getFeatures():
    request = QgsFeatureRequest()
    request.setDistanceWithin(line.geometry(), 0.001)
    points = list(point_layer.getFeatures(request))
    pt_points = [point.geometry().asPoint() for point in points]
    start = None
    end = None
for pt in line.geometry().asPolyline():
    try:
        index = pt_points.index(pt)
    except ValueError:
        continue
    if start is None:
        start = points[index]
        continue
    if end is None:
        end = points[index]
        dict_result[start["id_field"]].append(end["id_field"])
        dict_result[end["id_field"]].append(start["id_field"])
        start = end
        end = None


for point_key, point_values in dict_result.items():
    print(point_key, f"[{', '.join(point_values)}]")

result:

A [B]
B [A, C]
C [B, E, D]
E [C, F]
D [C]
F [E, G]
G [F]