first_point

Geometric functions first_point

The first_point function extracts the first point from each sequence (arc, polygon, or linestring).

syntax

first_point(sequences: E->Sequence<Point>) -> E->Point

definition

Returns the first point of each sequence geometry. For:

• Arcs/linestrings: The starting point of the line
• Polygons: The first vertex (which equals the last vertex for closed rings)
• Empty sequences: Returns undefined (null)

This is the counterpart to last_point.

arguments

argument	description	type
sequences	Attribute containing sequence geometries	E->Sequence

performance

Time complexity: O(n) where n is the number of sequences (constant time per sequence).

Memory efficient as it only accesses the first element of each sequence without loading entire geometries.

conditions

• Input must be a sequence type (arc, polygon, linestring)
• Result is undefined for empty sequences

example

unit<uint32> Roads: nrofrows = 100;
attribute<FPoint> geometry (poly, Roads);  // road linestrings

// Get start and end points of each road
attribute<FPoint> start_point (Roads) := first_point(geometry);
attribute<FPoint> end_point (Roads) := last_point(geometry);

// Check if roads form closed loops
attribute<Bool> is_closed (Roads) := start_point = end_point;

see also

• last_point - extracts last point
• sequence2points - extracts all points from sequences
• centroid - center of mass of geometry
• Geometric functions
• Sequence functions

since version

5.0