Convert a list of points to a polyline in Python.

2019-01-14

Let’s say you have a collection of four points like this:

points

To draw lines between the points, you need to convert the four points to three lines like this:

points

Expressed in numbers, you convert these points:

(0, 0.5)
(1, 1)
(2, 0)
(3, .5)

to these lines:

(0, 0.5) -> (1, 1)
(1, 1) -> (2, 0)
(2, 0) -> (3, 0.5)

Here is a recursive Python function that converts the list of points to a list of lines.

Create lines from points function

def create_lines_from_points(points, lines = []):
    if len(points) == 0:
        # no more points, return collected lines
        return lines
    else:
        if len(lines) == 0:
            if len(points) == 1:
                raise ValueError('Cannot create line from one point')
            # starting situation, return create_lines_from_points with args:
            # 1: remaining points after first two points
            # 2: line created from first two points
            return create_lines_from_points(points[2:], [(points[0], points[1])])
        else:
            # mid-list situation, return create_lines_from_points with args:
            # 1: remaining points
            # 2: collected lines plus a new line based on one point from
            #    the last line and a point from the list
            return create_lines_from_points(points[1:], lines + [(lines[-1][1], points[0])])

Usage:

points = [(0, 0.5), (1, 1), (2, 0), (3, 0.5)]
print(create_lines_from_points(points))

Output:

[((0, 0.5), (1, 1)), ((1, 1), (2, 0)), ((2, 0), (3, 0.5))]
[Finished in 0.031s]