find length between each point from array python

Question

suppose we have 8 point whose x,y coordinates are given as

[[[224  64]]
 [[ 62 381]]
 [[224 661]]
 [[568 661]]
 [[733 348]]
 [[650 205]]
 [[509 204]]
 [[509  64]]]

Suppose these are 8 points of a polygon and want to find the length of each side. For two points, I am able to find the length as

dx = math.abs(x2 - x1)
dy = math.abs(y2 - y1)
dist = math.sqrt((x2 - x1)**2 + (y2 - y1)**2)

How to find length of each side of polygon with above x, y coordinates?

Um, you just repeat that computation 8 times, presumably in a loop. And that's not an octagon. It's 5 sides of a hexagon. — Tim Roberts
– Tim Roberts, Commented Jun 7, 2021 at 17:38

Mad Physicist · Accepted Answer · 2021-06-07 17:51:01Z

2

Let's say your list is called coords. You have a numpy tag, so I am assuming you want a fast solution using numpy.

coords = [
 [224,  64],
 [ 62, 381],
 [224, 661],
 [568, 661],
 [733, 348],
 [650, 205],
 [509, 204],
 [509,  64]]

You will want to call np.diff on the successive elements, so to get the last side, you will want to replicate the first point at the end. You can do this as the same time as you convert your array to numpy:

vertices = np.concatenate((coords, coords[:1]), axis=0)

Now find the lengths of the sides:

sides = np.linalg.norm(np.diff(vertices, axis=0), axis=-1)

For your 2D case, you could also use np.hypot instead of np.linalg.norm:

sides = np.hypot(*np.diff(vertices, axis=0).T)

answered Jun 7, 2021 at 17:51

Mad Physicist

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Tim Roberts · Accepted Answer · 2021-06-07 18:34:38Z

1

import math
points = [
 [224,  64],
 [ 62, 381],
 [224, 661],
 [568, 661],
 [733, 348],
 [650, 205],
 [509, 204],
 [509,  64]]

for x,y in zip(points,points[1:]):
    d = math.sqrt((x[1]-y[1])*(x[1]-y[1]) + (x[0]-y[0])*(x[0]-y[0]))
    print(x, y, d)

Output:

[224, 64] [62, 381] 355.99578649191903
[62, 381] [224, 661] 323.4872485894923
[224, 661] [568, 661] 344.0
[568, 661] [733, 348] 353.82764165621654
[733, 348] [650, 205] 165.3420696616563
[650, 205] [509, 204] 141.00354605470034
[509, 204] [509, 64] 140.0

Here's what it looks like:

FOLLOWUP

Here's the code using your exact structure:

import numpy as np
import math
points = [
 [224,  64],
 [ 62, 381],
 [224, 661],
 [568, 661],
 [733, 348],
 [650, 205],
 [509, 204],
 [509,  64]]
points = np.array(points).reshape(8,1,2)
print(points)
for pt in zip(points,points[1:]):
    print( pt )
    x = pt[0][0]
    y = pt[1][0]
    d = math.sqrt((x[1]-y[1])*(x[1]-y[1]) + (x[0]-y[0])*(x[0]-y[0]))
    print(x, y, d)

Output:

[[[224  64]]
 [[ 62 381]]
 [[224 661]]
 [[568 661]]
 [[733 348]]
 [[650 205]]
 [[509 204]]
 [[509  64]]]
(array([[224,  64]]), array([[ 62, 381]]))
[224  64] [ 62 381] 355.99578649191903
(array([[ 62, 381]]), array([[224, 661]]))
[ 62 381] [224 661] 323.4872485894923
(array([[224, 661]]), array([[568, 661]]))
[224 661] [568 661] 344.0
(array([[568, 661]]), array([[733, 348]]))
[568 661] [733 348] 353.82764165621654
(array([[733, 348]]), array([[650, 205]]))
[733 348] [650 205] 165.3420696616563
(array([[650, 205]]), array([[509, 204]]))
[650 205] [509 204] 141.00354605470034
(array([[509, 204]]), array([[509,  64]]))
[509 204] [509  64] 140.0

edited Jun 7, 2021 at 18:34

answered Jun 7, 2021 at 17:42

Tim Roberts

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8 Comments

k_p Over a year ago

hii thanks for detailed solution but i m facing problem while looping the array which i have shown in question, my program is returning array like that from where i have to calculate the length of side ..hat you have shown is on list .

k_p Over a year ago

i have tried to convert the array to list by array.tolist() but its giving error as IndexError: list index out of range

Tim Roberts Over a year ago

What you have is am 8x1x2 array. So, each element is a 1-element list that contains a 2-element point. You'll need to handle that extra level. I'll edit the answer.

k_p Over a year ago

for x,y in zip(approx,approx[1:]): # print(x[0],y[0]) d = math.sqrt((x[0][1]-y[0][1])*(x[0][1]-y[0][1]) + (x[0][0]-y[0][0])*(x[0][0]-y[0][0])) print('length between point:',x[0],'and', y[0],'is', d) using this it was done thanks

Tim Roberts Over a year ago

Ah, because the + is doing vector addition, not list concat. I'm a little disappointed you couldn't look this up. Use for pt in zip(points,np.roll(points,-1,0)):

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