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Find centroid in a 2d array (list of lists)

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tillaart36 tillaart36 is offline

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Find centroid in a 2d array (list of lists)

Feb 19th, 2009

Hi,

my question is how I can find the centroid of group of cells that share the same value. Say I have this:

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grid = [[0, 0, 0, 0, 0],
[0, 1, 1, 1, 0],
[0, 1, 1, 0, 0],
[0, 1, 1, 1, 0],
[0, 0, 0 ,0, 0]]
grid = [[0, 0, 0, 0, 0],
[0, 1, 1, 1, 0],
[0, 1, 1, 0, 0],
[0, 1, 1, 1, 0],
[0, 0, 0 ,0, 0]]

I need to get the centroid in x and y = (1.875, 2)

I tried to iterate over all values and when this is 1 store all the x and y coordinates but im stucked as how to actually get the centroid of these points.

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jrcagle

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Re: Find centroid in a 2d array (list of lists)

Feb 22nd, 2009

You mean the center of mass?

Well, do it like the physicists do: Sum(moments) / Sum(masses):

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>>> def findCOM(grid):
    Mx = 0
    My = 0
    mass = 0
 
    for i in range(len(grid)):
       for j in range(len(grid[i])):
         if grid[i][j]:
            Mx += j
            My += i
            mass += 1
    COM = (float(Mx)/mass , float(My)/mass)
    return COM
 
>>> grid = [[0, 0, 0, 0, 0],
[0, 1, 1, 1, 0],
[0, 1, 1, 0, 0],
[0, 1, 1, 1, 0],
[0, 0, 0 ,0, 0]]
 
>>> findCOM(grid)
(1.875, 2.0)
>>>
>>> def findCOM(grid):
    Mx = 0
    My = 0
    mass = 0

    for i in range(len(grid)):
       for j in range(len(grid[i])):
         if grid[i][j]:
            Mx += j
            My += i
            mass += 1
    COM = (float(Mx)/mass , float(My)/mass)
    return COM

>>> grid = [[0, 0, 0, 0, 0],
[0, 1, 1, 1, 0],
[0, 1, 1, 0, 0],
[0, 1, 1, 1, 0],
[0, 0, 0 ,0, 0]]

>>> findCOM(grid)
(1.875, 2.0)
>>>

Jeff

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Re: Find centroid in a 2d array (list of lists)

Jun 18th, 2009

You could just take the average using pylab:

>>> import pylab

>>> grid = pylab.array[[0, 0, 0, 0, 0],
[0, 1, 1, 1, 0],
[0, 1, 1, 0, 0],
[0, 1, 1, 1, 0],
[0, 0, 0 ,0, 0]]

>>> ( pylab.average(pylab.array(grid.nonzero())[1,:]), pylab.average(pylab.array(grid.nonzero())[0,:]) )
(1.875, 2.0)