Python conversion between coordinates

Question

Are there functions for conversion between different coordinate systems?

For example, Matlab has [rho,phi] = cart2pol(x,y) for conversion from cartesian to polar coordinates. Seems like it should be in numpy or scipy.

Answer 1

Using numpy, you can define the following:

import numpy as np

def cart2pol(x, y):
    rho = np.sqrt(x**2 + y**2)
    phi = np.arctan2(y, x)
    return(rho, phi)

def pol2cart(rho, phi):
    x = rho * np.cos(phi)
    y = rho * np.sin(phi)
    return(x, y)

Answer 2

The existing answers can be simplified:

from numpy import exp, abs, angle

def polar2z(r,theta):
    return r * exp( 1j * theta )

def z2polar(z):
    return ( abs(z), angle(z) )

Or even:

polar2z = lambda r,θ: r * exp( 1j * θ )
z2polar = lambda z: ( abs(z), angle(z) )

Note these also work on arrays!

rS, thetaS = z2polar( [z1,z2,z3] )
zS = polar2z( rS, thetaS )

Answer 3

You can use the cmath module.

If the number is converted to a complex format, then it becomes easier to just call the polar method on the number.

import cmath
input_num = complex(1, 2) # stored as 1+2j
r, phi = cmath.polar(input_num)

Answer 4

If you can't find it in numpy or scipy, here are a couple of quick functions and a point class:

import math

def rect(r, theta):
    """theta in degrees

    returns tuple; (float, float); (x,y)
    """
    x = r * math.cos(math.radians(theta))
    y = r * math.sin(math.radians(theta))
    return x,y

def polar(x, y):
    """returns r, theta(degrees)
    """
    r = (x ** 2 + y ** 2) ** .5
    theta = math.degrees(math.atan2(y,x))
    return r, theta

class Point(object):
    def __init__(self, x=None, y=None, r=None, theta=None):
        """x and y or r and theta(degrees)
        """
        if x and y:
            self.c_polar(x, y)
        elif r and theta:
            self.c_rect(r, theta)
        else:
            raise ValueError('Must specify x and y or r and theta')
    def c_polar(self, x, y, f = polar):
        self._x = x
        self._y = y
        self._r, self._theta = f(self._x, self._y)
        self._theta_radians = math.radians(self._theta)
    def c_rect(self, r, theta, f = rect):
        """theta in degrees
        """
        self._r = r
        self._theta = theta
        self._theta_radians = math.radians(theta)
        self._x, self._y = f(self._r, self._theta)
    def setx(self, x):
        self.c_polar(x, self._y)
    def getx(self):
        return self._x
    x = property(fget = getx, fset = setx)
    def sety(self, y):
        self.c_polar(self._x, y)
    def gety(self):
        return self._y
    y = property(fget = gety, fset = sety)
    def setxy(self, x, y):
        self.c_polar(x, y)
    def getxy(self):
        return self._x, self._y
    xy = property(fget = getxy, fset = setxy)
    def setr(self, r):
        self.c_rect(r, self._theta)
    def getr(self):
        return self._r
    r = property(fget = getr, fset = setr)
    def settheta(self, theta):
        """theta in degrees
        """
        self.c_rect(self._r, theta)
    def gettheta(self):
        return self._theta
    theta = property(fget = gettheta, fset = settheta)
    def set_r_theta(self, r, theta):
        """theta in degrees
        """
        self.c_rect(r, theta)
    def get_r_theta(self):
        return self._r, self._theta
    r_theta = property(fget = get_r_theta, fset = set_r_theta)
    def __str__(self):
        return '({},{})'.format(self._x, self._y)

Answer 5

There is a better way to write a method to convert from Cartesian to polar coordinates; here it is:

import numpy as np
def polar(x, y) -> tuple:
  """returns rho, theta (degrees)"""
  return np.hypot(x, y), np.degrees(np.arctan2(y, x))

Answer 6

If your coordinates are stored as complex numbers you can use cmath

Answer 7

Mix of all the above answers which suits me:

import numpy as np

def pol2cart(r,theta):
    '''
    Parameters:
    - r: float, vector amplitude
    - theta: float, vector angle
    Returns:
    - x: float, x coord. of vector end
    - y: float, y coord. of vector end
    '''

    z = r * np.exp(1j * theta)
    x, y = z.real, z.imag

    return x, y

def cart2pol(x, y):
    '''
    Parameters:
    - x: float, x coord. of vector end
    - y: float, y coord. of vector end
    Returns:
    - r: float, vector amplitude
    - theta: float, vector angle
    '''

    z = x + y * 1j
    r,theta = np.abs(z), np.angle(z)

    return r,theta

Answer 8

In case, like me, you're trying to control a robot that accepts a speed and heading value based off of a joystick value, use this instead (it converts the radians to degrees:

def cart2pol(x, y):
    rho = np.sqrt(x**2 + y**2)
    phi = np.arctan2(y, x)
    return(rho, math.degrees(phi))

Answer 9

If you have an array of (x,y) coordinates or (rho, phi) coordinates you can transform them all at once with numpy.

The functions return an array of converted coordinates.

import numpy as np

def combine2Coord(c1, c2):
    return np.concatenate((c1.reshape(-1, 1), c2.reshape(-1, 1)), axis=1)

def cart2pol(xyArr):
    rho = np.sqrt((xyArr**2).sum(1))
    phi = np.arctan2(xyArr[:,1], xyArr[:,0])
    return combine2Coord(rho, phi)

def pol2cart(rhoPhiArr):
    x = rhoPhiArr[:,0] * np.cos(rhoPhiArr[:,1])
    y = rhoPhiArr[:,0] * np.sin(rhoPhiArr[:,1])
    return combine2Coord(x, y)

Answer 10

Do you care about speed? Use cmath, it's an order faster than numpy. And it's already included in any python since python 2!

Using ipython:

import cmath, numpy as np

def polar2z(polar):
    rho, phi = polar
    return rho * np.exp( 1j * phi )

def z2polar(z):
    return ( np.abs(z), np.angle(z) )


def cart2polC(xy):
    x, y = xy
    return(cmath.polar(complex(x, y))) # rho, phi

def pol2cartC(polar):
    rho, phi = polar
    z = rho * cmath.exp(1j * phi)
    return z.real, z.imag

def cart2polNP(xy):
    x, y = xy
    rho = np.sqrt(x**2 + y**2)
    phi = np.arctan2(y, x)
    return(rho, phi)

def pol2cartNP(polar):
    rho, phi = polar
    x = rho * np.cos(phi)
    y = rho * np.sin(phi)
    return(x, y)

xy = (100,100)
polar = (100,0)

%timeit cart2polC(xy)
%timeit pol2cartC(polar)

%timeit cart2polNP(xy)
%timeit pol2cartNP(polar)

%timeit z2polar(complex(*xy))
%timeit polar2z(polar)

373 ns ± 4.76 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
337 ns ± 0.976 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
4.3 µs ± 34.2 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)
3.41 µs ± 5.78 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)
3.4 µs ± 5.4 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)
1.39 µs ± 3.86 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)

Answer 11

Thinking about it in general, I would strongly consider hiding coordinate system behind well-designed abstraction. Quoting Uncle Bob and his book:

class Point(object)
    def setCartesian(self, x, y)
    def setPolar(self, rho, theta)
    def getX(self)
    def getY(self)
    def getRho(self)
    def setTheta(self)

With interface like that any user of Point class may choose convenient representation, no explicit conversions will be performed. All this ugly sines, cosines etc. will be hidden in one place. Point class. Only place where you should care which representation is used in computer memory.