# Better way to calculate double-scattering diffraction using cartesian product of arrays?

This script calculates points in reciprocal space for hexagonal 2D lattices, then uses the cartesian product from itertools to add each vector from one lattice to all of the vectors of the other in the line

np.array([a+b for a, b in list(itertools.product(p1.T, p2.T))])


It's slow right now because as written its instantiating millions of tiny numpy arrays.

I'm aware of:

and I suspect there's some way to do this, possibly using np.meshgrid or np.mgrid that's faster, uses less memory and looks cleaner, but I can not figure out how.

I will use the output in an optimization loop matching these positions to measured positions, so it needs to be callable several hundred times in a row, so reusing large array spaces rather than instantiating and garbage collecting them might have some advantages.

click for larger

import numpy as np
import matplotlib.pyplot as plt
import itertools

def rotatem(xy, rot):
r3o2, twopi, to_degs, to_rads = np.sqrt(3)/2., 2*np.pi, 180/np.pi, np.pi/180
c, s = [f(to_rads*rot) for f in (np.cos, np.sin)]
x, y = xy
xr = c*x - s*y
yr = c*y + s*x
return np.vstack((xr, yr))

def get_points(a=1.0, nmax=5, rot=0):
r3o2, twopi, to_degs, to_rads = np.sqrt(3)/2., 2*np.pi, 180/np.pi, np.pi/180
g = twopi / (r3o2 * a)
i = np.arange(-nmax, nmax+1)
I, J = [thing.flatten() for thing in np.meshgrid(i, i)]
keep = np.abs(I + J) <= nmax
I, J = [thing[keep] for thing in (I, J)]
xy = np.vstack((I+0.5*J, r3o2*J))
return g * rotatem(xy, rot=rot)

r3o2, twopi, to_degs, to_rads = np.sqrt(3)/2., 2*np.pi, 180/np.pi, np.pi/180

a1, a2, rot = 1.0, 2**0.2, 22

p1 = get_points(a=a1, nmax=20)
p2 = get_points(a=a2, nmax=20, rot=rot)
p3 = get_points(a=a2, nmax=20, rot=-rot)

d12 = np.array([a+b for a, b in list(itertools.product(p1.T, p2.T))])
d13 = np.array([a+b for a, b in list(itertools.product(p1.T, p3.T))])
d12, d13 = [d[((d**2).sum(axis=1)<4.)] for d in (d12, d13)]

if True:
plt.figure()
for d in (d12, d13):
plt.plot(*d.T, 'o', ms=2)
plt.gca().set_aspect('equal')
plt.show()

• This comment won't be a solution to the main question at hand, but it's important to understand that you don't need the two intermediate lists created in your first line of code. Instead, just write this: np.array(a+b for a, b in itertools.product(p1.T, p2.T)). The same point could be made about the creation of d12 and d13 in your larger code example.
– FMc
Sep 2, 2020 at 16:16
• @FMc While d12 = np.array(a+b for a, b in list(itertools.product(p1.T, p2.T))) is accepted, the next step d12 = np.array(d12[((d12**2).sum(axis=1)<4.)]) throws an exception "unsupported operand type(s) for ** or pow(): 'generator' and 'int"
– uhoh
Sep 2, 2020 at 16:57

You can replace:

d12 = np.array([a+b for a, b in list(itertools.product(p1.T, p2.T))])


with something like:

p1 = p1.T
p2 = p2.T
p3 = p3.T
d12 = p1[:,np.newaxis,:] + p2[np.newaxis,:,:]
d12 = my_d12.reshape((len(p1)*len(p2),2))


I find it most of the times easier to use the first index of an array for point_index and the second index for the dimensions, hence the .T's

With the use of the magic index np.newaxis at the right places you can create numpy array's of shape (M,N) with normal operators acting on arrays of shape (M) and (N).

With the reshape method d12 changed again to the shape in your original solution.

• Oh this is great! So simple; exactly what's needed. I've always used None in the place of np.newaxis thinking they did the same thing but never checked.
– uhoh
Sep 2, 2020 at 17:08
• @uhoh, you can also use None because np.newaxis is only an alias of None. I prefer to use np.newaxis because it is a heads-up that there is something funny going on with indexes and broadcasting rules. Sep 3, 2020 at 16:40
• Okay got it! I think I will switch to that as well since I've been soundly reprimanded for poor readability ;-)
– uhoh
Sep 3, 2020 at 17:18

Your code is pretty cluttered and you seem fixated on a 'less lines of code equals better code' mindset.

Firstly lets move r3o2 and friends out into the global scope as constants - UPPER_SNAKE_CASE variables in Python. This gets rid of 2 lines of code. Additionally it and makes:

• rotatem less confusing as now you're not defining 3 things you never use.
• get_points less confusing as now you're not defining 2 things you never use.

To get rotatem to look cleaner I'd then go on to:

1. Move the expression for defining xr and yr into the return.
2. Get rid of the clunky tuple unpacked comprehension; just write it out.
3. Use better names than c and s; cos and sin could be far more readable.
I moved them to the latter side of multiplication so the equation doesn't look like $$\\sin x - \cos y\$$

This makes rotatem pretty readable now.

def rotatem(xy, rot):
x, y = xy
return np.vstack((
x*cos - y*sin,
y*cos + x*sin,
))


To get get_points to look cleaner I'd then go on to:

1. Move the definition of g down to the bottom of the function. By defining it at the top of the function I'm having to read each and every line of code in the function to see if g is used. This is just a waste of time when you only use it in the return.
2. Rename:
• i to domain,
• I to i, and
• J to j.
3. Split the tuple in the expression for xy over multiple lines.
4. Change i to i[keep] and j to j[keep]; remove the tuple comprehension before this.
5. Add some whitespace around operators.

This makes get_points a bit more readable. But np.meshgrid is hampering this a bit.

def get_points(a=1.0, nmax=5, rot=0):
domain = np.arange(-nmax, nmax + 1)
i, j = [thing.flatten() for thing in np.meshgrid(domain, domain)]
keep = np.abs(i + j) <= nmax
xy = np.vstack((
i[keep] + 0.5 * j[keep],
R3O2 * j[keep],
))
return (TWO_PI / (R3O2 * a)) * rotatem(xy, rot=rot)


I still don't really understand what your code is doing, what are rotatem and get_points doing? You can explain this at the top of each function by using a docstring.

It's taken me a while to understand your code this much, and I still don't see myself understanding it all any time soon. You should really try to improve the readability of your code to the best it can be in the future so others don't just get bored.

In case you think my changes will decrease performance by anything important, my changes have a negligible impact on performance.

$python orig.py 0.0005879989994355128 4.213629218999813$ python peil.py
0.0005172819992367295 4.236298889999489

import numpy as np
import matplotlib.pyplot as plt
import itertools
import timeit

R3O2 = np.sqrt(3) / 2.
TWO_PI = 2 * np.pi  # You could just call this tau.
TO_DEGS = 180 / np.pi

def rotatem(xy, rot):
"""Explain what rotatem does."""
x, y = xy
cos = np.cos(rot)
sin = np.sin(rot)
return np.vstack((
x*cos - y*sin,
y*cos + x*sin,
))

def get_points(a=1.0, nmax=5, rot=0):
"""Explain what get_points does."""
domain = np.arange(-nmax, nmax + 1)
i, j = [thing.flatten() for thing in np.meshgrid(domain, domain)]
keep = np.abs(i + j) <= nmax
xy = np.vstack((
i[keep] + 0.5 * j[keep],
R3O2 * j[keep],
))
return (TWO_PI / (R3O2 * a)) * rotatem(xy, rot=rot)

def main(a1, a2, rot):
timer = timeit.perfcounter()
start = timer()

p1 = get_points(a=a1, nmax=20)
p2 = get_points(a=a2, nmax=20, rot=TO_RADS * rot)
p3 = get_points(a=a2, nmax=20, rot=TO_RADS * -rot)

mid = timer()

d12 = np.array([a+b for a, b in list(itertools.product(p1.T, p2.T))])
d13 = np.array([a+b for a, b in list(itertools.product(p1.T, p3.T))])
d12, d13 = [d[((d**2).sum(axis=1)<4.)] for d in (d12, d13)]

stop = timer()
print(mid - start, stop - start)

if __name__ == '__main__':
main(1.0, 2**0.2, 22)

• "I still don't really understand what your code is doing..." & "...my changes have a negligible impact on performance." Got it.
– uhoh
Sep 3, 2020 at 12:36
• @uhoh If you'd have followed my answer before I'd posted it then you'd get a better answer. Instead due to you not caring about readability or explaining what your code does it's just a waste of our time. Sep 3, 2020 at 12:52
• @uhoh You can also fix this by writing docstrings and improving the readability of your code so your code is accessible to anyone that reads it. Even more so when you put it up for critique. Sep 3, 2020 at 13:12