Create python array from first element of array 1, first element of array 2, second element of array 1, second element of array 2, etc

Question

I am attempting to create a state vector representing the positions and velocities of a series of particles at a given time, for a simulation. I have created individual vectors x,y,vx,vy which give the value of that variable for each particle. Is there a good way of automatically combining them into one array, which contains all the info for particle one, followed by all the info for particle two etc etc)? Thanks

Answer 1

Do you mean like this?

x = [0, 1, 2]
y = [3, 4, 5]
vx = [6, 7, 8]
vy = [9, 10, 11]

c = zip(x, y, vx, vy)
print(c)  # -> [(0, 3, 6, 9), (1, 4, 7, 10), (2, 5, 8, 11)]

if you're using Python 3, you would need to use c = list(zip(x, y, vx, vy)) .

If you don't want the values for each particle grouped into a tuple like that for some reason, the result could be flattened:

c = [item for group in zip(x, y, vx, vy) for item in group]
print(c)  # -> [0, 3, 6, 9, 1, 4, 7, 10, 2, 5, 8, 11]

However , I would recommend just "naming" the tuples instead:

from collections import namedtuple

Particle = namedtuple('Particle', 'x, y, vx, vy')
c = [Particle._make(group) for group in zip(x, y, vx, vy)]
print(c)

Output:

[Particle(x=0, y=3, vx=6, vy=9),
 Particle(x=1, y=4, vx=7, vy=10),
 Particle(x=2, y=5, vx=8, vy=11)]

That way you can reference the fields by name — ie c[1].x — which could make subsequent code and calculations a lot more readable.