Question

I’m making a gravity simulator and I need to calculate the resultant force acting upon each body.

In order to do this, I need to iterate through every pair of bodies in a dictionary (id: instance of Body class) and get the gravitational force between those two bodies. Then, I would add up all the forces and get the resultants.

But, how do I iterate over each pair of items in a dictionary only once in Python? If the celestial bodies were kept in a list, it would be simple:

for i in range(len(bodies)):
    for j in range(len(bodies) - i - 1):
        k = j - i + 1
        b1 = bodies[i]
        b2 = bodies[k]

Answer 1

values() and itertools‘ combinations are ideal for this use case.

from itertools import combinations
for a, b in combinations(bodies.values(), 2):
    print a, b