How work to with an array of arrays and how initialize multiple arrays in Numpy?

I have to write an ABM (agent-based model) project in Python and I need to initialize 50 agents which will each contain a different set of numbers. I cannot use a matrix of 50 rows because each agent (each row) can have a different quantity of elements so the vector of each agent has not the same length: when certain conditions for agent_i occur in the algorithm then a number calculated by the algorithm is added to its vector. The simplest way would be to write manually every one like this

agent_1 = np.array([])
agent_2 = np.array([])
agent_3 = np.array([])
...

but of course I can’t. I don’t know if exist a way to automatically initialize with a loop, something like

for i in range(50):
    agent_i = np.array([])

If it exist, It would be useful because then, when certain conditions occur in the algorithm, I could add a calculated numbers to agent_i:

agent_i = np.append(agent_i, result_of_algorithm)

Maybe another way is to use an array of arrays

[[agent_1_collection],[agent_2_collection], ... , [agent_50_collection]]

Once again, I don’t know how to initialize an array of arrays and I don’t know how to add a number to a specific array: in fact I think it can’t be done like this (assume, for simplicity, that I have this little array of only 3 agents and that I know how it is done):

vect = np.array([[1],[2,3],[4,5,6]])
result_of_algorithm_for_agent_2 = ...some calculations that we assume give as result... = 90 
vect[1] = np.append(vect[1], result_of_algorithm_for_agent_2)

output:

array([[1], array([ 2,  3, 90]), [4, 5, 6]], dtype=object)

why it changes in that way?

Do you have any advice on how to manipulate arrays of arrays? For example, how to add elements to a specific point of a sub-array (agent)?
Thanks.

Answer

List of Arrays

You can create a list of arrays:

agents = [np.array([]) for _ in range(50)]

And then to append values to some agent, say agents[0], use:

items_to_append = [1, 2, 3]  # Or whatever you want.
agents[0] = np.append(agents[0], items_to_append)

List of Lists

Alternatively, if you don’t need to use np.arrays, you can use lists for the agents values. In that case you would initialize with:

a = [[] for _ in range(50)]

And you can append to agents[0] with either

single_value = 1  # Or whatever you want.
agents[0].append(single_value)

Or with

items_to_append = [1, 2, 3]  # Or whatever you want
agents[0] += items_to_append