Appending to an array in a loop - Python

Question

Please see this question for streamlined version with no clutter

I am writing a gravity simulation. The structure of my code is:

A solution array - this contains the (already computed) solution for the system of masses. Because of the nature of solving differential equations in Python, all the relevant values (each component of position and velocity for every mass at every time value) are in a the solution array.
An array masses. The objects here contain all the information about every mass including the arrays x_historywhich I am trying to update below.
No. of dimensions d. Here I have used 2 for simplicity.

I have the following code:

d = 2
for i in range(len(masses)): #  loop through masses
    for k in range(len(solution)): # loop through solution to obtain values at each timestep
        x = []  #  d dimensional x vector at current timestep
        #  fill the x-vector
        for j in range(d):
            x.append(solution[k][d * i + j])
        masses[i].x_history.append(x)

The idea is simply to loop through the objects in the masses array, abstract the relevant information from another array (x arrays representing vectors), and add it to the x_history arrays in the mass objects. The problem is (I have checked this by debugging) in the lines:

masses[i].x_history.append(x)

Rather than appending to masses[i].x_history for the current value of i, the code appends it to the x_history array for all the objects in the masses list, meaning that at the end of the outer loop every object in the masses list contains all the information rather than only its own.

Does anyone have any idea as to why this might be the case? Am I accidentally vectorizing this operation or something?

Why not just append them directly to `self.masses[i].(x/v)_history`? Or use [`extend`](https://www.tutorialspoint.com/python/list_extend.htm) — meowgoesthedog, Mar 04 '19 at 11:37
Sorry, could you expand? I do not know what you mean by 'directly'. I have done a fair amount of programming in other languages but not Python — bnosnehpets, Mar 04 '19 at 11:39
As in `self.masses[i].x_history.append(solution[k]...` instead of `x.append(solution[k]...`. — meowgoesthedog, Mar 04 '19 at 11:39
Can you please try to [remove the clutter from the code](https://stackoverflow.com/help/mcve) to let others reproduce the actual problem you have? — moooeeeep, Mar 04 '19 at 11:39
@meowgoesthedog because I am making smaller arrays from the larger arrays solution array, and then adding them — bnosnehpets, Mar 04 '19 at 11:42
Does not (self.masses[i].x_history).append(x) and work (self.masses[i].v_history).append(v) work? — zabop, Mar 04 '19 at 11:55

K Surya Kumar · Answer 1 · 2019-03-04T12:51:48.227

0

Seems you made the loop correct but appending the value to self.masses[i].x_history at wrong indented position. You have to move that last two line statements one intended (four space) back.

edited Mar 04 '19 at 12:51

answered Mar 04 '19 at 11:44

K Surya Kumar

65
8

I'm pretty sure it is in the right place - I am appending many arrays within x_array – bnosnehpets Mar 04 '19 at 11:47
You probably wanted to write intend -> indent. – VPfB Mar 04 '19 at 12:02
I agree with you. seems the last two appends should be outdented to loop only a the first for. – Arootin Aghazaryan Mar 04 '19 at 12:15
@VPfB ThankU :-) – K Surya Kumar Mar 04 '19 at 12:55

Appending to an array in a loop - Python

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