CODEWITHSUNDEEP
pythonloopsdictionary
hex93
hex93

Reputation: 359

Why is my dict being overwritten in this loop in python?

I have a dict, coords_dict, in a strange format. Which is currently being used to store a set of Cartesian coordinate points (x,y,z). The structure of the dict (which is unfortunately out of my control) is as follows.

The keys of the dict are a series of z values of a plane, and each entry consists of a single element list, which itself is a list of lists containing the coordinate points. For example, two elements in the dict can be specified as

coords_dict['3.5']=[[[1.62,2.22,3.50],[4.54,5.24,3.50]]]
coords_dict['5.0']=[[[0.33,6.74,5.00],[2.54,12.64,5.00]]]

So, I now want to apply some translational shift to all coordinate points in this dict by some shift vector [-1,-1,-1], i.e. I want all x, y, and z coordinates to be 1 less than they were before (rounded to 2 decimal places). And I want to assign the result of this translation to a new dictionary, coords_dict_translated, while also updating the dict keys to match the z locations of all points

My attempt at a solution is below

import numpy as np
shift_vector=[-1,-1,-1]
coords_dict_translated={}
for key,plane in coords_dict.items(): #iterate over dictionary, k are keys representing each plane
    key=str(float(key)+shift_vector[2]) #the new key should match the z location
    #print(key)
    for point_index in range(0,len(plane[0])): #loop over points in this plane
        plane[0][point_index]=list(np.around(np.array(plane[0][point_index])
                                   +np.array(shift_vector),decimals=2)) #add shift vector to all points    
    coords_dict_translated[key]=plane

However, I notice that if I do this that that the original values of coords_dict are also changing. I want coords_dict to stay the same but return a completely new and entirely separate dict. I am not quite sure where the issue lies, I have tried using for key,plane in list(coords_dict.items()): as well but this did not work. Why does this loop change the values of the original dictionary?

Upvotes: 0

Views: 91

Answers (2)

Josh Sharkey
Josh Sharkey

Reputation: 1038

when you are iterating over the dictionary in the for loop you are referencing the elements in your list/array:

for key,plane in coords_dict.items(): #iterate over dictionary, k are keys representing each plane

If you don't want to change the items, you should just make a copy of the variable you are using instead of setting plane directly:

import copy
key=str(float(key)+shift_vector[2]) #the new key should match the z location
    #print(key)
    c = copy.deepcopy(plane)
    for point_index in range(0,len(plane[0])): #loop over points in this plane
        c[0][point_index]=list(np.around(np.array(plane[0][point_index])
                                   +np.array(shift_vector),decimals=2)) #add shift vector to all points    
    coords_dict_translated[key] = c

Upvotes: 1

Joffan
Joffan

Reputation: 1480

The most likely issue here is that you have a list that is being referenced from two different variables. This can happen even using .copy() when you have nested structure (as you do here).

If this is the problem, you can probably overcome it by using need to make sure you are making a (deep) copy of lists you want to update independently. copy.deepcopy will iteratively make copies of lists within lists etc. to avoid double references to lower-level lists.

(comment made into answer).

Upvotes: 1

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