Create a dictionary in a loop

Question

I have 2 lists that I want to convert them into a dict with key and values. I managed to do so but there are too many steps so I would like to know if there's a simpler way of achieving this. Basically I would like to create the dict directly in the loop without having the extra steps bellow. I just started working with python and I don't quite understand all the datatypes that it provides. The jName form can be modified if needed.

jName=["Nose", "Neck", "RShoulder", "RElbow", "RWrist", "LShoulder", "LElbow", "LWrist", "RHip",
        "RKnee","RAnkle","LHip", "LKnee", "LAnkle", "REye", "LEye", "REar", "LEar"]
def get_joints(subset, candidate):
    joints_per_skeleton = [[] for i in range(len(subset))]
    # for each detected skeleton
    for n in range(len(subset)):
        # for each joint
        for i in range(18):
            cidx = subset[n][i]
            if cidx != -1:
                y = candidate[cidx.astype(int), 0]
                x = candidate[cidx.astype(int), 1]
                joints_per_skeleton[n].append((y, x))
            else:
                joints_per_skeleton[n].append(None)
    return joints_per_skeleton

joints = get_joints(subset,candidate)
print joints

Here is the output of the joints list of list

[[None, (48.0, 52.0), (72.0, 50.0), None, None, (24.0, 55.0), (5.0, 105.0), None, (63.0, 159.0), (57.0, 221.0), (55.0, 281.0), (28.0, 154.0), (23.0, 219.0), (23.0, 285.0), None, (25.0, 17.0), (55.0, 18.0), (30.0, 21.0)]]

Here I defined a function to create the dictionary from the 2 lists

def create_dict(keys, values):
    return dict(zip(keys, values))

my_dict = create_dict(jointsName, joints[0])

Here is the result:

{'LAnkle': (23.0, 285.0),
 'LEar': (30.0, 21.0),
 'LElbow': (5.0, 105.0),
 'LEye': (25.0, 17.0),
 'LHip': (28.0, 154.0),
 'LKnee': (23.0, 219.0),
 'LShoulder': (24.0, 55.0),
 'LWrist': None,
 'Neck': (48.0, 52.0),
 'Nose': None,
 'RAnkle': (55.0, 281.0),
 'REar': (55.0, 18.0),
 'RElbow': None,
 'REye': None,
 'RHip': (63.0, 159.0),
 'RKnee': (57.0, 221.0),
 'RShoulder': (72.0, 50.0),
 'RWrist': None}

I'm not very clear on your question (what are subset and candidate?) but you might want to look at dictionary comprehensions. You could also make an empty dict, d = {} and then fill it as you loop — mitoRibo
– mitoRibo, Commented Jun 13, 2018 at 0:34
I just want to create the dictionary, as shown in the last part of the code in the for loop to ditch the extra processing step. — AMayer
– AMayer, Commented Jun 13, 2018 at 5:58

mitoRibo · Accepted Answer · 2018-06-13 06:54:23Z

I think defaultdict could help you. I made my own example to show that you could predefine the keys and then go through a double for loop and have the values of the dict be lists of potentially different sizes. Please let me know if this answers your question:

from collections import defaultdict
import random
joint_names = ['hip','knee','wrist']
num_skeletons = 10

d = defaultdict(list)

for skeleton in range(num_skeletons):
    for joint_name in joint_names:
        r1 = random.randint(0,10)
        r2 = random.randint(0,10)
        if r1 > 4:
            d[joint_name].append(r1*r2)

print d

Output:

defaultdict(<type 'list'>, {'hip': [0, 5, 30, 36, 56], 'knee': [35, 50, 10], 'wrist': [27, 5, 15, 64, 30]})

As a note I found it very difficult to read through your code since there were some variables that were defined before the snippet you posted.

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