I have a python dictionary where i need to find the children of the nodes if available

Question

I have the following dictionary representing id:parent. {1: 2, 3: 1}.

I need to loop and check if id == parent then it is a child.

For example here: 1=1 so 3 is the child of 1.

I will append it to the dictionary accordingly. Any ideas?

 d={1:2, 3:1}
      for node in d:

so you want to make a new dict of `id:child` – Joe Iddon Jun 04 '18 at 16:09 — Joe Iddon, Jun 04 '18 at 16:09
yes, or append the child to the original dictionary – user3057109 Jun 04 '18 at 16:27 — user3057109, Jun 04 '18 at 16:27

score 1 · Accepted Answer · answered Jun 04 '18 at 16:27

dict1={1:2,2:3,3:1,4:1,5:2}
result={}
for key in dict1.keys():
    result[key]=[]
    for item in dict1.items():
        if key==item[1]:
            result[key].append(item[0])
print(result)  

output:
   {1: [3, 4], 2: [1, 5], 3: [2], 4: [], 5: []}

If you don't want to have those ids with no child, then you can write in the following way.

dict1={1:2,2:3,3:1,4:1,5:2}
result={}
for key in dict1.keys():
    for item in dict1.items():
        if key==item[1]:
            if key not in result:
                result[key]=[]
            result[key].append(item[0])
print(result)
output:
{1: [3, 4], 2: [1, 5], 3: [2]}

This is `O(n^2)` though... Still gets it done though – Joe Iddon Jun 04 '18 at 17:16 — Joe Iddon, Jun 04 '18 at 17:16

Joe Iddon · Answer 2 · 2018-06-04T17:30:16.317

1

An O(n) solution would be:

child_parent = {1:2, 3:1, 4:1, 5:2, 1:5, 6:5, 7:2}
parent_children = {}
for child, parent in child_parent.items():
    parent_children.setdefault(parent, []).append(child)

giving:

{5: [1, 6], 1: [3, 4], 2: [5, 7]}

And, for ease of evaluation, the data represents the following tree:

edited Jun 04 '18 at 17:30

answered Jun 04 '18 at 17:24

Joe Iddon

20,101
7
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Thank you! Can python display the tree you drew? – user3057109 Jun 04 '18 at 18:01
@user3057109 Not natively, but checkout:https://stackoverflow.com/questions/15675261/displaying-a-tree-in-ascii – Joe Iddon Jun 04 '18 at 18:14

I have a python dictionary where i need to find the children of the nodes if available

2 Answers2