Python, appending from a list to a dependant on the value in the list

Question

Morning all, I have a little query which I am struggling to find a way to program efficiently, what I would like to do is have a two dimensional list [[1, 1, 2, 1], [1, 2, 3, 4]] and I would like to check through the [0] index of this list and if the same numbers appears more than once I would like to appended it to separate list. So in my example list above I would like to output Source1List = [1,2,4] Source2List = [3] The index [0] of the two dimensional list is always changing whilst index [1] will never change. I have began with something like this but cant get it working currently

SourceSelection = [[1 ,2 , 1, 1], [1, 2, 3, 4]]
DisplayGroup1= []
DisplayGroup2= []
DisplayGroup3= []
DisplayGroup4= []
Groups= [DisplayGroup1,DisplayGroup2,DisplayGroup3, DisplayGroup4 ]
for source in SourceSelection[0]:
 #is there a way to grab what position source sits in the first list without knowing what number is it?
 Groups[source -1].append(SourceSelection[1][Pull the position that source sits in first list])

I sadly wont know what numbers are in the list of index [0] so i dont think i can use the .index function without an error?

Thank you in advance

• Andrew Wei: Is there any significance of SourceSelection[1] or is it purely there for indexing purposes?
• A9M: It is purpely for indexing purposes

Answer 1

Based on your description of the problem, you can do something like this, where you use a dictionary instead of a list of lists.

from collections import defaultdict
SourceSelection = [[1, 2, 1, 1], [1, 2, 3, 4]]
# Use dictionary because we don't know what's in index 0 of SourceSelection
Groups = defaultdict(list)
for i in range(len(SourceSelection[0])):
 source = SourceSelection[0][i]
 Groups[source].append(SourceSelection[1][i])
# Pass to remove entries with only 1
to_remove = set()
for g in Groups:
 if len(Groups[g]) == 1:
 to_remove.add(g)
for g in to_remove:
 del Groups[g]
# Print results
for g in sorted(Groups):
 print(g, Groups[g])

If you need to use a list of lists, you can just generate the initial list by finding the number of distinct elements and creating a map that stores which number maps to which index.