Combining nested lists according to first element

Question

So I have three lists:

lst1 = [('test1', 0.2), ('test7', 0.2)]
lst2 = [('test1', 5.2), ('test2', 11.1), ('test7', 0.2)]
lst3 = [('test1', 19.2), ('test2', 12.1), ('test7', 19.2), ('test9', 15.1)]

What I want to do is to go through the lists and create the following tuple:

[(test1, 0.2, 5.2, 19.2), (test2, 0.0, 11.1, 12.1), (test7, 0.2, 0.2, 19.2), (test9, 0.0, 0.0, 15.1)]

I have tried to solve this through multiple methods but no luck, any help is welcomed!

It would be helpful to also include the code that you've written till now. Also, your problem is similar to [Merge multiple dictionaries](https://stackoverflow.com/questions/5946236/how-to-merge-multiple-dicts-with-same-key) and [Merge python dictionaries](https://stackoverflow.com/questions/2365921/merging-python-dictionaries) except instead of key and value in dictionaries, you have a list of key value pairs. — algrebe, Nov 29 '20 at 19:05

Daniel Lenz · Answer 1 · 2020-11-29T19:14:07.907

3

If you really want to use test1 etc. as unique keys, you might be better off using a dictionary.

I'd recommend the following: Use itertools.chain to combine the lists that you want to iterate over, and use a default dictionary that you simply append items to.

import itertools as it
from collections import defaultdict

lst1 = [('test1', 0.2), ('test7', 0.2)]
lst2 = [('test1', 5.2), ('test2', 11.1), ('test7', 0.2)]
lst3 = [('test1', 19.2), ('test2', 12.1), ('test7', 19.2), ('test9', 15.1)]


mydict = defaultdict(list)

for key, value in it.chain(lst1, lst2, lst3):
  mydict[key].append(value)

print(mydict)

> defaultdict(
        <class 'list'>,
        {'test1': [0.2, 5.2, 19.2],
        'test7': [0.2, 0.2, 19.2],
        'test2': [11.1, 12.1],
        'test9': [15.1]}
)

edited Nov 29 '20 at 19:14

answered Nov 29 '20 at 19:08

Daniel Lenz

3,334
17
36

This is not the output I want really, how can I make the output `{'test1': [0.2, 5.2, 19.2], 'test7': [0.2, 0.2, 19.2], 'test2': [0.0, 11.1, 12.1], 'test9': [0.0, 0.0, 15.1]}` – apol96 Nov 29 '20 at 21:27
How do you decide which lists get an extra `0.0` as first element and which don't? – Daniel Lenz Nov 30 '20 at 09:15

algrebe · Accepted Answer · 2020-11-30T23:45:50.593

lst1 = [('test1', 0.2), ('test7', 0.2)]
lst2 = [('test1', 5.2), ('test2', 11.1), ('test7', 0.2)]
lst3 = [('test1', 19.2), ('test2', 12.1), ('test7', 19.2), ('test9', 15.1)]
# create a list containing all these lists
lst_of_lists = [lst1, lst2, lst3]
# Create a dictionary where keys are the test1, test2, test3...
# and the values are [0.0, 0.0, ....] as many as the number of lists
output_dict = {
    name:[0.0]*len(lst_of_lists) for lst in lst_of_lists
    for (name, value) in lst
}

# Now, 'test1' in lst1 has to modify output_dict['test1'][0]
#      'test1' in lst2 modifies output_dict['test1'][1]
# and so on...
for idx, lst in enumerate(lst_of_lists):
    for key, value in lst:
        output_dict[key][idx] = value

# this gives you a dictionary
print("output_dict: ", output_dict)

# you can convert it back to a list with .items(), and since you want it sorted
print("dict_as_list: ", sorted(output_dict.items()))

# since you want the keys and values together in the same tuple
output_list = [tuple([key] + val_list) for key, val_list in output_dict.items()]
# and sort that
output_list = sorted(output_list)
print("output_list: ", output_list)

Output:

output_dict:  {'test1': [0.2, 5.2, 19.2], 'test7': [0.2, 0.2, 19.2], 'test2': [0.0, 11.1, 12.1], 'test9': [0.0, 0.0, 15.1]}
dict_as_list:  [('test1', [0.2, 5.2, 19.2]), ('test2', [0.0, 11.1, 12.1]), ('test7', [0.2, 0.2, 19.2]), ('test9', [0.0, 0.0, 15.1])]
output_list:  [('test1', 0.2, 5.2, 19.2), ('test2', 0.0, 11.1, 12.1), ('test7', 0.2, 0.2, 19.2), ('test9', 0.0, 0.0, 15.1)]

Combining nested lists according to first element

2 Answers2