Taking identical 1st element of a list and assigning it as 1st element of the list in python

Question

I am trying to Taking identical 1st element of a list and assigning it as 1st element of the list. I was told it can be done by using defaultdict from collections module but is there a way i can do this without using the Collections library.

What i have:

mapping = [['Tom', 'BTPS 1.500 625', 0.702604], ['Tom', 'BTPS 2.000 1225', 0.724939], ['Max', 'OBL 0.0 421', 0.766102], ['Max', 'DBR 3.250 721', 0.887863]]

What i am looking to do:

mapping = [['Tom',[ 'BTPS 1.500 625', 0.702604], [ 'BTPS 2.000 1225', 0.724939]],['Max',[ 'OBL 0.0 421', 0.766102],['DBR 3.250 721', 0.887863]]]

Answer 1

You should use a dict / defaultdict to group the data by name, using the first element which is the key as the name, slicing the rest of the data and appending that as a value:

from collections import defaultdict

d = defaultdict(list)
for sub in mapping:
     d[sub[0]].append(sub[1:])

print(d)

Which would give you:

defaultdict(<type 'list'>, {'Max': [['OBL 0.0 421', 0.766102], ['DBR 3.250 721', 0.887863]], 'Tom': [['BTPS 1.500 625', 0.702604], ['BTPS 2.000 1225', 0.724939]]})

Or if the order matters, use an OrderedDict :

from collections import OrderedDict

d = OrderedDict()
for sub in mapping:
     d.setdefault(sub[0],[]).append(sub[1:])

That gives you:

OrderedDict([('Tom', [['BTPS 1.500 625', 0.702604], ['BTPS 2.000 1225', 0.724939]]), ('Max', [['OBL 0.0 421', 0.766102], ['DBR 3.250 721', 0.887863]])])

Without any imports, just use a regular dict again using dict.setdefault :

d = {}
for sub in mapping:
     d.setdefault(sub[0],[]).append(sub[1:])

print(d)

Using setdefault, if the key is not in the dict it gets added with a list as the value, if it does exist it just appends the value.

Answer 2

You can loop over the names in mapping and add to dictionary.

mapping = [['Tom', 'BTPS 1.500 625', 0.702604], ['Tom', 'BTPS 2.000 1225', 0.724939], ['Max', 'OBL 0.0 421', 0.766102], ['Max', 'DBR 3.250 721', 0.887863]]

#using dictionary to store output
mapping_dict=dict()

for items in mapping:
if items[0] in mapping_dict:
    mapping_dict[items[0]].append([items[1],items[2]])
else:
    mapping_dict[items[0]]=[items[1],items[2]]

print mapping_dict

Output: {'Max': ['OBL 0.0 421', 0.766102, ['DBR 3.250 721', 0.887863]], 'Tom': ['BTPS 1.500 625', 0.702604, ['BTPS 2.000 1225', 0.724939]]}