CODEWITHSUNDEEP
python
Animesh D
Animesh D

Reputation: 5002

Searching list of lists

I have a list of data like this

data = [['A',11],['A',22],['B',14],['C',23],['C',31],['B',12]]

I put the unique items in an other list like this:

search=[]
for item in data:
    if search.count(item[0]) == 0:
        search.append(item[0])

# search contains ['A', 'B', 'C']

How do I search the list data with the list search and produce output like?

['A', [11,22]]
['B', [14,12]]
['C', [23,31]]

Upvotes: 1

Views: 151

Answers (6)

georg
georg

Reputation: 215039

You can also use a comprehension:

grouped_data = dict((x[0],list(y[1] for y in data if y[0]==x[0])) for x in data)

Note, that unlike other solutions, this one has quadratic complexity, don't use it on very large lists.

Upvotes: 1

eumiro
eumiro

Reputation: 213075

data = [['A',11],['A',22],['B',14],['C',23],['C',31],['B',12]]
d = {}
for k, v in data:
    d.setdefault(k, []).append(v)

for item in sorted(d.iteritems()):
    print list(item)

prints

['A', [11, 22]]
['B', [14, 12]]
['C', [23, 31]]

but consider keeping the d dictionary:

{'A': [11, 22], 'B': [14, 12], 'C': [23, 31]}

It will allow you to access the values more easily.

Upvotes: 2

smessing
smessing

Reputation: 4350

Create a dictionary of key value pairs, where each key is a first element in a tuple in data (you can only have unique keys in a dictionary), and the value is a list of all items that are in tuples with the key.

map = {}
for item in data:
  if map.has_key(item[0]):
    map[item[0]].append(item[1])
  else:
    map[item[0]] = []
    map[item[0]].append(item[1])

>>> map
{'A': [11, 22], 'C': [23, 31], 'B': [14, 12]}

Upvotes: 1

tzaman
tzaman

Reputation: 47850

I'd use a defaultdict, like so:

from collections import defaultdict

dd = defaultdict(list)
for key, val in data:
    dd[key].append(val)

# dd.iteritems() will give you your output.

Upvotes: 5

Marcin
Marcin

Reputation: 49886

You should use a dict for this task (or an OrderedDict if necessary). Please read the python documentation: http://docs.python.org/library/stdtypes.html#mapping-types-dict

Upvotes: 1

Sven Marnach
Sven Marnach

Reputation: 602635

Use a dictionary:

d = {}
for k, v in data:
    d.setdefault(k, []).append(v)
print d

prints

{'A': [11, 22], 'C': [23, 31], 'B': [14, 12]}

You can get a list of all keys with d.keys().

Instead of building the list data, it would probably be better to use a dictionary right from the start.

(Someone else will recommend a defaultdict for this soon. I prefer a dict in this case, but it doesn't matter too much.)

Upvotes: 6

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