efficient list mapping in python

Question

Hi everyone,

I have the following input:

input = [(dog, dog, cat, mouse), (cat, ruby, python, mouse)]

and trying to have the following output:

outputlist = [[0, 0, 1, 2], [1, 3, 4, 2]]

outputmapping = {0:dog, 1:cat, 2:mouse, 3:ruby, 4:python, 5:mouse}

Any tips on how to handle given with scalability in mind (var input can get really large).

Answer 1

Here is one possible solution, although it isn't the greatest. It could be made slightly more efficient if you know how many elements each entry in the list will have before-hand, by pre-allocating them.

labels=[];
label2index={};
outputlist=[];
for group in input:
    current=[];
    for label in group:
       if label not in label2index:
           label2index[label]=len(labels);
           labels.append(label);
       current.append(label2index[label]);
    outputlist.append(current);

outputmapping={};
for idx, val in enumerate(labels):
    outputmapping[idx]=val;

Answer 2

You probably want something like:

import collections
import itertools

def build_catalog(L):
    counter = itertools.count().next
    names = collections.defaultdict(counter)
    result = []
    for t in L:
        new_t = [ names[item] for item in t ]
        result.append(new_t)
    catalog = dict((name, idx) for idx, name in names.iteritems())
    return result, catalog

Using it:

>>> input = [('dog', 'dog', 'cat', 'mouse'), ('cat', 'ruby', 'python', 'mouse')]
>>> outputlist, outputmapping = build_catalog(input)
>>> outputlist
[[0, 0, 1, 2], [1, 3, 4, 2]]
>>> outputmapping
{0: 'dog', 1: 'cat', 2: 'mouse', 3: 'ruby', 4: 'python'}

Answer 3

This class will automatically map objects to increasing integer values:

class AutoMapping(object):
    def __init__(self):
        self.map = {}
        self.objects = []

    def __getitem__(self, val):
        if val not in self.map:
            self.map[val] = len(self.objects)
            self.objects.append(val)
        return self.map[val]

Example usage, for your input:

>>> input = [('dog', 'dog', 'cat', 'mouse'), ('cat', 'ruby', 'python', 'mouse')]
>>> map = AutoMapping()
>>> [[map[x] for x in y] for y in input]
[[0, 0, 1, 2], [1, 3, 4, 2]]
>>> map.objects
['dog', 'cat', 'mouse', 'ruby', 'python']
>>> dict(enumerate(map.objects))
{0: 'dog', 1: 'cat', 2: 'mouse', 3: 'ruby', 4: 'python'}

Answer 4

I had the same problem quite often in my projects, so I wrapped up a class some time ago that does exactly this:

class UniqueIdGenerator(object):
    """A dictionary-like class that can be used to assign unique integer IDs to
    names.

    Usage:

    >>> gen = UniqueIdGenerator()
    >>> gen["A"]
    0
    >>> gen["B"]
    1
    >>> gen["C"]
    2
    >>> gen["A"]      # Retrieving already existing ID
    0
    >>> len(gen)      # Number of already used IDs
    3
    """

    def __init__(self, id_generator=None):
        """Creates a new unique ID generator. `id_generator` specifies how do we
        assign new IDs to elements that do not have an ID yet. If it is `None`,
        elements will be assigned integer identifiers starting from 0. If it is
        an integer, elements will be assigned identifiers starting from the given
        integer. If it is an iterator or generator, its `next` method will be
        called every time a new ID is needed."""
        if id_generator is None:
            id_generator = 0
        if isinstance(id_generator, int):
            import itertools
            self._generator = itertools.count(id_generator)
        else:
            self._generator = id_generator
        self._ids = {}

    def __getitem__(self, item):
        """Retrieves the ID corresponding to `item`. Generates a new ID for `item`
        if it is the first time we request an ID for it."""
        try:
            return self._ids[item]
        except KeyError:
            self._ids[item] = self._generator.next()
            return self._ids[item]

    def __len__(self):
        """Retrieves the number of added elements in this UniqueIDGenerator"""
        return len(self._ids)

    def reverse_dict(self):
        """Returns the reversed mapping, i.e., the one that maps generated IDs to their
        corresponding items"""
        return dict((v, k) for k, v in self._ids.iteritems())

    def values(self):
        """Returns the list of items added so far. Items are ordered according to
        the standard sorting order of their keys, so the values will be exactly
        in the same order they were added if the ID generator generates IDs in
        ascending order. This hold, for instance, to numeric ID generators that
        assign integers starting from a given number."""
        return sorted(self._ids.keys(), key = self._ids.__getitem__)

Usage example:

>>> input = [(dog, dog, cat, mouse), (cat, ruby, python, mouse)]
>>> gen = UniqueIdGenerator()
>>> outputlist = [[gen[x] for x in y] for y in input]
[[0, 0, 1, 2], [1, 3, 4, 2]]
>>> print outputlist
>>> outputmapping = gen.reverse_dict()
>>> print outputmapping
{0: 'dog', 1: 'cat', 2: 'mouse', 3: 'ruby', 4: 'python'}