mygpo/counter.py

   1 # from http://code.activestate.com/recipes/576611-counter-class/
   2 from operator import itemgetter
   3 from heapq import nlargest
   4 from itertools import repeat, ifilter
   5
   6 class Counter(dict):
   7     '''Dict subclass for counting hashable objects.  Sometimes called a bag
   8     or multiset.  Elements are stored as dictionary keys and their counts
   9     are stored as dictionary values.
  10
  11     >>> Counter('zyzygy')
  12     Counter({'y': 3, 'z': 2, 'g': 1})
  13
  14     '''
  15
  16     def __init__(self, iterable=None, **kwds):
  17         '''Create a new, empty Counter object.  And if given, count elements
  18         from an input iterable.  Or, initialize the count from another mapping
  19         of elements to their counts.
  20
  21         >>> c = Counter()                           # a new, empty counter
  22         >>> c = Counter('gallahad')                 # a new counter from an iterable
  23         >>> c = Counter({'a': 4, 'b': 2})           # a new counter from a mapping
  24         >>> c = Counter(a=4, b=2)                   # a new counter from keyword args
  25
  26         '''
  27         self.update(iterable, **kwds)
  28
  29     def __missing__(self, key):
  30         return 0
  31
  32     def most_common(self, n=None):
  33         '''List the n most common elements and their counts from the most
  34         common to the least.  If n is None, then list all element counts.
  35
  36         >>> Counter('abracadabra').most_common(3)
  37         [('a', 5), ('r', 2), ('b', 2)]
  38
  39         '''
  40         if n is None:
  41             return sorted(self.iteritems(), key=itemgetter(1), reverse=True)
  42         return nlargest(n, self.iteritems(), key=itemgetter(1))
  43
  44     def elements(self):
  45         '''Iterator over elements repeating each as many times as its count.
  46
  47         >>> c = Counter('ABCABC')
  48         >>> sorted(c.elements())
  49         ['A', 'A', 'B', 'B', 'C', 'C']
  50
  51         If an element's count has been set to zero or is a negative number,
  52         elements() will ignore it.
  53
  54         '''
  55         for elem, count in self.iteritems():
  56             for _ in repeat(None, count):
  57                 yield elem
  58
  59     # Override dict methods where the meaning changes for Counter objects.
  60
  61     @classmethod
  62     def fromkeys(cls, iterable, v=None):
  63         raise NotImplementedError(
  64             'Counter.fromkeys() is undefined.  Use Counter(iterable) instead.')
  65
  66     def update(self, iterable=None, **kwds):
  67         '''Like dict.update() but add counts instead of replacing them.
  68
  69         Source can be an iterable, a dictionary, or another Counter instance.
  70
  71         >>> c = Counter('which')
  72         >>> c.update('witch')           # add elements from another iterable
  73         >>> d = Counter('watch')
  74         >>> c.update(d)                 # add elements from another counter
  75         >>> c['h']                      # four 'h' in which, witch, and watch
  76         4
  77
  78         '''
  79         if iterable is not None:
  80             if hasattr(iterable, 'iteritems'):
  81                 if self:
  82                     self_get = self.get
  83                     for elem, count in iterable.iteritems():
  84                         self[elem] = self_get(elem, 0) + count
  85                 else:
  86                     dict.update(self, iterable) # fast path when counter is empty
  87             else:
  88                 self_get = self.get
  89                 for elem in iterable:
  90                     self[elem] = self_get(elem, 0) + 1
  91         if kwds:
  92             self.update(kwds)
  93
  94     def copy(self):
  95         'Like dict.copy() but returns a Counter instance instead of a dict.'
  96         return Counter(self)
  97
  98     def __delitem__(self, elem):
  99         'Like dict.__delitem__() but does not raise KeyError for missing values.'
 100         if elem in self:
 101             dict.__delitem__(self, elem)
 102
 103     def __repr__(self):
 104         if not self:
 105             return '%s()' % self.__class__.__name__
 106         items = ', '.join(map('%r: %r'.__mod__, self.most_common()))
 107         return '%s({%s})' % (self.__class__.__name__, items)
 108
 109     # Multiset-style mathematical operations discussed in:
 110     #       Knuth TAOCP Volume II section 4.6.3 exercise 19
 111     #       and at http://en.wikipedia.org/wiki/Multiset
 112     #
 113     # Outputs guaranteed to only include positive counts.
 114     #
 115     # To strip negative and zero counts, add-in an empty counter:
 116     #       c += Counter()
 117
 118     def __add__(self, other):
 119         '''Add counts from two counters.
 120
 121         >>> Counter('abbb') + Counter('bcc')
 122         Counter({'b': 4, 'c': 2, 'a': 1})
 123
 124
 125         '''
 126         if not isinstance(other, Counter):
 127             return NotImplemented
 128         result = Counter()
 129         for elem in set(self) | set(other):
 130             newcount = self[elem] + other[elem]
 131             if newcount > 0:
 132                 result[elem] = newcount
 133         return result
 134
 135     def __sub__(self, other):
 136         ''' Subtract count, but keep only results with positive counts.
 137
 138         >>> Counter('abbbc') - Counter('bccd')
 139         Counter({'b': 2, 'a': 1})
 140
 141         '''
 142         if not isinstance(other, Counter):
 143             return NotImplemented
 144         result = Counter()
 145         for elem in set(self) | set(other):
 146             newcount = self[elem] - other[elem]
 147             if newcount > 0:
 148                 result[elem] = newcount
 149         return result
 150
 151     def __or__(self, other):
 152         '''Union is the maximum of value in either of the input counters.
 153
 154         >>> Counter('abbb') | Counter('bcc')
 155         Counter({'b': 3, 'c': 2, 'a': 1})
 156
 157         '''
 158         if not isinstance(other, Counter):
 159             return NotImplemented
 160         _max = max
 161         result = Counter()
 162         for elem in set(self) | set(other):
 163             newcount = _max(self[elem], other[elem])
 164             if newcount > 0:
 165                 result[elem] = newcount
 166         return result
 167
 168     def __and__(self, other):
 169         ''' Intersection is the minimum of corresponding counts.
 170
 171         >>> Counter('abbb') & Counter('bcc')
 172         Counter({'b': 1})
 173
 174         '''
 175         if not isinstance(other, Counter):
 176             return NotImplemented
 177         _min = min
 178         result = Counter()
 179         if len(self) < len(other):
 180             self, other = other, self
 181         for elem in ifilter(self.__contains__, other):
 182             newcount = _min(self[elem], other[elem])
 183             if newcount > 0:
 184                 result[elem] = newcount
 185         return result