文章目錄

• • 1. 簡介
  • 2. 使用
  • 3. 分析

1. 簡介

Python 標準模塊 collections 中的類 Counter 是字典類 dict 的一個子類，該類一般用于對可哈希對象進行計數，有時也被稱為多重集合。在 Counter 類的對象中，數據存儲方式為：可哈希對象作為鍵，而可哈希對象的計數作為值進行存儲。

2. 使用

創建 Counter 對象：

>>> from collections import Counter >>> c = Counter('abcdeabcdabcaba')

獲得出現頻率最高的元素：

>>> c.most_common(3) [('a', 5), ('b', 4), ('c', 3)]

列出所有的鍵：

>>> sorted(c) ['a', 'b', 'c', 'd', 'e']

列出所有元素：

>>> ''.join(sorted(c.elements())) 'aaaaabbbbcccdde'

計算所有鍵的頻率和：

>>> sum(c.values()) 15

獲得某可哈希對象的計數：

>>> c['a'] 5

根據可迭代對象更新計數：

>>> for elem in 'shazam': ... c[elem] += 1>>> c['a'] 7

刪除某可迭代對象：

>>> del c['b'] >>> c['b'] 0

合并兩個 Counter 對象：

>>> d = Counter('simsalabim') >>> c.update(d) >>> c['a'] 9 >>> c['i'] 2

清空 Counter 對象：

>>> c.clear() >>> c['a'] 0

需要特別注意的是，當某可哈希對象的計數被設為 $0$ 或減至 $0$ 后，該對象將仍然保存在 Counter 對象中直到該對象被刪除或 Counter 對象本身被清空：

>>> c = Counter('aaabbc') >>> c['b'] -= 2 >>> c.most_common() [('a', 3), ('c', 1), ('b', 0)] >>> del c['b'] >>> c.most_common() [('a', 3), ('c', 1)]

關于 Counter 對象的其他詳細使用，請見 https://www.demo2s.com/python/python-collections-counter.html 。

3. 分析

######################################################################## ### Counter ########################################################################def _count_elements(mapping, iterable):'Tally elements from the iterable.'mapping_get = mapping.getfor elem in iterable:mapping[elem] = mapping_get(elem, 0) + 1try: # Load C helper function if availablefrom _collections import _count_elements except ImportError:passclass Counter(dict):'''Dict subclass for counting hashable items. Sometimes called a bagor multiset. Elements are stored as dictionary keys and their countsare stored as dictionary values.>>> c = Counter('abcdeabcdabcaba') # count elements from a string>>> c.most_common(3) # three most common elements[('a', 5), ('b', 4), ('c', 3)]>>> sorted(c) # list all unique elements['a', 'b', 'c', 'd', 'e']>>> ''.join(sorted(c.elements())) # list elements with repetitions'aaaaabbbbcccdde'>>> sum(c.values()) # total of all counts15>>> c['a'] # count of letter 'a'5>>> for elem in 'shazam': # update counts from an iterable... c[elem] += 1 # by adding 1 to each element's count>>> c['a'] # now there are seven 'a'7>>> del c['b'] # remove all 'b'>>> c['b'] # now there are zero 'b'0>>> d = Counter('simsalabim') # make another counter>>> c.update(d) # add in the second counter>>> c['a'] # now there are nine 'a'9>>> c.clear() # empty the counter>>> cCounter()Note: If a count is set to zero or reduced to zero, it will remainin the counter until the entry is deleted or the counter is cleared:>>> c = Counter('aaabbc')>>> c['b'] -= 2 # reduce the count of 'b' by two>>> c.most_common() # 'b' is still in, but its count is zero[('a', 3), ('c', 1), ('b', 0)]'''# References:# http://en.wikipedia.org/wiki/Multiset# http://www.gnu.org/software/smalltalk/manual-base/html_node/Bag.html# http://www.demo2s.com/Tutorial/Cpp/0380__set-multiset/Catalog0380__set-multiset.htm# http://code.activestate.com/recipes/259174/# Knuth, TAOCP Vol. II section 4.6.3def __init__(self, iterable=None, /, **kwds):'''Create a new, empty Counter object. And if given, count elementsfrom an input iterable. Or, initialize the count from another mappingof elements to their counts.>>> c = Counter() # a new, empty counter>>> c = Counter('gallahad') # a new counter from an iterable>>> c = Counter({'a': 4, 'b': 2}) # a new counter from a mapping>>> c = Counter(a=4, b=2) # a new counter from keyword args'''super(Counter, self).__init__()self.update(iterable, **kwds)def __missing__(self, key):'The count of elements not in the Counter is zero.'# Needed so that self[missing_item] does not raise KeyErrorreturn 0def most_common(self, n=None):'''List the n most common elements and their counts from the mostcommon to the least. If n is None, then list all element counts.>>> Counter('abracadabra').most_common(3)[('a', 5), ('b', 2), ('r', 2)]'''# Emulate Bag.sortedByCount from Smalltalkif n is None:return sorted(self.items(), key=_itemgetter(1), reverse=True)return _heapq.nlargest(n, self.items(), key=_itemgetter(1))def elements(self):'''Iterator over elements repeating each as many times as its count.>>> c = Counter('ABCABC')>>> sorted(c.elements())['A', 'A', 'B', 'B', 'C', 'C']# Knuth's example for prime factors of 1836: 2**2 * 3**3 * 17**1>>> prime_factors = Counter({2: 2, 3: 3, 17: 1})>>> product = 1>>> for factor in prime_factors.elements(): # loop over factors... product *= factor # and multiply them>>> product1836Note, if an element's count has been set to zero or is a negativenumber, elements() will ignore it.'''# Emulate Bag.do from Smalltalk and Multiset.begin from C++.return _chain.from_iterable(_starmap(_repeat, self.items()))# Override dict methods where necessary@classmethoddef fromkeys(cls, iterable, v=None):# There is no equivalent method for counters because the semantics# would be ambiguous in cases such as Counter.fromkeys('aaabbc', v=2).# Initializing counters to zero values isn't necessary because zero# is already the default value for counter lookups. Initializing# to one is easily accomplished with Counter(set(iterable)). For# more exotic cases, create a dictionary first using a dictionary# comprehension or dict.fromkeys().raise NotImplementedError('Counter.fromkeys() is undefined. Use Counter(iterable) instead.')def update(self, iterable=None, /, **kwds):'''Like dict.update() but add counts instead of replacing them.Source can be an iterable, a dictionary, or another Counter instance.>>> c = Counter('which')>>> c.update('witch') # add elements from another iterable>>> d = Counter('watch')>>> c.update(d) # add elements from another counter>>> c['h'] # four 'h' in which, witch, and watch4'''# The regular dict.update() operation makes no sense here because the# replace behavior results in the some of original untouched counts# being mixed-in with all of the other counts for a mismash that# doesn't have a straight-forward interpretation in most counting# contexts. Instead, we implement straight-addition. Both the inputs# and outputs are allowed to contain zero and negative counts.if iterable is not None:if isinstance(iterable, _collections_abc.Mapping):if self:self_get = self.getfor elem, count in iterable.items():self[elem] = count + self_get(elem, 0)else:super(Counter, self).update(iterable) # fast path when counter is emptyelse:_count_elements(self, iterable)if kwds:self.update(kwds)def subtract(self, iterable=None, /, **kwds):'''Like dict.update() but subtracts counts instead of replacing them.Counts can be reduced below zero. Both the inputs and outputs areallowed to contain zero and negative counts.Source can be an iterable, a dictionary, or another Counter instance.>>> c = Counter('which')>>> c.subtract('witch') # subtract elements from another iterable>>> c.subtract(Counter('watch')) # subtract elements from another counter>>> c['h'] # 2 in which, minus 1 in witch, minus 1 in watch0>>> c['w'] # 1 in which, minus 1 in witch, minus 1 in watch-1'''if iterable is not None:self_get = self.getif isinstance(iterable, _collections_abc.Mapping):for elem, count in iterable.items():self[elem] = self_get(elem, 0) - countelse:for elem in iterable:self[elem] = self_get(elem, 0) - 1if kwds:self.subtract(kwds)def copy(self):'Return a shallow copy.'return self.__class__(self)def __reduce__(self):return self.__class__, (dict(self),)def __delitem__(self, elem):'Like dict.__delitem__() but does not raise KeyError for missing values.'if elem in self:super().__delitem__(elem)def __repr__(self):if not self:return '%s()' % self.__class__.__name__try:items = ', '.join(map('%r: %r'.__mod__, self.most_common()))return '%s({%s})' % (self.__class__.__name__, items)except TypeError:# handle case where values are not orderablereturn '{0}({1!r})'.format(self.__class__.__name__, dict(self))# Multiset-style mathematical operations discussed in:# Knuth TAOCP Volume II section 4.6.3 exercise 19# and at http://en.wikipedia.org/wiki/Multiset## Outputs guaranteed to only include positive counts.## To strip negative and zero counts, add-in an empty counter:# c += Counter()## Rich comparison operators for multiset subset and superset tests# are deliberately omitted due to semantic conflicts with the# existing inherited dict equality method. Subset and superset# semantics ignore zero counts and require that p≤q ∧ p≥q → p=q;# however, that would not be the case for p=Counter(a=1, b=0)# and q=Counter(a=1) where the dictionaries are not equal.def __add__(self, other):'''Add counts from two counters.>>> Counter('abbb') + Counter('bcc')Counter({'b': 4, 'c': 2, 'a': 1})'''if not isinstance(other, Counter):return NotImplementedresult = Counter()for elem, count in self.items():newcount = count + other[elem]if newcount > 0:result[elem] = newcountfor elem, count in other.items():if elem not in self and count > 0:result[elem] = countreturn resultdef __sub__(self, other):''' Subtract count, but keep only results with positive counts.>>> Counter('abbbc') - Counter('bccd')Counter({'b': 2, 'a': 1})'''if not isinstance(other, Counter):return NotImplementedresult = Counter()for elem, count in self.items():newcount = count - other[elem]if newcount > 0:result[elem] = newcountfor elem, count in other.items():if elem not in self and count < 0:result[elem] = 0 - countreturn resultdef __or__(self, other):'''Union is the maximum of value in either of the input counters.>>> Counter('abbb') | Counter('bcc')Counter({'b': 3, 'c': 2, 'a': 1})'''if not isinstance(other, Counter):return NotImplementedresult = Counter()for elem, count in self.items():other_count = other[elem]newcount = other_count if count < other_count else countif newcount > 0:result[elem] = newcountfor elem, count in other.items():if elem not in self and count > 0:result[elem] = countreturn resultdef __and__(self, other):''' Intersection is the minimum of corresponding counts.>>> Counter('abbb') & Counter('bcc')Counter({'b': 1})'''if not isinstance(other, Counter):return NotImplementedresult = Counter()for elem, count in self.items():other_count = other[elem]newcount = count if count < other_count else other_countif newcount > 0:result[elem] = newcountreturn resultdef __pos__(self):'Adds an empty counter, effectively stripping negative and zero counts'result = Counter()for elem, count in self.items():if count > 0:result[elem] = countreturn resultdef __neg__(self):'''Subtracts from an empty counter. Strips positive and zero counts,and flips the sign on negative counts.'''result = Counter()for elem, count in self.items():if count < 0:result[elem] = 0 - countreturn resultdef _keep_positive(self):'''Internal method to strip elements with a negative or zero count'''nonpositive = [elem for elem, count in self.items() if not count > 0]for elem in nonpositive:del self[elem]return selfdef __iadd__(self, other):'''Inplace add from another counter, keeping only positive counts.>>> c = Counter('abbb')>>> c += Counter('bcc')>>> cCounter({'b': 4, 'c': 2, 'a': 1})'''for elem, count in other.items():self[elem] += countreturn self._keep_positive()def __isub__(self, other):'''Inplace subtract counter, but keep only results with positive counts.>>> c = Counter('abbbc')>>> c -= Counter('bccd')>>> cCounter({'b': 2, 'a': 1})'''for elem, count in other.items():self[elem] -= countreturn self._keep_positive()def __ior__(self, other):'''Inplace union is the maximum of value from either counter.>>> c = Counter('abbb')>>> c |= Counter('bcc')>>> cCounter({'b': 3, 'c': 2, 'a': 1})'''for elem, other_count in other.items():count = self[elem]if other_count > count:self[elem] = other_countreturn self._keep_positive()def __iand__(self, other):'''Inplace intersection is the minimum of corresponding counts.>>> c = Counter('abbb')>>> c &= Counter('bcc')>>> cCounter({'b': 1})'''for elem, count in self.items():other_count = other[elem]if other_count < count:self[elem] = other_countreturn self._keep_positive()

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