{"id":75892,"date":"2020-01-31T14:01:23","date_gmt":"2020-01-31T08:31:23","guid":{"rendered":"https:\/\/techvidvan.com\/tutorials\/?p=75892"},"modified":"2020-01-31T14:01:23","modified_gmt":"2020-01-31T08:31:23","slug":"python-counter","status":"publish","type":"post","link":"https:\/\/techvidvan.com\/tutorials\/python-counter\/","title":{"rendered":"Python Counter – Learn to create, access, and assign counters in Python"},"content":{"rendered":"

In this article, we will talk about Counter<\/strong> in Python. This is a class in the collections module<\/strong>.<\/p>\n

Other classes are defaultdict<\/strong>, OrderedDict<\/strong> and namedtuple<\/strong>. Let’s first discuss the concept of Python counter.<\/p>\n

What is a Counter in Python?<\/h3>\n

A counter<\/strong> is a container<\/strong> in the collections module<\/strong>. It is a subclass<\/strong> of dict<\/strong> and it is an unordered collection<\/strong> in which elements<\/strong> and their counts<\/strong> are stored<\/strong> as a dictionary<\/strong>.<\/p>\n

Code:<\/strong><\/p>\n

>>> from collections import Counter\n>>> issubclass(Counter,dict)<\/pre>\n
Output:<\/strong><\/p>\n
True<\/div>\n
Creating a Python Counter<\/h3>\n
We can create<\/strong> a counter<\/strong> with a parameter<\/strong> that can be:<\/p>\n
    \n
  • A sequence<\/strong> of items – like a list<\/strong> or tuple<\/strong> or string<\/strong>.<\/li>\n
  • Dictionary<\/strong> with keys<\/strong> and counts<\/strong>.<\/li>\n
  • Keyword arguments<\/strong> with string<\/strong> names<\/strong> and counts<\/strong>.<\/li>\n<\/ul>\n
    Let\u2019s take an example.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> Counter([1, 2, 1, 3, 4, 2, 1, 1])<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({1: 4, 2: 2, 3: 1, 4: 1})<\/div>\n
    Code:<\/strong><\/p>\n
    >>> Counter({1:4, 2:2, 3:1, 4:1})<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({1: 4, 2: 2, 3: 1, 4: 1})<\/div>\n
    Code:<\/strong><\/p>\n
    >>> Counter(A=3, B=2, C=1, D=4)<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘D’: 4, ‘A’: 3, ‘B’: 2, ‘C’: 1})<\/div>\n
    Updating a Counter in Python<\/h3>\n
    If you don\u2019t know the elements<\/strong> to add<\/strong> in a counter<\/strong>, you can create<\/strong> an empty counter<\/strong> and add<\/strong> them later<\/strong>.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> nums=Counter()\n>>> nums.update([1,2,1,3,4,2,1,1])\n>>> nums<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({1: 4, 2: 2, 3: 1, 4: 1})<\/div>\n
    The update()<\/strong> method lets us add sequences<\/strong> or dictionaries<\/strong> or keyword arguments<\/strong> later.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> nums.update({1:2,2:2})\n>>> nums<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({1: 6, 2: 4, 3: 1, 4: 1})<\/div>\n
    It does not replace the counts<\/strong> in the counter, it adds<\/strong> to them<\/strong>.<\/p>\n
    Accessing Elements in Python Counter<\/h3>\n
    To get the list<\/strong> of elements<\/strong> in the counter<\/strong> we can use the elements()<\/strong> method. It returns an iterator<\/strong> object for the values<\/strong> in the Counter.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> c=Counter(A=3, B=2, C=1, D=4)\n>>> c.elements()<\/pre>\n
    Output:<\/strong><\/p>\n
    <itertools.chain object at 0x00000210EF4C4248><\/div>\n
    Code:<\/strong><\/p>\n
    >>> for elem in c.elements():\n  print(elem)<\/pre>\n
    Output:<\/strong><\/p>\n
    A
    \nA
    \nA
    \nB
    \nB
    \nC
    \nD
    \nD
    \nD
    \nD<\/div>\n
    To delete<\/strong> an element<\/strong>, you can use the del keyword<\/strong>.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> del c['B']\n>>> c<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘D’: 4, ‘A’: 3, ‘C’: 1})<\/div>\n
    Accessing Counts in Python Counter<\/h3>\n
    We can use indexing<\/strong> to access the counts<\/strong> in a Python counter<\/strong>. Whatever element<\/strong> we want to get the count<\/strong> of, we will use it as the index<\/strong>.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> nums[2]<\/pre>\n
    Output:<\/strong><\/p>\n
    4<\/div>\n
    Code:<\/strong><\/p>\n
    >>> c=Counter(A=3, B=2, C=1, D=4)\n>>> c['B']<\/pre>\n
    Output:<\/strong><\/p>\n
    2<\/div>\n
    Code:<\/strong><\/p>\n
    >>> c['E']<\/pre>\n
    Output:<\/strong><\/p>\n
    0<\/div>\n
    E<\/strong> is not<\/strong> in the counter object<\/strong>, so it returns 0. <\/strong>This raises a KeyError<\/strong> for dictionaries<\/strong>. We can also get the counts<\/strong> of all items<\/strong> in a sequence<\/strong>.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> for val in 'ABCD':\n  print(val, c[val])<\/pre>\n
    Output:<\/strong><\/p>\n
    A 3
    \nB 2
    \nC 1
    \nD 4<\/div>\n
    Reassigning Counts in Python Counter<\/h3>\n
    The update()<\/strong> method adds<\/strong> to the counts<\/strong> in the Python counter<\/strong> but does not<\/strong> replace<\/strong> them. If you want to replace<\/strong> them, you can use the assignment operator<\/strong>.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> c=Counter(A=3, B=2, C=1, D=4)\n>>> c['B']=4\n>>> c<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘B’: 4, ‘D’: 4, ‘A’: 3, ‘C’: 1})<\/div>\n
    You can also clear the counts<\/strong> with the clear()<\/strong> method.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> c.clear()\n>>> c<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter()<\/div>\n
    Getting Most Common Values in Python Counter<\/h3>\n
    The most_common()<\/strong> method gives us the most common<\/strong> values in the Python counter<\/strong>. These are the values<\/strong> with the highest counts<\/strong> in the counter<\/strong>.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> c=Counter(A=3, B=2, C=1, D=4, E=2, F=4)\n>>> c.most_common()<\/pre>\n
    Output:<\/strong><\/p>\n
    [(‘D’, 4), (‘F’, 4), (‘A’, 3), (‘B’, 2), (‘E’, 2), (‘C’, 1)]<\/div>\n
    Code:<\/strong><\/p>\n
    >>> c.most_common(1)<\/pre>\n
    Output:<\/strong><\/p>\n
    [(‘D’, 4)]<\/div>\n
    Code:<\/strong><\/p>\n
    >>> c.most_common(2)<\/pre>\n
    Output:<\/strong><\/p>\n
    [(‘D’, 4), (‘F’, 4)]<\/div>\n
    D<\/strong> and F<\/strong> are the most common values<\/strong> in this counter<\/strong>.<\/p>\n
    If we do not<\/strong> provide<\/strong> a parameter value<\/strong>, it prints<\/strong> all values<\/strong> and their counts<\/strong>.<\/p>\n
    Python Counter Arithmetic<\/h3>\n
    Like we add<\/strong> the integers 2<\/strong> and 3<\/strong>, we can perform counter arithmetic<\/strong> on counters<\/strong>.<\/p>\n
    Let\u2019s take two counters<\/strong>.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> c1=Counter(A=3, B=2, C=1, D=4, E=2)\n>>> c2=Counter(A=2, B=3, C=1, D=5, E=1, F=4)<\/pre>\n
    Code:<\/strong><\/p>\n
    >>> c1+c2<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘D’: 9, ‘A’: 5, ‘B’: 5, ‘F’: 4, ‘E’: 3, ‘C’: 2})<\/div>\n
    Code:<\/strong><\/p>\n
    >>> c1-c2<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘A’: 1, ‘E’: 1})<\/div>\n
    Code:<\/strong><\/p>\n
    >>> c2-c1<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘F’: 4, ‘B’: 1, ‘D’: 1})<\/div>\n
    Counters<\/strong> can have negative counts<\/strong> too.<\/p>\n
    The subtract()<\/strong> method gives us negative counts<\/strong> too but the – operator doesn\u2019t<\/strong>. Now we can add c2<\/strong> back to c1<\/strong>.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> c1.__add__(c2)<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘D’: 4, ‘A’: 3, ‘B’: 2, ‘E’: 2, ‘C’: 1})<\/div>\n
    We can \u2018and\u2019<\/strong> and \u2018or\u2019<\/strong> counters.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> c1&c2<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘A’: 1, ‘E’: 1})<\/div>\n
    Code:<\/strong><\/p>\n
    >>> c1|c2<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘D’: 5, ‘F’: 4, ‘B’: 3, ‘A’: 2, ‘C’: 1, ‘E’: 1})<\/div>\n
    But we cannot XOR<\/strong> them.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> c1^c2<\/pre>\n
    Output:<\/strong><\/p>\n
    Traceback (most recent call last):
    \n\u00a0\u00a0<\/span>File “<pyshell#50>”, line 1, in <module>
    \n\u00a0\u00a0\u00a0\u00a0<\/span>c1^c2
    \nTypeError: unsupported operand type(s) for ^: ‘Counter’ and ‘Counter’<\/div>\n
    We can also use the unary<\/strong> – operator<\/strong> with a counter<\/strong>.<\/p>\n
    Code:<\/strong><\/p>\n
    >>> c1<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘A’: 1, ‘E’: 1, ‘C’: 0, ‘B’: -1, ‘D’: -1, ‘F’: -4})<\/div>\n
    Code:<\/strong><\/p>\n
    >>> -c1<\/pre>\n
    Output:<\/strong><\/p>\n
    Counter({‘F’: 4, ‘B’: 1, ‘D’: 1})<\/div>\n
    B<\/strong>, D<\/strong>, and F<\/strong> had negative counts<\/strong> so now their counts<\/strong> are positive<\/strong>. And it doesn\u2019t<\/strong> show the counts<\/strong> for A<\/strong>, C<\/strong>, and E<\/strong> because they are negative<\/strong>.<\/p>\n
    Summary<\/h3>\n
    In this Python<\/strong> counter<\/strong> tutorial, we learned about counters<\/strong> in Python<\/strong>. We saw how to create<\/strong> them, update<\/strong> them, access elements<\/strong>, access counts<\/strong>, reassign counts<\/strong>, get the most common values<\/strong> and perform counter arithmetic<\/strong>.<\/p>\n","protected":false},"excerpt":{"rendered":"
    In this article, we will talk about Counter in Python. This is a class in the collections module. Other classes are defaultdict, OrderedDict and namedtuple. Let’s first discuss the concept of Python counter. What...<\/p>\n","protected":false},"author":1,"featured_media":76077,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1053],"tags":[1514,1515,1516,1517,1518,1519,1520,1521],"class_list":["post-75892","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-python","tag-accessing-counts-in-python-counter","tag-accessing-elements-in-python-counter","tag-counter-in-python","tag-creating-a-python-counter","tag-python-counter","tag-python-counter-arithmetic","tag-reassigning-counts-in-python-counter","tag-updating-a-counter-in-python"],"yoast_head":"\nPython Counter - Learn to create, access, and assign counters in Python - TechVidvan<\/title>\n<meta name=\"description\" content=\"Learn the full concept of counters in Python. 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