Python|计算作为列表的字典值中的项数

原文:https://www . geesforgeks . org/python-计数-字典中的项目数-值-即列表/

在 Python 中，字典是一个无序的、可变的、有索引的集合。字典是用花括号写的，它们有键和值。它用于散列特定的密钥。

一本字典有多对key:value 。可以有多个对，其中对应于一个键的值是一个列表。为了检查该值是否是一个列表，我们使用 Python 中内置的[isinstance()](https://www.geeksforgeeks.org/python-isinstance-method/)方法。

isinstance()方法取两个参数:

object - object to be checked
classinfo - class, type, or tuple of classes and types


无论对象是否是给定类的实例，它都返回一个布尔值。

让我们讨论不同的方法来计算字典值(即列表)中的项数。

方法#1 使用in 运算符

# Python program to count number of items
# in a dictionary value that is a list.
def main():
    # defining the dictionary
    d = {'A' : [1, 2, 3, 4, 5, 6, 7, 8, 9],
        'B' : 34,
        'C' : 12,
        'D' : [7, 8, 9, 6, 4] }
    # using the in operator
    count = 0
    for x in d:
        if isinstance(d[x], list):
            count += len(d[x])
    print(count)
# Calling Main    
if __name__ == '__main__':
    main()


Output:

14


方法 2: 使用列表理解

# Python program to count number of items
# in a dictionary value that is a list.
def main():
    # defining the dictionary
    d = {'A' : [1, 2, 3, 4, 5, 6, 7, 8, 9],
        'B' : 34,
        'C' : 12,
        'D' : [7, 8, 9, 6, 4] }
    # using list comprehension
    print(sum([len(d[x]) for x in d if isinstance(d[x], list)]))
if __name__ == '__main__':
    main()


Output:

14


方法 3: 使用 dict.items()

# Python program to count number of items
# in a dictionary value that is a list.
def main():
    # defining the dictionary
    d = { 'A' : [1, 2, 3, 4, 5, 6, 7, 8, 9],
        'B' : 34,
        'C' : 12,
        'D' : [7, 8, 9, 6, 4] }
    # using dict.items()
    count = 0
    for key, value in d.items():
        if isinstance(value, list):
              count += len(value)
    print(count)
if __name__ == '__main__':
    main()


Output:

14


方法#4: 使用enumerate()

# Python program to count number of items
# in a dictionary value that is a list.
def main():
    # defining the dictionary
    d = {'A' : [1, 2, 3, 4, 5, 6, 7, 8, 9],
        'B' : 34,
        'C' : 12,
        'D' : [7, 8, 9, 6, 4] }
    # using enumerate()
    count = 0
    for x in enumerate(d.items()):
        # enumerate function returns a tuple in the form
        # (index, (key, value)) it is a nested tuple
        # for accessing the value we do indexing x[1][1]
        if isinstance(x[1][1], list):
            count += len(x[1][1])
    print(count)
if __name__ == '__main__':
    main()


Output:

14