In Python, dictionary is a collection which is unordered, changeable and indexed. Dictionaries are written with curly brackets, and they have keys and values. It is used to hash a particular key. A dictionary has multiple key:value pairs. There can be multiple pairs where value corresponding to a key is a list. To check that the value is a list or not we use the isinstance() method which is inbuilt in Python. isinstance() method takes two parameters:
object - object to be checked classinfo - class, type, or tuple of classes and types
It return a boolean whether the object is an instance of the given class or not. Let’s discuss different methods to count number of items in a dictionary value that is a list. Method #1 Using in operator
Python3
# 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() |
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Time Complexity: O(n), where n is the number of key-value pairs in the dictionary d. This is because the code iterates through each key-value pair in the dictionary using the for loop and for each key-value pair, it checks if the value is a list using the isinstance() function. If the value is a list, it increments the count by the length of the list. Finally, it prints the count.
Auxiliary Space: O(n), because it only uses a fixed amount of memory to store the dictionary d, the count, and the temporary variables x. It does not use any data structures that grow in proportion to the size of the input.
Method #2: Using list comprehension
Python3
# 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() |
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Time complexity: O(n), where n is the number of key-value pairs in the dictionary.
Auxiliary space: O(n), to store the keys and values in dictionary.
Method #3: Using dict.items()
Python3
# 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() |
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Method #4: Using enumerate()
Python3
# 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() |
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Method#5: Using values(),find() and type().
Python3
# 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] } # values() - to fetch values of dictionary count = 0 for x in list(d.values()): # type() - returns type of a variable type1=str(type(x)) # find() - returns position of a specified value if found else returns -1 if(type1.find('list')!=-1): count+=len(x) print(count)if __name__ == '__main__': main() |
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Method #6: Using Recursive method.
This function takes a dictionary d as its input, and recursively searches through all the values in the dictionary. If a value is a list, it adds the length of the list to the count. If a value is another dictionary, it calls the count_list_items function recursively on that dictionary and adds the result to the count.
Python3
def count_list_items(d): count = 0 for v in d.values(): if isinstance(v, list): count += len(v) elif isinstance(v, dict): count += count_list_items(v) return countdef 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] } count=count_list_items(d) print(count)if __name__ == '__main__': main() |
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The time complexity of this function is O(N), where N is the total number of elements in all the lists in the dictionary. This is because the function needs to examine each element in each list exactly once.
The space complexity of this function is also O(N), where N is the total number of elements in all the lists in the dictionary. This is because the function needs to keep track of the count, as well as the call stack for each level of recursion.
