Python 找到数组中最不频繁的元素

在本文中，我们将学习使用Python程序获取数组中最不频繁的元素。

使用的方法

以下是完成此任务的各种方法:

• 使用sort()函数（原生的方法）

• 使用Hashing

• 使用Counter()函数

方法1：使用sort()函数（原生的方法）

运行两个循环是一个简单的解决办法。外部循环逐一选择每个元素。内部循环计算所选元素的频率，并将其与到目前为止达到的最小值进行比较。此解决方案的时间复杂度为O(n^2)。

排序是一个更好的解决方案。首先对数组进行排序，然后线性遍历数组，按照下面的代码可以找到最不频繁的元素。

示例

以下程序使用sort()函数返回输入数组/列表中最不频繁的元素。

# creating a function for returning the least frequent element
def leastFrequencyElement(inputList, listLength):
   # sorting the given input list
   inputList.sort()
   # Setting the minimum frequency(minimum count) as length of list + 1
   minimumCount = listLength + 1
   resultElement = -1
   # Variable to count the frequency
   currentCount = 1
   # Looping from 1st index to the length of the list
   for k in range(1, listLength):
      # Check if the previous element is equal to the current element
      if (inputList[k] == inputList[k - 1]):
         #Increase the frequency of the current element by 1
            currentCount = currentCount + 1
      else:
         # Check if the current Count is less than the minimum Count
            if (currentCount < minimumCount):
               #If it is true then set the minimum count as current Count value
                  minimumCount = currentCount
                  # Store this previous element as the least frequent element
                  resultElement = inputList[k - 1]
            # Resetting the current Count as 1
            currentCount = 1
   # checking whether the last element is less frequent
      if (currentCount < minimumCount):
         minimumCount = currentCount
         resultElement = inputList[listLength - 1]
   # returning the least frequent element
      return resultElement
# input list
inputList = [6, 5, 5, 4, 4, 2, 2, 2, 1, 1]
print("Given List is:", inputList)
# getting list length
listLength = len(inputList)
# calling the leastFrequencyElement function by passing
# input list and list length as arguments
print("Least frequent element in the input list is:")
print(leastFrequencyElement(inputList, listLength))

输出

执行上述程序后，将生成以下输出 –

Given List is: [6, 5, 5, 4, 4, 2, 2, 2, 1, 1]
Least frequent element in the input list is:
6

方法2：使用哈希

应用 哈希 是一种 有效的解决方案。 在这种方法中，我们创建一个哈希表并存储元素及其频率计数作为键值对。然后，遍历哈希表并打印具有最小值的键。

示例

以下程序使用哈希计算返回输入数组/列表中出现频率最低的元素 –

# creating a function for returning the least frequent element
# by accepting the input list and input list length as arguments
def leastFrequencyElement(inputList, listLength):
   # Take a dictionary as a hash table
   HashTable = dict()
   # Loop in the given list
   for k in range(listLength):
      # Check if the list element in the hashtable
         if inputList[k] in HashTable.keys():
            # If it is present then increase the frequency by 1
               HashTable[inputList[k]] += 1
         else:
            # Else create a new key with the frequency as 1
               HashTable[inputList[k]] = 1
   #  Setting the minimum frequency(minimumCount) as length of list + 1
   minimumCount = listLength + 1
   resultElement = -1
   # Iterating the hashtable(dictionary)
   for k in HashTable:
      # Check if the minimum count is greater or equal to the frequency of the key
         if (minimumCount >= HashTable[k]):
            #If it is true then this key will be the current least frequent element
               resultElement = k
            # Set the minimum count as the current key frequency value
               minimumCount = HashTable[k]

   # returning the least frequent element
   return resultElement
# input list
inputList = [3, 10, 3, 1, 5, 5, 4, 4, 2, 2, 2, 1, 1]
# getting list length
listLength = len(inputList)
print("Given List is:", inputList)
print("Least frequent element in the input list is:")
# calling the leastFrequencyElement function by passing
# input list and list length as arguments
print(leastFrequencyElement(inputList, listLength))

输出

在执行上述程序时，将会生成以下输出 –

Given List is: [3, 10, 3, 1, 5, 5, 4, 4, 2, 2, 2, 1, 1]
Least frequent element in the input list is:
10

方法3：使用Counter()函数

Counter() 函数（返回以键值对形式表示的单词频率）

示例

以下程序使用Counter()函数返回输入数组/列表中出现频率最低的元素 –

# importing Counter from the collections module
from collections import Counter
# creating a function for returning the least frequent element
# by accepting the input list and input list length as arguments
def leastFrequencyElement(inputList, listLength):
   # getting the frequency of all elements of a list
   hashTable = Counter(inputList)
   # Setting the minimum frequency(minimumCount) as length of list + 1
   minimumCount = listLength + 1
   # Variable to store the resultant least frequent element
   resultElement = -1
   # iterating the hash table
   for k in hashTable:
         # Check if the minimum count is greater or equal to the frequency of the key
            if (minimumCount >= hashTable[k]):
               # If it is true then this key will be the current least frequent element
                  resultElement = k
            # Set the minimum count as the current key frequency value
            minimumCount = hashTable[k]
# returning the least frequent element
   return resultElement
# input list
inputList = [3, 10, 3, 1, 5, 5, 4, 4, 2, 2, 2, 1, 1]
# getting list length
listLength = len(inputList)
print("The Given List is:", inputList)
print("Least frequent element in the input list is:")
# calling the leastFrequencyElement function by passing
# input list and list length as arguments
print(leastFrequencyElement(inputList, listLength))

输出

在执行时，上述程序将生成以下输出 –

The Given List is: [3, 10, 3, 1, 5, 5, 4, 4, 2, 2, 2, 1, 1]
Least frequent element in the input list is:
10

结论

本文中，我们学习了三种不同的方法来找到给定列表中出现频率最低的元素。我们还学习了如何在Python中执行哈希操作，可以用它来获取所有独特的元素、在O(1)时间进行搜索等等。我们学习了如何使用Counter()函数来执行哈希操作。