Python reduce()

Description

The reduce(fun,seq) function is used to apply a particular function passed in its argument to all of the list elements mentioned in the sequence passed along.This function is defined in “functools” module.

Working :

At first step, first two elements of sequence are picked and the result is obtained.
Next step is to apply the same function to the previously attained result and the number just succeeding the second element and the result is again stored.
This process continues till no more elements are left in the container.
The final returned result is returned and printed on console.

reduce() 函数会对参数序列中元素进行累积。

函数将一个数据集合（链表，元组等）中的所有数据进行下列操作：用传给 reduce 中的函数 function（有两个参数）先对集合中的第 1、2 个元素进行操作，得到的结果再与第三个数据用 function 函数运算，最后得到一个结果。

Example

# python code to demonstrate working of reduce()

# importing functools for reduce()

import functools

# initializing list

lis = [ 1 , 3, 5, 6, 2, ]

# using reduce to compute sum of list

print ("The sum of the list elements is : ",end="")

print (functools.reduce(lambda a,b : a+b,lis))

# using reduce to compute maximum element from list

print ("The maximum element of the list is : ",end="")

print (functools.reduce(lambda a,b : a if a > b else b,lis))

Output:

The sum of the list elements is : 17
The maximum element of the list is : 6

Detail

Using Operator Functions

reduce() can also be combined with operator functions to achieve the similar functionality as with lambda functions and makes the code more readable.

# python code to demonstrate working of reduce() 
# using operator functions 
  
# importing functools for reduce() 
import functools 
  
# importing operator for operator functions 
import operator 
  
# initializing list 
lis = [ 1 , 3, 5, 6, 2, ] 
  
# using reduce to compute sum of list 
# using operator functions 
print ("The sum of the list elements is : ",end="") 
print (functools.reduce(operator.add,lis)) 
  
# using reduce to compute product 
# using operator functions 
print ("The product of list elements is : ",end="") 
print (functools.reduce(operator.mul,lis)) 
  
# using reduce to concatenate string 
print ("The concatenated product is : ",end="") 
print (functools.reduce(operator.add,["holy","fucking","shit"])) 

Output

The sum of the list elements is : 17
The product of list elements is : 180
The concatenated product is : holyfuckingshit

reduce() vs accumulate()

Both reduce() and accumulate() can be used to calculate the summation of a sequence elements. But there are differences in the implementation aspects in both of these.

reduce() is defined in “functools” module, accumulate() in “itertools” module.
reduce() stores the intermediate result and only returns the final summation value. Whereas, accumulate() returns a iterator containing the intermediate results. The last number of the iterator returned is summation value of the list.
reduce(fun,seq) takes function as 1st and sequence as 2nd argument. In contrast accumulate(seq,fun) takes sequence as 1st argument and function as 2nd argument.

# python code to demonstrate summation  
# using reduce() and accumulate() 
  
# importing itertools for accumulate() 
import itertools 
  
# importing functools for reduce() 
import functools 
  
# initializing list  
lis = [ 1, 3, 4, 10, 4 ] 
  
# priting summation using accumulate() 
print ("The summation of list using accumulate is :",end="") 
print (list(itertools.accumulate(lis,lambda x,y : x+y))) 
  
# priting summation using reduce() 
print ("The summation of list using reduce is :",end="") 
print (functools.reduce(lambda x,y:x+y,lis)) 

Output:

The summation of list using accumulate is :[1, 4, 8, 18, 22]
The summation of list using reduce is :22