reduce() function (from the functools module) applies a function cumulatively to the elements of an iterable and returns a single final value. It processes elements step-by-step, combining two elements at a time until only one result remains.

Example: In this example, reduce() combines all strings in a list into one sentence.

from functools import reduce

a = ["Geeks", "for", "Geeks"]

r = reduce(lambda result, current: result + " " + current, a)

print(r)

Geeks for Geeks

Explanation:

• result stores the value accumulated from the previous step.
• current represents the next element from the list.
• First, "Geeks" and "for" are combined to produce "Geeks for".

Syntax

reduce(function, iterable[, initializer])

Parameters:

• function: A function that takes two arguments and returns a single value.
• iterable: The sequence to be reduced (list, tuple, etc.).
• initializer (optional): A starting value that is placed before first element.

Returns: Returns a single final value after processing all elements.

Examples

Example 1: Here, the code adds all numbers in a list using reduce(). The function combines two numbers at a time.

from functools import reduce

a = [2, 4, 6, 8]

r = reduce(lambda result, current: result + current, a)

print(r)

20

Explanation:

• result represents the sum calculated up to the current step.
• current represents the next number from the list.
• First, 2 + 4 gives 6.
• Then, 6 + 6 gives 12.
• Finally, 12 + 8 gives 20.
• Therefore, the final result is 20.

Example 2: Here, the code finds the largest number from a list using reduce().

from functools import reduce

a = [5, 9, 3, 12, 7]

r = reduce(
    lambda result, current: result if result > current else current,
    a
)

print(r)

12

Explanation:

• lambda x, y: x if x > y else y compares two values at each step.
• Keeps the larger one, resulting in the maximum value 12.

Example 3: This example uses functools.reduce() with built-in functions from operator module to perform sum, product and string concatenation on lists.

import functools
import operator
a = [1, 3, 5, 6, 2]

print(functools.reduce(operator.add, a))
print(functools.reduce(operator.mul, a)) 
print(functools.reduce(operator.add, ["geeks", "for", "geeks"]))

17
180
geeksforgeeks

Explanation:

• functools.reduce(operator.add, a) adds all numbers in the list 1+3+5+6+2 = 17.
• functools.reduce(operator.mul, a) multiplies all numbers in the list 1*3*5*6*2 = 180.
• functools.reduce(operator.add, ["geeks", "for", "geeks"]) concatenates all strings in list "geeksforgeeks"

Difference Between reduce() and accumulate()

accumulate() function (from itertools) and reduce() both apply a function cumulatively to items in a sequence. However, accumulate() returns an iterator of intermediate results, while reduce() returns only final value.

Example: This code demonstrates how accumulate() from itertools module works it performs cumulative operations and returns all intermediate results instead of just a single final value.

from itertools import accumulate
from operator import add

a = [1, 2, 3, 4, 5]
res = accumulate(a, add)
print(list(res))

[1, 3, 6, 10, 15]

Explanation: accumulate(a, add) - Adds elements cumulatively:

• Step 1: 1
• Step 2: 1 + 2 = 3
• Step 3: 3 + 3 = 6
• Step 4: 6 + 4 = 10
• Step 5: 10 + 5 = 15

Let's understand the difference between accumulate() and reduce() more clearly with the help of the table below: