chapter-13.5-reduce.md

Chapter 13.5: The reduce() Function

Table of Contents

• Chapter 13.5: The reduce() Function
  • Introduction to reduce function

Introduction to reduce function

The reduce function performs calculation on all elements by iterating on items of an iterable. For example, we can find out the product of all numbers of a list. The function reduce can be imported from functools builtin library.

The first parameter of the reduce function takes a function as a parameter that takes exactly 2 arguments. To reduce the iterable it first takes first 2 elements and performs operation. The result of them is then fed to the function again with another element until the iterable gets exhausted.

from functools import reduce

numbers = [1, 12, 30, 24, 8, 11, 15, ]
result = reduce(lambda x, y: x * y, numbers)
print(result)  # 11404800

Explanation of the above function:

function: lambda x, y: x * y

• step1: lambda => 1 * 12 = 12
• step2: lambda => 12 * 30 = 360
• step3: lambda => 360 * 24 = 8640
• step4: lambda => 8640 * 8 = 69120
• step5: lambda => 69120 * 11 = 760320
• step6: lambda => 760320 * 15 = 11404800

Note: We might mistakenly think that we could find out sum of squares with the reduce function by passing lambda as lambda x,y: x**2 + y**2 but it end up giving very large number since it finds out square of first 2 numbers and in the next iteration, it again finds out the square of the past result and add it with the square of the next occurrence.

Explanation:

from functools import reduce

numbers = [1, 2, 3, 4, 5]
result = reduce(lambda x, y: x ** 2 + y ** 2, numbers)
print(result)  # 1373609

here we might think that the reduce function does 1 + 4 + 9 + 16 + 25 but it does something like below:

[{(1 + 4) ** 2 + 9} ** 2 + 16] ** 2 + 25 which will be equal to 1373609

• step 1: lambda => 1 ** 2 + 2 ** 2 = 1 + 4 = 5
• step 2: lambda => 5 ** 2 + 3 ** 2 = 25 + 9 = 34
• step 3: lambda => 34 ** 2 + 4 ** 2 = 1156 + 16 = 1172
• step 4: lambda => 1172 ** 2 + 5 ** 2 = 1,373,584 + 25 = 1373609