# How To Use Reduce Function In Python

The aim of this pageđź“ťis to document the `reduce()`

from the standard `functools`

Python library. Reduce is a standard technique within functional programming - in other languages, it is known as `fold`

or `accumulate`

and typically combined with `map`

and `filter`

as brilliantly illustrated by prof. Abelson in the SICP Lecture on Streams.

## 1. DEFINE

- Repeatedly applies a function of two arguments to an interim accumulator value
- Each element of the series, in turn, updating (accumulating) the interim value with the result of the called function
- The initial value can be the first element in the input or an optional argument
- The final accumulated â€” or
*reduced*value is returned

## 2. EXAMPLE: SUMMATION OF A SEQUENCE

`from functools import reduce`

import operator

reduce(operator.add, [1, 2, 3, 4, 5])

numbers = [1, 2, 3, 4, 5]

accumulator = operator.add(numbers[0], numbers[1])

for item in numbers[2:]:

accumulator = operator.add(accumulator, item)

print(accumulator)

>>> 15

## 3. PRINTING OUT REDUCTION

`def mul(x, y):`

print(f"mul {x} {y}")

return x * y

reduce(mul, range(1, 10))

"""

>>> reduce(mul, range(1, 10))

mul 1 2

mul 2 3

mul 6 4

mul 24 5

mul 120 6

mul 720 7

mul 5040 8

mul 40320 9

362880

"""

## 4. EMPTY SEQUENCE RAISES A TYPE ERROR

- I am using the
`mul`

function defined above

`>>> reduce(mul, [])`

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: reduce() of empty sequence with no initial value

## 5. SINGLE-ITEM ELEMENTS NEVER CALL REDUCE

`>>> reduce(mul, [1])`

1

## 6. OPTIONAL: INITIAL VALUE ARG

- this value is conceptually added to the beginning of the input sequence
- it, therefore, serves as the first accumulator value
- this means it is returned if the input sequence is empty
- useful if not sure that the input will have any values

`>>> import operator`

>>> values = [1,2,3]

>>> reduce(operator.add, values, 0)

6

- take care when selecting the initial value â€” the correct value depends on the function you are applying
- i.e. do not take initial value
`0`

for multiplication :)

`>>> values = [1,2,3]`

>>> reduce(mul, values, 0)

mul 0 1

mul 0 2

mul 0 3

0

>>> reduce(mul, values, 1)

mul 1 1

mul 1 2

mul 2 3

6