CODEWITHSUNDEEP
python-3.xlistiterationreducefunctools
anantdark
anantdark

Reputation: 425

How can i store each iteration of a reduce function in a list?

In the below code, the output is 38, and i want a separate list with the output [34,36,38].

from functools import *
nums = [0, 34, 2, 2]
sum_num = reduce(lambda a, b : a+b, nums)

As the reduce function adds 0 and 34, i need to append this value in a separate list, now in the second iteration i need to get 34 + 2 appended to the list. and at last 38 will be appended to the list. what piece of code do i need to add to get the desired output?

Upvotes: 3

Views: 724

Answers (2)

Miriam
Miriam

Reputation: 2711

According to the docs, the reduce function is roughly equivalent to:

def reduce(function, iterable, initializer=None):
    it = iter(iterable)
    if initializer is None:
        value = next(it)
    else:
        value = initializer
    for element in it:
        value = function(value, element)
    return value

So, to get each sum as we move through the process, we can use the function:

def reduce(function, iterable):
    it = iter(iterable)
    value = next(it)
    values = []
    for element in it:
        value = function(value, element)
        values.append(value)
    return values

(Simplified since the initializer parameter is not used)

Upvotes: 0

Tim Peters
Tim Peters

Reputation: 70592

You need a different function. itertools.accumulate() generates all the intermediate results functools.reduce() produces under the covers:

>>> from itertools import accumulate
>>> nums = [0, 34, 2, 2]
>>> list(accumulate(nums))
[0, 34, 36, 38]

It uses addition by default. Or you can pass whatever other 2-argument function you want instead:

>>> list(accumulate(nums, lambda a, b: a + b)) # same as the default
[0, 34, 36, 38]
>>> list(accumulate(nums, lambda a, b: a + 2*b))
[0, 68, 72, 76]

If you don't want the 0 at the start, you'll have to get rid of that yourself; e.g.,

>>> f = accumulate(nums)
>>> next(f)  # throw out first result
0
>>> list(f)  # and make a list out of what remains
[34, 36, 38]

Upvotes: 6

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