How to use for cycles in functional programming?

Question

Good evening, I would like to ask the community for help to understand the following, I have been experimenting with the functional programming method, and I have found basic functions such as map () reduce () filter (), but what I would like to know is what happens with the for cycles within functional programming.

For example, in this code I have to perform many for cycles to find a result:

test = {'one':1,'two':{'two-two':{'two-two-two':222,'three-three-three':333}},'three':3}

for i in test.items():
    if i[0]=="two":
        for s in i[1].items():
            if s[0] == "two-two":
                for a in s[1].items():
                    print(a[1])

Basically I do not know what would apply in this case, it would be a map () or a filter () or something else I hope you can help me

Answer 1

I got, here you have used functional programming term to introduce the use of map(), filter() and reduce() in your code but you should not use it here for this scenario as functional programming refers to the implementation of your problem by using functions (modular design).

In your case, you cannot use filter(), reduce() to get the expected result as these functions does not provide you a flexible way to control the program's control.

You can try something like this but I don't want you to use that, you may get None if the condition is not satisfied in case of map(). Using filter() / reduce() does not make sense.

Here, I have tried to make it working as you expect.

>>> def f(tup):
...     items = []
...     if tup[0] == 'two':
...         for s in tup[1].items():
...             if s[0] == "two-two":
...                 for a in s[1].items():
...                     print(a[1])
...                     items.append(a[1])
...         return items
...     else:
...         return None
...
>>> test
{'one': 1, 'two': {'two-two': {'two-two-two': 222, 'three-three-three': 333}},
'three': 3}
>>>
>>> out = list(map(f, test.items()))
222
333
>>> out
[None, [222, 333], None]
>>>
>>> out[1]
[222, 333]
>>>

map(), filter() are bascially used to work on iterables like list, tuple, dictionary, set etc. and produce another iterables by performaing opeartions on items. filter() allows us to filter data (picking even numbers from a list).

reduce() is bascially used to work on iterables and reducing them to a single value (e.g. getting sum of list of numbers).

Initializations

>>> l = [9, 4, 3, 2, 1]
>>> d = {'first': 'Rishikesh', 'last': 'Agrawani'}
>>> t = (3, 4, 5)
>>>

Using map()

>>> # Using map()
...
>>> map_obj = map(lambda n: n ** 2, l)
>>> map_obj
<map object at 0x0000016DAF88B6D8>
>>>
>>> squares = list(map_obj)  # list(map(lambda n: n ** 2, l))
>>> squares
[81, 16, 9, 4, 1]
>>>
>>> details = {k + '-name': v for k, v in d.items()}
>>> details
{'first-name': 'Rishikesh', 'last-name': 'Agrawani'}
>>>
>>> details = dict(map(lambda tup: (tup[0] + '_name', tup[1]), d.items()))
>>> details
{'first_name': 'Rishikesh', 'last_name': 'Agrawani'}
>>>

Using filter()

>>> # Using filter() - let's filter even numbers from list
...
>>> filter_obj = filter(lambda n: n % 2 == 0, l)
>>> filter_obj
<filter object at 0x0000016DAF88B908>
>>>
>>> evens = list(filter_obj)
>>> evens
[4, 2]
>>>

Using reduce()

>>> # Using reduce() - finding sum of al numbers in a list
... # i.e. reducing list of values to a single value
...
>>> from functools import reduce
>>>
>>> total = reduce(lambda n1, n2: n1 + n2, l)
>>> total
19
>>>

Answer 2

You can use a forEach() function to apply some callback function to each item in a collection

A basic implementation would look like this:

   function forEach(callback) {
      for(index=0;index<len_of_collection;index++) {
         callback(collection[index], index, collection);
      }
   }

You would need to make sure your collection implements this method for you to be able to call it from the collection like so:

some_collection.forEach(myCustomCallback)

Or inline:

some_collection.forEach(item => console.log(item))

Please excuse the JavaScript, I know the question was in Python but the concept is not language specific.

EDIT: https://www.geeksforgeeks.org/python-map-function/

map() function returns a list of the results after applying the given function to each item of a given iterable (list, tuple etc.)

Your example does not show modification. Therefore, map symantically would not be correct. If what you want is to apply some function without modification, explore options similar to forEach.