I am trying to learn the functional programming way of doing things in python. I am trying to serialize a list of strings in python using the following code
S = ["geeks", "are", "awesome"]
reduce(lambda x, y: (str(len(x)) + '~' + x) + (str(len(y)) + '~' + y), S)
I am expecting:
5~geeks3~are7~awesome
But I am seeing:
12~5~geeks3~are7~awesome
Can someone point out why? Thanks in advance!
reduce function on each current iteration relies on previous item/calculation (the nature of all reduce routines), that's why you got 12 at the start of the resulting string: on the 1st pass the item was 5~geeks3~are with length 12 and that was used/prepended on next iteration.
Instead, you can go with simple consecutive approach:
lst = ["geeks", "are", "awesome"]
res = ''.join('{}~{}'.format(str(len(s)), s) for s in lst)
print(res) # 5~geeks3~are7~awesome
You need to add the initializer parameter - an empty string to the reduce() function. It will be the first argument passed to the lambda function before the values from the list.
from functools import reduce
S = ["geeks", "are", "awesome"]
reduce(lambda x, y: x + f'{len(y)}~{y}', S, '')
# 5~geeks3~are7~awesome
Equivalent to:
((('' + '5~geeks') + '3~are') + '7~awesome')
# 5~geeks3~are7~awesome
The reduce function is for aggregation. What you're trying to do is mapping instead.
You can use the map function for the purpose:
''.join(map(lambda x: str(len(x)) + '~' + x, S))
This returns:
5~geeks3~are7~awesome
pyton3.7+ using fstring.>>> S = ["geeks", "are", "awesome"]
>>> ''.join(f'{len(s)}~{s}' for s in S)
'5~geeks3~are7~awesome'
