So in Python sum([]) will yield 0, which is pretty important.
Whereas in ruby [].reduce(:+) will give nil, of course the ternary operator isn't a replacement, because:
(my_complicated_mapping).empty? ? 0 : (my_complicated_mapping).reduce(:+)
Will call my_complicated_mapping twice. Therefore the obvious method is:
res = my_complicated_mapping
res = (res.empty? ? 0 : res.reduce(:+))
I think there must be a neater way to do this though.
With reduce you can specify the initial value as the first parameter e.g.
my_complicated_mapping.reduce(0, :+)
then if the list is empty you'll get 0 instead of nil. You can check the different alternative ways of using reduce here.
or if using a block for reduce it would be:
my_complicated_mapping.reduce(0) { |sum, n| sum + n }
i.e. a single parameter with with initial value and supplying your block.
It's important to understand why reduce returns nil in the case of the empty array: If you don't specify an explicit initial value for memo, then the first element of the array is used as the initial value of memo, which of course doesn't exist in the empty case.
reduce instead of map at first was because the { |sum, i| } block was assigning the first value of the range to sum - which was not a practical behaviour, specifying a value to reduce appears to fix that. Although that's even more magical! Not sure why it was designed like that (e.g. (1..4).reduce(0) {} will yield 4 iterations, where as (1..4).reduce {} yields only 3 iterations with 1 assigned to sum.
foldl :: (a -> b -> a) -> a -> [b] -> a and foldl1 :: (a -> a -> a) -> [a] -> a, in Scala, they are GenTraversableOnce[+A].foldLeft[B](z: B)(op: (B, A) ⇒ B): B and TraversableOnce[+A].reduceLeft[B >: A](op: (B, A) ⇒ B): B. In Ruby, they are overloaded into one method. But still, they are two distinct operations: in the general case, the accumulator can be of a different type than the elements and the folding operator has type (ACC, EL) -> ACC. In the restricted case, the accumulator has the same type as(EL, EL) -> EL, plus, reducing an empty list is an error.You can use to_i to convert nil to 0
[].reduce(:+).to_i
# => 0
Incase Array contains float values, using to_f can help:
[].reduce(:+).to_f
# => 0.0
Why not give 0 as initial parameter? Like
my_complicated_mapping.reduce(0, :+)
nil.to_i #=> 0, so you could tackto_iontoreduce, but I wouldn't recommend it. Just do as @mikej suggests.