reduce vs foldLeft A big big difference, not mentioned in any other stackoverflow answer relating to this topic clearly, is that reduce should be given a commutative monoid, i.e. an operation that is both commutative and associative. This means the operation can be parallelized. This distinction is very important for Big Data / MPP / … Read more
You can transfer a fold into an infix operator notation (writing in between): This example fold using the accumulator function x fold x [A, B, C, D] thus equals A x B x C x D Now you just have to reason about the associativity of your operator (by putting parentheses!). If you have a … Read more
How folds differ seems to be a frequent source of confusion, so here’s a more general overview: Consider folding a list of n values [x1, x2, x3, x4 … xn ] with some function f and seed z. foldl is: Left associative: f ( … (f (f (f (f z x1) x2) x3) x4) …) … Read more
In addition to what Lee said, you can define reduce in terms of fold, but not (easily) the other way round: let reduce f list = match list with | head::tail -> List.fold f head tail | [] -> failwith “The list was empty!” The fact that fold takes an explicit initial value for the … Read more
The Pythonic way of summing an array is using sum. For other purposes, you can sometimes use some combination of reduce (from the functools module) and the operator module, e.g.: def product(xs): return reduce(operator.mul, xs, 1) Be aware that reduce is actually a foldl, in Haskell terms. There is no special syntax to perform folds, … Read more
The recursion for foldr f x ys where ys = [y1,y2,…,yk] looks like f y1 (f y2 (… (f yk x) …)) whereas the recursion for foldl f x ys looks like f (… (f (f x y1) y2) …) yk An important difference here is that if the result of f x y can … Read more
fold takes an initial value, and the first invocation of the lambda you pass to it will receive that initial value and the first element of the collection as parameters. For example, take the following code that calculates the sum of a list of integers: listOf(1, 2, 3).fold(0) { sum, element -> sum + element … Read more
In general, all 6 fold functions apply a binary operator to each element of a collection. The result of each step is passed on to the next step (as input to one of the binary operator’s two arguments). This way we can cumulate a result. reduceLeft and reduceRight cumulate a single result. foldLeft and foldRight … Read more
Few things to mention here, before giving the actual answer: Your question doesn’t have anything to do with left, it’s rather about the difference between reducing and folding The difference is not the implementation at all, just look at the signatures. The question doesn’t have anything to do with Scala in particular, it’s rather about … Read more