What's difference between reduce and fold ?

# announcements

[Yogesh Gosavi]

What's difference between reduce and fold ?

[hudsonb]

fold is much more general. It allows you to choose what the type of the accumulator is and its initial value

[hudsonb]

With reduce accumulator type is fixed, it is always the same type as the element type.

[hudsonb]

If for example you want to find the max value in an IntArray, you could use reduce because you are reducing to the same type (Int)

[hudsonb]

If instead you wanted to count how many times each value occurs in the IntArray, you would have to use fold cause you are reducing to a Map<Int, Int> (or similar structure) and not just simply an Int

[Yogesh Gosavi]

So basically fold usage that initial value twice?

[Shawn]

What do you mean by “usage that initial value twice”

[hudsonb]

No... not sure what you mean. It's really all about the accumulator. In fold, you may use any type you want for an accumulator. That's not true in reduce, the accumulator type is fixed.

[nekomatic]

fold relays on a accumulator, its execution is sequential, reduce relays on the concept of identity and associativity and does not require operations to be executed in any particular order

[nekomatic]

so fold scans through your collection and appends each value to the accumulator, reduce picks any two adjacent values and applies the operation >>replacing<< (but not really) the values with the result then repeats. The difference is that reduce can be done in parallel, fold not really.

[nekomatic]

simply speaking fold is a line of operations, reduce is a tree