If you have a collection of numbers that you want to sum or multiply together, Scala’s collection classes all define both a sum
operation and a product
operation:
Array  sum 
product 

List  sum 
product 
Seq  sum 
product 
Set  sum 
product 
But what if, for some reason, you want to implement sum
or product
yourself? Well, it turns out that sum
and product
are both just special cases of a more general operation that the collection modules also define: reduce. The reduce
operation takes all the elements in a collection and combines them in some way to produce a single value. It uses a binary operation to combine the first two values. Then it takes the result of that first combination and combines it with the next element in the collection, and then the next, and so on through the end of the collection.
Here’s an illustration of how to use reduce
to implement a product
operation:
The code looks like this:
val product = List(2,5,3,6) reduce { (x,y) => x * y } // product: Int = 180
You can be even more succinct by employing the underscore, Scala’s shorthand for the lambda’s input arguments:
val product = List(2,5,3,6).reduce(_*_) // product: Int = 180
Another special case of reduce
is mkString
, e.g., List.mkString
—also known as a join
operation—which allows you to concatenate a sequence of values together into a string with a separator between each value. Here’s how it works:
See how similar it is to the product
operation above? If you should want to implement mkString
yourself, you could use reduce
like this:
val joined = List("do","mi","sol","do") reduce { (x,y) => x + "" + y } // joined: String = domisoldo
Or again, with the shorthand:
val joined = List("do","mi","sol","do"). reduce(_ + "" + _) // joined: String = domisoldo
Here is the documentation on reduce
in each of the collection classes:
Now a few of caveats about reduce
:

You cannot use
reduce
on an empty collection. You will get an exception if you do, so make sure you check to make sure your collection is not empty before you pass it toreduce
. (Scala provides an alternative,reduceOption
, that does not throw an exception, but represents the result as anOption
.) 
The result of a
reduce
operation is always the same type as the elements in the collection. In other words, you can only reduce a collection of typeA
to a value of typeA
. You cannot reduce a collection of typeA
to a value of typeB
. 
Finally, order matters if the order of your binary operation matters. In other words, with addition and multiplication, order does not matter. That is,
a + b
is the same asb + a
. (Mathematicians say that such a function is commutative.) But with something like subtraction, order does matter. In other words,a âˆ’ b
does not necessarily produce the same result asb âˆ’ a
. So make sure that your binary operation applies the reduction to its operands in the correct order.
Having noted these caveats, next week, we will cover the fold
operation. It operates almost the same way as reduce
in that it walks a collection, applying a binary operation to each element. The fold
operation cannot help us with that last point—order still matters—but it can handle an empty collection and, if necessary, produce a result that is of a type different from the type of the source collection’s elements.
2 replies on “Scala Saturday – The reduce Method”
Thanks for you post! I like your explanation with images =) . Please, I would like sum of elements in list of list:
val list1 = List((1,2),(3,4),(5,6))
I would like a new list with 3 element:
Result: (1+2, 3+4, 5+6)ï»¿
Greetings from Peru!!!
Hi, @EJayo.
Here’s what you’re after:
The
reduce
operation is not what you want here becausereduce
combines all of the elements together. The operation you described does not combine all the elements, but rather it modifies each element. That’s not a job forreduce
, butmap
.Hope this helps!