Steps

1. How to initialize a sequence of donut prices

The code below show how to initialize a Sequence of type Double to represent donut prices..

println("Step 1: How to initialize a sequence of donut prices")
val donutPrices: Seq[Double] = Seq(1.5, 2.0, 2.5)
println(s"Elements of donutPrices = $donutPrices")

You should see the following output when you run your Scala application in IntelliJ:

Step 1: How to initialize a sequence of donut prices
Elements of donutPrices = List(1.5, 2.0, 2.5)

2. How to find the sum of the elements using reduceLeft function

The code below shows how to use the reduceLeft method to find the sum of the Sequence of type Double from Step 1, that is, the sum of donut prices.

println("\nStep 2: How to find the sum of the elements using reduceLeft function")
val sum: Double = donutPrices.reduceLeft(_ + _)
println(s"Sum of elements from donutPrices = $sum")

You should see the following output when you run your Scala application in IntelliJ:

Step 2: How to find the sum of the elements using reduceLeft function
Sum of elements from donutPrices = 6.0

3. How to find the sum of elements using reduceLeft function explicitly

In Step 2, we used the reduceLeft method along with the wildcard operator. The example below is similar to the one in Step 2 except that we are using the reduce method explicitly to capture the operator function.

println("\nStep 3: How to find the sum of elements using reduceLeft function explicitly")
val sum1: Double = donutPrices.reduceLeft((a, b) => a + b)
println(s"Sum of elements from donutPrices by calling reduceLeft function explicitly= $sum1")

You should see the following output when you run your Scala application in IntelliJ:

Step 3: How to find the sum of elements using reduceLeft function explicitly
Sum of elements from donutPrices by calling reduceLeft function explicitly= 6.0

4. How to find the cheapest donut using reduceLeft function

We can also use the reduceLeft method to find the minimum element in the Sequence of donut prices as shown below.

println("\nStep 4: How to find the cheapest donut using reduceLeft function")
println(s"Cheapest donut price = ${donutPrices.reduceLeft(_ min _)}")

You should see the following output when you run your Scala application in IntelliJ:

Step 4: How to find the cheapest donut using reduceLeft function
Cheapest donut price = 1.5

5. How to find the most expensive donut using reduceLeft function

Similar to Step 4, we can use the reduceLeft method to find the maximum element in the Sequence of donut prices.

println("\nStep 5: How to find the most expensive donut using reduceLeft function")
println(s"Most expensive donut price = ${donutPrices.reduceLeft(_ max _)}")

You should see the following output when you run your Scala application in IntelliJ:

Step 5: How to find the most expensive donut using reduceLeft function
Most expensive donut price = 2.5

6. How to initialize a Sequence of donuts

The reduce method is of course not limited to Sequence of type Double. Let's create a Sequence of type Double to represent a Sequence of donuts.

println("\nStep 6: How to initialize a Sequence of donuts")
val donuts: Seq[String] = Seq("Plain Donut", "Strawberry Donut", "Glazed Donut")
println(s"Elements of donuts = $donuts")

You should see the following output when you run your Scala application in IntelliJ:

Step 6: How to initialize a Sequence of donuts
Elements of donuts = List(Plain Donut, Strawberry Donut, Glazed Donut)

7. How to concatenate the elements from the sequence using reduceLeft function

In the mkString method tutorial, we showed how to conveniently create String representation of collection. The code below will instead use the reduceLeft method to concatenate the String elements from the Sequence of donuts.

println("\nStep 7: How to concatenate the elements from the sequence using reduceLeft function")
println(s"Elements of donuts sequence concatenated = ${donuts.reduceLeft((left, right) => left + ", " + right)}")

You should see the following output when you run your Scala application in IntelliJ:

Step 7: How to concatenate the elements from the sequence using reduceLeft function
Elements of donuts sequence concatenated = Plain Donut, Strawberry Donut, Glazed Donut

8. How to declare a value function to concatenate donut names

Let's redo the example from Step 7 and show how to declare a value function which will encapsulate combining donut elements of type String.

println("\nStep 8: How to declare a value function to concatenate donut names")
val concatDonutNames: (String, String) => String = (left, right) => {
 left + ", " + right
}
println(s"Value function concatDonutNames = $concatDonutNames")

You should see the following output when you run your Scala application in IntelliJ:

Step 8: How to declare a value function to concatenate donut names
Value function concatDonutNames = <function2>

9. How to pass a function to reduceLeft function

With the value function from Step 8, we can now pass it through to the reduceLeft method to concatenate our donut String elements as follows.

println("\nStep 9: How to pass a function to reduceLeft function")
println(s"Elements of donuts sequence concatenated by passing function to the reduceLeft function = ${donuts reduceLeft concatDonutNames}")

You should see the following output when you run your Scala application in IntelliJ:

Step 9: How to pass a function to reduceLeft function
Elements of donuts sequence concatenated by passing function to the reduceLeft function = Plain Donut, Strawberry Donut, Glazed Donut

10. How to use reduceLeftOption to avoid exception if the collection is empty

You should note that calling the reduceLeft method on an empty collection will throw an exception. To avoid an exception being thrown, you can make use of the convenient reduceLeftOption method instead.

println("\nStep 10: How to use reduceLeftOption to avoid exception if the collection is empty")
println(s"Using reduceLeftOption will NOT throw any exception = ${Seq.empty[String].reduceLeftOption(_ + ", " + _)}")

You should see the following output when you run your Scala application in IntelliJ:

Step 10: How to use reduceLeftOption to avoid exception if the collection is empty 
Using reduceLeftoption will NOT throw any exception = None

NOTE:

• If you are unsure on how to use option in Scala, feel free to visit our tutorial on using option.