This is a page from my book, Functional Programming, Simplified
Next, let’s see if we can use a filtering clause inside of a for expression with the Sequence code we have so far.
When I paste the current Sequence class and this code into the Scala REPL:
val ints = Sequence(1,2,3,4,5)
val res = for {
i <- ints
if i > 2
} yield i*2
I see the following error message:
This is a page from my book, Functional Programming, Simplified
Getting Sequence to work as a generator in a simple for loop was cool, but does adding foreach let Sequence also work when I add yield? Let’s see.
When I paste this code into the REPL:
val ints = Sequence(1,2,3)
for {
i <- ints
} yield i*2
I see this error message:
This is a page from my book, Functional Programming, Simplified
So far I have this Sequence class:
case class Sequence[A](initialElems: A*) {
private val elems = scala.collection.mutable.ArrayBuffer[A]()
// initialize
elems ++= initialElems
}
With that code I can create new Sequence instances like this:
val strings = Sequence("a", "b", "c")
val nums = Sequence(1, 2, 3, 4, 5)
Next, I’ll modify Sequence so I can use it as a generator in a for loop.
This is a page from my book, Functional Programming, Simplified
The best way I know to demonstrate how the Scala for expression works is for us to build our own collection class.
To keep things simple I’m going to create a custom class as a “wrapper” around an existing Scala collection class. The reason for this is that I want you to focus on the effects that writing map, flatMap, withFilter, and foreach methods have on how the class works in a for expression — not on writing the gory internals of a collection class.
As a quick note today, to shuffle/randomize a list in Scala, use this technique:
scala.util.Random.shuffle(List(1,2,3,4))
Here’s what this approach looks like in the Scala REPL:
Besides having a bad memory, I haven’t been able to work with Scala much recently, so I’ve been putting together this list of for loop examples.
This page is a work in progress, and as of tonight I haven’t tested some of the examples, but ... if you’re looking for some Scala for loop examples — technically called a for-comprehension or for-expression — I hope these examples are helpful.
This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 12.10, “How to list subdirectories beneath a directory in Scala.”
You want to generate a list of subdirectories in a given directory.
Use a combination of the Java File class and Scala collection methods:
This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 3.11, “How to use pattern matching in Scala match expressions.”
You need to match one or more patterns in a match expression, and the pattern may be a constant pattern, variable pattern, constructor pattern, sequence pattern, tuple pattern, or type pattern.
This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 3.3, “How to use a 'for' loop with embedded 'if' statements (guards).”
You want to add one or more conditional clauses to a for loop, typically to filter out some elements in a collection while working on the others.
Add an if statement after your generator, like this:
This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 3.2, “How to use Scala for loops with multiple counters.”
You want to create a loop with multiple counters, such as when iterating over a multi‐dimensional array.
You can create a for loop with two counters like this: