[Scala] For-Yield with Guards

mj park

2 min readJan 10, 2021

Highlights

How to generate a list with For-Yield
Can I apply “guards” in For-Yield ?

Problem

I want to generate a list of variables with certain conditions.

Solution

Before I introduce “For-Yield with guards”, let’s revisit the basic for-yield loop and see what it does

Basic For-Yield Loop

In case you are new to yield, you can think of it as below

When the for-yield loop begins, it will create a temporary bucket that is the same type as the initial collection.
For each iteration, the output is store in the bucket.
When all iterations are finished, the contents of the bucket are returned

val list = for (i <- 0 to 5) yield i// this will return Vector(0,1,2,3,4,5)

For-Yield Loop with Guards

In addition to “yielding” a new collection from the initial collection, we can also apply “guards” to for-yield loop. This is typically used when you want to filter out the values based on specific condition(s).

My preferred syntax is to use curly brackets, { }

for {
  CONDITION(S)
} yield SOMETHING

Example code below:

# Case 1 - using a ternary operator// creating a list of even numbers
val foo = for (i <- 1 to 10 if (i % 2 == 0)) yield i// re-writing the code using "Guards"
val bar = for {
  i <- 1 to 10
  if (i % 2 == 0)
} yield i// multiple conditions
val woo = for {
  i <- 1 to 100
  if (i % 2 == 0)
  if (i > 10 && i < 40)
  if (i % 4 == 0)
} yield i

Well, hope this helps everyone out there who is enthusiastic about Scala. Thanks for reading! :)