For-comprehensions

Scala supports a very rich For loop construct known as For-comprehension. We can use it like Java's extended For loop, as shown here:

scala> val numList = List(1,2,3,4,5) 
numList: List[Int] = List(1, 2, 3, 4, 5) 
 
scala> for(num <- numList) yield num+1 
res7: List[Int] = List(2, 3, 4, 5, 6)  

When we observe the preceding example, we realize that For-comprehension results in the same container or object with a new set of elements.

We can use For-comprehension to replace the operation that uses both filter and map, as shown here:

scala> val numList = List(1,2,3,4,5) 
numList: List[Int] = List(1, 2, 3, 4, 5) 
 
scala> numList.filter(a => a %2 == 0).map(b => b*b) 
res8: List[Int] = List(4, 16) 
 
scala> for(num <- numList if num % 2 == 0) yield num*num 
res10: List[Int] = List(4, 16) 

Here, For-comprehension is picking one element at a time from numList, checking the condition num % 2 == 0 (even number) and performing the square root operation, that is, num*num.

A combination of these two Higher-Order Functions, that is, filter and map, also does the same.