Comprehensions have the structure
for (enumerators) yield e, wherever enumerators refers to a semicolon-separated list of enumerators. Enumerator is either a generator that introduces new variables, or it's a
filter. A comprehension evaluates the body
e for every binding generated by the enumerators and returns a sequence of those values.
These definitions lead us to the for comprehension ideas of generators, filters, and definitions. A Scala for comprehension will contain the subsequent 3 expressions:
- Generators
- Filters
- Definitions
Syntax:
for {
b <- books // generator
n = b.name // definition
if (n startsWith "To") // filter
} yield
Generators have below form:
pattern <- expression
For example
b <- books In this expression the value b iterates over all of the elements contained in books.
Below are two more things about generators -
- Each for comprehension begins with a generator.
- for comprehensions will be multiple generators.
For comprehension definitions have below syntax:
pattern = expression
For example
n = b.name the variable n is bound to the value b.name. That statement has a similar result as writing this code outside of a for comprehension.
val n = b.name
For comprehension filters have below form:
if (expression)
Expression have the type Boolean.
Filters drop all elements from the iteration that which expression returns false, as like given code. For example
if (n startsWith "Ca") any value
n that does not start with the string
Ca will be dropped during the iteration process.
Let's discuss some examples.
Example #1: With
yield
Output :
Scala
Java
In above example, the for loop used with a
yield statement actually creates a List. Because we said yield
language.name, it’s a
List[String].
language <- LanguageBase is our generator and
if (language.article >=20 && language.article < 30) could be a guard that filters out article those don't seem to be in between 20 to 30.
Example #2: Without
yield
We can omit yield in comprehension. In that case, comprehension will return Unit. This can be helpful just in case we would like to perform side-effects. Here’s a program like the above one, without using yield.
Output:
(2, 2)
(2, 3)
(3, 2)
(3, 3)
In above example,
a = 4. In the first iteration,
i = 0 and
j = 0 so
i * j is not greater than equal to
a and therefore nothing is yielded.
j gets incremented 3 more times before
i gets incremented to 1.
