Merge pull request #754 from travissarles/sequence-comp

Rewrote sequence comprehensions section of tour

Author: travissarles

tutorials/tour/_posts/2017-02-13-for-comprehensions.md

@@ -0,0 +1,51 @@
+---
+layout: tutorial
+title: For Comprehensions
+
+disqus: true
+
+tutorial: scala-tour
+categories: tour
+num: 17
+next-page: generic-classes
+previous-page: extractor-objects
+---
+
+Scala offers a lightweight notation for expressing *sequence comprehensions*. Comprehensions have the form `for (enumerators) yield e`, where `enumerators` refers to a semicolon-separated list of enumerators. An *enumerator* is either a generator which introduces new variables, or it is a filter. A comprehension evaluates the body `e` for each binding generated by the enumerators and returns a sequence of these values.
+
+Here's an example:
+
+```tut
+case class User(val name: String, val age: Int)
+
+val userBase = List(new User("Travis", 28),
+  new User("Kelly", 33),
+  new User("Jennifer", 44),
+  new User("Dennis", 23))
+
+val twentySomethings = for (user <- userBase if (user.age >=20 && user.age < 30))
+  yield user.name  // i.e. add this to a list
+
+twentySomethings.foreach(name => println(name))  // prints Travis Dennis
+```
+The `for` loop used with a `yield` statement actually creates a `List`. Because we said `yield user.name`, it's a `List[String]`. `user <- userBase` is our iterator and `if (user.age >=20 && user.age < 30)` is a guard that filters out users who are in their 20s.
+
+Here is a more complicated example using two generators. It computes all pairs of numbers between `0` and `n-1` whose sum is equal to a given value `v`:
+
+```tut
+def foo(n: Int, v: Int) =
+   for (i <- 0 until n;
+        j <- i until n if i + j == v)
+   yield (i, j)
+
+foo(10, 10) foreach {
+  case (i, j) =>
+    print(s"($i, $j) ")  // prints (1, 9) (2, 8) (3, 7) (4, 6) (5, 5)
+}
+
+```
+Here `n == 10` and `v == 10`. On the first iteration, `i == 0` and `j == 0` so `i + j != v` and therefore nothing is yielded. `j` gets incremented 9 more times before `i` gets incremented to `1`. Without the `if` guard, this would simply print the following:
+```
+
+(0, 0) (0, 1) (0, 2) (0, 3) (0, 4) (0, 5) (0, 6) (0, 7) (0, 8) (0, 9) (1, 1) ...
+```