Value assignment inside for-loop in Scala - scala

Question

Is there any difference between this code:
for(term <- term_array) {
val list = hashmap.get(term)
...
}
and:
for(term <- term_array; val list = hashmap.get(term)) {
...
}
Inside the loop I'm changing the hashmap with something like this
hashmap.put(term, string :: list)
While checking for the head of list it seems to be outdated somehow when using the second code snippet.

Answer 1

The difference between the two is, that the first one is a definition which is created by pattern matching and the second one is a value inside a function literal. See Programming in Scala, Section 23.1 For Expressions:
for {
p <- persons // a generator
n = p.name // a definition
if (n startsWith "To") // a filter
} yield n
You see the real difference when you compile sources with scalac -Xprint:typer <filename>.scala:
object X {
val x1 = for (i <- (1 to 5); x = i*2) yield x
val x2 = for (i <- (1 to 5)) yield { val x = i*2; x }
}
After code transforming by the compiler you will get something like this:
private[this] val x1: scala.collection.immutable.IndexedSeq[Int] =
scala.this.Predef.intWrapper(1).to(5).map[(Int, Int), scala.collection.immutable.IndexedSeq[(Int, Int)]](((i: Int) => {
val x: Int = i.*(2);
scala.Tuple2.apply[Int, Int](i, x)
}))(immutable.this.IndexedSeq.canBuildFrom[(Int, Int)]).map[Int, scala.collection.immutable.IndexedSeq[Int]]((
(x$1: (Int, Int)) => (x$1: (Int, Int) #unchecked) match {
case (_1: Int, _2: Int)(Int, Int)((i # _), (x # _)) => x
}))(immutable.this.IndexedSeq.canBuildFrom[Int]);
private[this] val x2: scala.collection.immutable.IndexedSeq[Int] =
scala.this.Predef.intWrapper(1).to(5).map[Int, scala.collection.immutable.IndexedSeq[Int]](((i: Int) => {
val x: Int = i.*(2);
x
}))(immutable.this.IndexedSeq.canBuildFrom[Int]);
This can be simplified to:
val x1 = (1 to 5).map {i =>
val x: Int = i * 2
(i, x)
}.map {
case (i, x) => x
}
val x2 = (1 to 5).map {i =>
val x = i * 2
x
}

Answer 2

Instantiating variables inside for loops makes sense if you want to use that variable the for statement, like:
for (i <- is; a = something; if (a)) {
...
}
And the reason why your list is outdated, is that this translates to a foreach call, such as:
term_array.foreach {
term => val list= hashmap.get(term)
} foreach {
...
}
So when you reach ..., your hashmap has already been changed. The other example translates to:
term_array.foreach {
term => val list= hashmap.get(term)
...
}