Nested iteration in Scala

What is the difference (if any) between two code fragments below?

Example from Ch7 of Programming i Scala

def grep(pattern: String) = 
  for (
    file <- filesHere
    if file.getName.endsWith(".scala");
    line <- fileLines(file)
    if line.trim.matches(pattern)
  ) println(file + ": " + line.trim)

and this one

def grep2(pattern: String) = 
  for (
    file <- filesHere
    if file.getName.endsWith(".scala")
  ) for (
    line <- fileLines(file)
    if line.trim.matches(pattern)
  ) println(file + ": " + line.trim)

Or

for (i <- 1 to 2开发者_StackOverflow)
  for (j <- 1 to 2)
    println(i, j)

and

for (
  i <- 1 to 2;
  j <- 1 to 2
) println(i, j)

In this case there is no difference. However when using yield there is:

for (
  i <- 1 to 2;
  j <- 1 to 2
) yield (i, j)

Will give you a sequence containing (1,1), (1,2), (2,1) and (2,2).

for (i <- 1 to 2)
  for (j <- 1 to 2)
    yield (i, j)

Will give you nothing, because it generates the sequence (i,1), (i,2) on each iteration and then throws it away.

Sometimes it is also useful to output a multi dimensional collection (for example a matrix of table):

for (i <- 1 to 2) yield for (j <- 1 to 2) yield (i, j)

Will return:

Vector(Vector((1,1), (1,2)), Vector((2,1), (2,2)))