Nested iteration in Scala

Question

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

and

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

I think the variants only differ by syntax. Section 6.19 of the Scala Language Specification (v. 2.8) defines how for loops are rewritten. http://www.scala-lang.org/sites/default/files/linuxsoft_archives/docu/files/ScalaReference.pdf#page=97 — mkneissl, Sep 03 '10 at 11:06
Search for questions about Scala and yield. One of them should explain exactly how `for` works in Scala. — Daniel C. Sobral, Sep 03 '10 at 18:40

score 36 · Accepted Answer · answered Sep 03 '10 at 11:03

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.

score 11 · Answer 2 · answered Oct 06 '14 at 09:15

11

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)))

answered Oct 06 '14 at 09:15

Mishael Rosenthal

619
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12

Nested iteration in Scala

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