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)
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)))
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