3.3. Using a for Loop with Embedded if Statements (Guards)
Problem
You want to add one or more conditional clauses to a for
loop, typically to filter out some
elements in a collection while working on the others.
Solution
Add an if
statement after your
generator, like this:
// print all even numbers
scala> for (i <- 1 to 10 if i % 2 == 0) println(i)
2
4
6
8
10
or using the preferred curly brackets style, like this:
for
{
i
<-
1
to
10
if
i
%
2
==
0
}
println
(
i
)
These if
statements are
referred to as filters, filter expressions, or
guards, and you can use as many guards as are
needed for the problem at hand. This loop shows a hard way to print the
number 4
:
for
{
i
<-
1
to
10
if
i
>
3
if
i
<
6
if
i
%
2
==
0
}
println
(
i
)
Discussion
Using guards with for
loops can
make for concise and readable code, but you can also use the traditional
approach:
for
(
file
<-
files
)
{
if
(
hasSoundFileExtension
(
file
)
&&
!
soundFileIsLong
(
file
))
{
soundFiles
+=
file
}
}
However, once you become comfortable with Scala’s for
loop syntax, I think you’ll find it makes
the code more readable, because it separates the looping and filtering
concerns from the business logic:
for
{
file
<-
files
if
passesFilter1
(
file
)
if
passesFilter2
(
file
)
}
doSomething
(
file
)
As a final note, because guards are generally intended to filter
collections, you may want to use one of the many filtering methods that
are available to collections (filter
,
take
, drop
, etc.) instead of a for
loop, depending on your needs.
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