How to get the element index when mapping an array in Scala?
Solution 1
Depends whether you want convenience or speed.
Slow:
a.zipWithIndex.map{ case (s,i) => myFn(s,i) }
Faster:
for (i <- a.indices) yield myFn(a(i),i)
{ var i = -1; a.map{ s => i += 1; myFn(s,i) } }
Possibly fastest:
Array.tabulate(a.length){ i => myFn(a(i),i) }
If not, this surely is:
val b = new Array[Whatever](a.length)
var i = 0
while (i < a.length) {
b(i) = myFn(a(i),i)
i += 1
}
(In Scala 2.10.1 with Java 1.6u37, if "possibly fastest" is declared to take 1x time for a trivial string operation (truncation of a long string to a few characters), then "slow" takes 2x longer, "faster" each take 1.3x longer, and "surely" takes only 0.5x the time.)
Solution 2
A general tip: Use .iterator
method liberally, to avoid creation of intermediate collections, and thus speed up your computation. (Only when performance requirements demand it. Or else don't.)
scala> def myFun(s: String, i: Int) = s + i
myFun: (s: String, i: Int)java.lang.String
scala> Array("nami", "zoro", "usopp")
res17: Array[java.lang.String] = Array(nami, zoro, usopp)
scala> res17.iterator.zipWithIndex
res19: java.lang.Object with Iterator[(java.lang.String, Int)]{def idx: Int; def idx_=(x$1: Int): Unit} = non-empty iterator
scala> res19 map { case (k, v) => myFun(k, v) }
res22: Iterator[java.lang.String] = non-empty iterator
scala> res22.toArray
res23: Array[java.lang.String] = Array(nami0, zoro1, usopp2)
Keep in mind that iterators are mutable, and hence once consumed cannot be used again.
An aside: The map
call above involves de-tupling and then function application. This forces use of some local variables. You can avoid that using some higher order sorcery - convert a regular function to the one accepting tuple, and then pass it to map
.
scala> Array("nami", "zoro", "usopp").zipWithIndex.map(Function.tupled(myFun))
res24: Array[java.lang.String] = Array(nami0, zoro1, usopp2)
Solution 3
What about this? I think it should be fast and it's pretty. But I'm no expert on Scala speed...
a.foldLeft(0) ((i, x) => {myFn(x, i); i + 1;} )
Ivan
Updated on May 31, 2020Comments
-
Ivan almost 4 years
Let's consider a simple mapping example:
val a = Array("One", "Two", "Three") val b = a.map(s => myFn(s))
What I need is to use not
myFn(s: String): String
here, butmyFn(s: String, n: Int): String
, wheren
would be the index ofs
ina
. In this particular case myFn would expect the second argument to be 0 for s == "One", 1 for s == "Two" and 2 for s == "Three". How can I achieve this? -
Daniel C. Sobral over 12 yearsI love reading your answers on speed stuff, but it can be so depressing sometimes... :-)
-
Ivan over 12 years"iterators are mutable, and hence once consumed cannot be used again" - I've once spent some time debugging why doesn't a program work to find an iterator to get empty once used :-)
-
gzm0 almost 11 yearsHow does the speed of
a.view.zipWithIndex.map{ case (s,i) => myFn(s,i) }
compare to your solutions? -
Rex Kerr almost 11 years@gzm0 - Even slower (by 2-5x) than the slowest if you actually need all the values. If you only need a few, then the view is better.