How to print a value outside a for loop in scala? - scala

Question

I am new to scala and would like to know way to access val which is defined inside for loop and would like to write that val to a file outside for loop.
def time[A](logFile: String, description: String)(job: => A): Unit = {
var totalDuration: Long = 0
for (i <- 1 to 3) {
val currentTime = System.currentTimeMillis
val result = job
val finalTime = System.currentTimeMillis
val duration = finalTime - currentTime
totalDuration = if (totalDuration == 0) duration else totalDuration.min(duration)
}
val pw = new PrintWriter(new FileOutputStream(new File(logFile),true))
pw.write(description + " " + result + " " + totalDuration +" ms"+"\n")
pw.flush()
pw.close
}
In the above code i am calculating my result which contains the length of bytes read from other function and would like to calculate the time it takes to read the total bytes. I would like to iterate 3 times and take the minimum of all the three. The val result contains the bytes length which also needs to be written in a file. i get a error because i am accessing the result val outside the scope of for loop. Can someone please help me solve this error. How can i access the val result outside for loop to write it to a file ?
Thanks in advance!!

Answer 1

While your question is answered, the for loop is not in it's typical form, which would look more like this:
def time[A] (logFile: String, description: String) (job: => A): Unit = {
val (result, totalDuration): (A, Long) = (for { i <- 1 to 3
currentTime = System.currentTimeMillis
result = job
finalTime = System.currentTimeMillis
duration = finalTime - currentTime
} yield (result, duration)).minBy {case (r, d) => d}
val pw = new PrintWriter (new FileOutputStream (new File (logFile), true))
pw.write (description + " " + result + " " + totalDuration +" ms"+"\n")
pw.flush()
pw.close
}
If I understood your code correctly. I don't know whether a side effect yields to different results for each job invocation.
I missed the internal discussion of the for-loop invention/definition, why the keyword val should be omitted here, but it is quiet handy.
What is more important, is, that you usually have all temporary assignments in the for (...(here part)...) { not here}. The consequences of round or curly braces in the first part aren't totally clear to me, but if you use round parens, you have terminate most statements with a semicolon.
scala> for (i <- 1 to 3;
| j <- 4 to 5;
| k = j-i;
| l = k/2) yield l * l;
res2: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 4, 1, 1, 0, 1)
Note that neither i, nor j, k, l where declared as val or var.
scala> for {i <- 1 to 3
| j <- 4 to 5
| k = j-i
| l = k/2} yield l * l;
res3: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 4, 1, 1, 0, 1)
You will find multiple questions here, which explain, how a for-loop is and can be translatet to a flatMap/map-combination, eventually with filter:
scala> for {i <- 1 to 3
| j <- 4 to 5
| k = j-i
| if (k > 1)
| l = k/2 } yield l * l;
res5: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 4, 1, 1, 1)
And instead of yielding just one value, you can yield a tuple, and assign by
val (a:X, b:Y) = (for ..... yield (aa, bb))
The
yield (result, duration)).minBy {case (r, d) => d}
takes a tuple (result, duration) and selects the minimum by duration, but yields both values.

Answer 2

you can use yield. Yield will return data from for loop after completion of loop and use that data accordingly. Since I do not have your code. see this example
val j = for (i <- 1 to 10) yield i
println(j)
output of j will be
Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

Answer 3

For replacing your for loop to calculate the total duration of some jobs that are executed sequentially, you could use foldLeft

Answer 4

Make use of yield or you can use any of the fold method. Or if you are good with recursion you can use a tailrec method which returns what is Desired. Yield is the most simplest way to do it.
val answer = for(i<- 1 to 10) yield i
println(answer) // Vector(1,2,3,4,5,6,7,8,9,10)