Functional style for this Scala code

Question

A friend of mine is learning Scala and wrote this simple code to keep track of the longest line in a file:

val longest = (filename:String) => {
  val is = new FileInputStream(filename)
  val buf = new Array[Byte](1024)
  var longest=0 //keep track of the longest line
  var lastPos=0
  var read=0
  try {
    read = is.read(buf)
    while (read > 0) {
      for (i<-0 until read) {
        if (buf(i) == '\n') {
          val size=i-lastPos-1
          lastPos=i
          if (size>longest) {
            longest=size
          }
        }
      }
      lastPos-=buf.length
      read=is.read(buf)
    }
  } finally {
    is.close()
  }
  longest
}

I'm new to Scala too, but I'm pretty sure there's a lot of room for flatMaps and other functions in this code.

Could someone post a functional version of this?

Ben James · Accepted Answer · 2011-08-14 16:42:04Z

24

An alternative implementation:

def longest(filename: String) =
  Source.fromFile(filename).getLines.map(_.size).max

Brief explanation:

getLines returns an iterator of the lines in the file;
map(_.size), equivalent to map(line => line.size), returns a new iterator of the line lengths
max returns the greatest line length.

edited Aug 14, 2011 at 16:42

answered Aug 14, 2011 at 16:27

Ben James

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5 Comments

OscarRyz Over a year ago

How efficient is this code. I may "guess" it iterates the file ( or keeps it in memory ) three times, 1 for getLines, 1 for map , and last one to get the max, while the original code does only one pass, but I may be wrong of course.

Ben James Over a year ago

The file will only be iterated once. When you call map, it does not pull any values from the iterator; it stores the _.size function and creates a new iterator which will apply that function to each output of the first iterator when called. Calling max finally forces the iteration.

OscarRyz Over a year ago

Got it, that's why it is called BufferedSource :) What about .max?

Ben James Over a year ago

I meant to say max at the end of that comment, edited it now.

OscarRyz Over a year ago

So.. that makes it 1 iteration total? :-/

missingfaktor · Accepted Answer · 2011-08-14 16:32:30Z

13

val longest = (filename: String) =>
  io.Source.fromFile(filename).getLines.maxBy(_.length).length

edited Aug 14, 2011 at 16:32

answered Aug 14, 2011 at 16:25

missingfaktor

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4 Comments

Tomasz Nurkiewicz Over a year ago

Admittedly your solution with maxBy is more pleasant than mine, +1.

OscarRyz Over a year ago

Hello @missingfaktor this looks more efficient isn't? I understand while it read the lines it applies the _.length code but I don't get why does it returns "length" instead of max , shouldn't that be ...minBy(_.length)[0] or something like that?

missingfaktor Over a year ago

@Oscar: Ben's code should be just as fast (given that getLines returns an iterator).

missingfaktor Over a year ago

@Oscar: getLines gives you an Iterator[String]. maxBy returns the line for which the function _.length returns maximum value i.e. the longest line. At last, we take the length of the longest line.

Tomasz Nurkiewicz · Accepted Answer · 2011-08-14 16:25:28Z

5

Yes, this code is painfully imperative. In Scala the rough equivalent would be (!):

def longest(fileName: String) = 
  Source.fromFile(fileName).getLines().max(Ordering.fromLessThan[String](_.size < _.size)).size

Guess it wouldn't hurt to provide some explanation:

def longest(fileName: String) = Source.
    fromFile(fileName).     //file contents abstraction
    getLines().     //iterator over lines
    max(        //find the max element in iterated elements
        Ordering.fromLessThan[String](_.size < _.size)  //however, use custom comparator by line size
    ).size  //max() will return the line, we want the line length

Of course TMTOWTDI in Scala.

edited Aug 14, 2011 at 16:25

answered Aug 14, 2011 at 16:19

Tomasz Nurkiewicz

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