CODEWITHSUNDEEP
scalainputstreamio
ARKBAN
ARKBAN

Reputation: 3477

What is the proper way to code a read-while loop in Scala?

What is the "proper" of writing the standard read-while loop in Scala? By proper I mean written in a Scala-like way as opposed to a Java-like way.

Here is the code I have in Java:

MessageDigest md = MessageDigest.getInstance( "MD5" );
InputStream input = new FileInputStream( "file" );
byte[] buffer = new byte[1024];
int readLen;
while( ( readLen = input.read( buffer ) ) != -1 )
    md.update( buffer, 0, readLen );
return md.digest();

Here is the code I have in Scala:

val md = MessageDigest.getInstance( hashInfo.algorithm )
val input = new FileInputStream( "file" )
val buffer = new Array[ Byte ]( 1024 )
var readLen = 0
while( readLen != -1 )
{
    readLen = input.read( buffer )
    if( readLen != -1 )
        md.update( buffer, 0, readLen )
}
md.digest

The Scala code is correct and works, but feels very un-Scala-ish. For one it is a literal translation of the Java code, taking advantage of none of the advantages of Scala. Further it is actually longer than the Java code! I really feel like I'm missing something, but I can't figure out what.

I'm fairly new to Scala, and so I'm asking the question to avoid falling into the pitfall of writing Java-style code in Scala. I'm more interested in the Scala way to solve this kind of problem than in any specific helper method that might be provided by the Scala API to hash a file.

(I apologize in advance for my ad hoc Scala adjectives throughout this question.)

Upvotes: 26

Views: 11584

Answers (3)

Ant Kutschera
Ant Kutschera

Reputation: 6588

What about a curried function? You 11 lines of Scala code become:

val md = MessageDigest.getInstance(hashInfo.algorithm)
val input = new FileInputStream("file")
iterateStream(input){ (data, length) => 
    md.update(data, 0, length)
}
md.digest

The iterateStream function on line 3, which you could add to a library is:

def iterateStream(input: InputStream)(f: (Array[Byte], Int) => Unit){
    val buffer = new Array[Byte](512)
    var curr = input.read(buffer)
    while(curr != -1){
        f(buffer, curr)
        curr = input.read(buffer)
    }
}

The ugly duplicated code (where the input is read) ends up in the library, well tested and hidden away from the programmer. I feel that the first block of code is less complex than the Iterator.continually solution.

Upvotes: 0

michael.kebe
michael.kebe

Reputation: 11085

What Rex Kerr suggests in his comment is the following:

val md = MessageDigest.getInstance("MD5")
val input = new FileInputStream("foo.txt")
val buffer = new Array[ Byte ]( 1024 )
Stream.continually(input.read(buffer))
  .takeWhile(_ != -1)
  .foreach(md.update(buffer, 0, _))
md.digest

The key is the Stream.continually. It gets an expression which is evaluated continually, creating an infinite Stream of the evaluated expression. The takeWhile is the translation from the while-condition. The foreach is the body of the while-loop.

Upvotes: 9

Sandor Murakozi
Sandor Murakozi

Reputation: 4412

Based on Rex's post that he mentioned:

Stream.continually(input.read(buffer)).takeWhile(_ != -1).foreach(md.update(buffer, 0, _))

You should replace the var readLen + while {...} lines with it, it produces the same result.

As Rex mentioned, it works with scala 2.8.

Upvotes: 30

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