FOR, WHILE Is Too Easy, Let's Go Looping

With several 10k code in Erlang, I'm familiar with functional style coding, and I found I can almost rewrite any functions in Erlang to Scala, in syntax meaning.

Now, I have some piece of code written in Java, which I need to translate them to Scala. Since "for", "while", or "do" statement is so easy in Java, I can find a lot of them in Java code. The problem is, should I keep them in the corresponding "for", "while", "do" in Scala, or, as what I do in Erlang, use recursive function call, or, "loop"?

I sure choose to loop, and since Scala supports recursive function call on functions defined in function body (Erlang doesn't), I choose define these functions' name as "loop", and I tried to write code let "loop" looks like a replacement of "for", "while" etc.

Here's a piece of code that is used to read number string and convert to double, only piece of them.

The Java code: 

public class ReadNum {

    private double readNumber(int fstChar, boolean isNeg) {
        StringBuilder out = new StringBuilder(22);
        out.append(fstChar);
        
        double v = '0' - fstChar;
        // the maxima length of number stirng won't exceed 22
        for (int i = 0; i < 22; i++) {
            int c = getChar();
            switch (c) {
                case '0':
                case '1':
                case '2':
                case '3':
                case '4':
                case '5':
                case '6':
                case '7':
                case '8':
                case '9':
                    v = v * 10 - (c - '0');
                    out.append(c);
                    continue;
                case '.':
                    out.append('.');
                    return readFrac(out, 22 - i);
                case 'e':
                case 'E':
                    out.append(c);
                    return readExp(out, 22 - i);
                default:
                    if (c != -1) backup(1);
                    if (!isNeg) return v; else return -v
            }
        }
        return 0;
    }
}

The Scala code: 

class ReadNum {
   private
   def readNumber(fstChar:Char, isNeg:Boolean) :Double = {
      val out = new StringBuilder(22)
      out.append(fstChar)

      val v:Double = '0' - fstChar
      def loop(c:Char, v:Double, i:Int) :Double = c match {
         // the maxima length of number stirng won't exceed 22
         case _ if i > 21 =>
            0
         case '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' =>
            out.append(c)
            val v1 = v * 10 - (c - '0')
            loop(getChar, v1, i + 1)
         case '.' =>
            out.append('.')
            readFrac(out, 22 - i)
         case 'e' | 'E' =>
            out.append(c)
            readExp(out, 22 - i)
         case _ =>
            if (c != -1) backup(1)
            if (isNeg) v else -v
      }; loop(getChar, v, 1)
   }
}

As you can see in line 25, the loop call is put at the position immediately after the "loop" definition, following "}; ", I don't put it to another new line, it makes me aware of the "loop" function is just used for this call.

And yes, I named all these embedded looping function as "loop", every where.

  • Posted: 2008-10-21 09:00 (Updated: 2010-01-05 10:39)
  • Author: dcaoyuan
  • Categories: Java Scala

Comments

1. Hynek (Pichi) Vychodil -- 2008-10-21 09:00

It's wrong idea to try rewrite broken code. What happens with v value when return readFrac(out, 22 - i) is called?

2. Caoyuan -- 2008-10-22 09:00

What happens with v value when return readFrac(out, 22 - i) is called?

When readFrac(out, 22-i), it will break from loop function, and return the result of readFrac

3. Jesse Eichar -- 2008-10-22 09:00

Do you know how this compiles in scala? Does it compile down to a loop or are we in danger of blowing out the stack?

4. Caoyuan -- 2008-10-22 09:00

Jesse,

Scala supports Tail-recursion optimization. Maybe by analyzing the AST, before generate binary class bytes.

5. wangwei -- 2008-10-26 09:00

hi, caoyuan

This is wangwei, asking something irrelevant to your topic :P

How do you make syntax coloring for Scala code in your web page? When I copy from Netbeans and paste to other editors the sytanx coloring seems to be lost. (copying from Eclipse keeps it though.)

BTW, Netbeans 6.5 RC1 cames out, I tried to install your Scala plug-in but failed with a common (but strange) error of dependency, saying: "The plugin Common Scripting Language API is requested in version >= 1.5 but only 1.8.1.1.6 was found."

Have you already tested your plug-in for 6.5 RC1 or any plans?

thanks in advance

6. Caoyuan -- 2008-10-27 09:00

Hi Wangwei,

The syntax highlighting in this page is via JavaScript?. Although NetBeans support export code to html, with token level highlighting reserved, I like to put pure code in page and let js to highlight it.

For 6.5 RC1, Scala plugins at plugins portal is not compatible, you'll need to change your Update Center to developer version as  http://wiki.netbeans.org mentioned. But I recommended to wait for NetBeans 6.5 offical release.