• Justin Johansson (24/24) Oct 16 2009 What's wrong with this simple symbol table class?
• Bill Baxter (18/42) Oct 16 2009 You query presence of a key in an AA using 'in'
• Justin Johansson (3/4) Oct 16 2009 Thank you Bill .. esp. the tip to avoid double lookup.
• Manfred_Nowak (3/4) Oct 16 2009 shouldn't the compiler sort this out?
• Bill Baxter (5/8) Oct 16 2009 I'm not really sure what you mean, but I think the answer is that
• downs (3/24) Oct 16 2009 Slightly shorter form:
• bearophile (6/7) Oct 16 2009 In LDC both forms (and several other ones, if the usage of the pointer i...

Justin Johansson <no spam.com> writes:

What's wrong with this simple symbol table class?

class Symbol
{
   private char[] id;
   private static Symbol[char[]] symtab;

   private this( string id) {
      this.id = id;
   }

   static Symbol opCall( char[] id) {
      Symbol sym = symtab[id];  // *** ArrayBoundsError here

      if (sym is null) {
         sym = symtab[id] = new Symbol( id);
      }

      return sym;
   }
}


void main() {
  auto sym = Symbol( "foo");
}

Running gives Error: ArrayBoundsError

Does symtab need to be initialized in, say, a static if.
The reference documentation on associative arrays does not suggest that it
needs to be.

Thanks for all help.
Justin

Bill Baxter <wbaxter gmail.com> writes:

You query presence of a key in an AA using 'in'

  if (id in symtab) {
     Symbol sym =3D symtab[id];
     ...
  } else {
      ..
  }

Or this avoids a double lookup if the symbol is present:

  Symbol* pSym =3D id in symtab;
  if (pSym !is null) {
         Symbol sym =3D *pSym;
         ...
  } else {
         ...
  }

--bb

On Fri, Oct 16, 2009 at 1:37 PM, Justin Johansson <no spam.com> wrote:
 What's wrong with this simple symbol table class?

 class Symbol
 {
 =A0 private char[] id;
 =A0 private static Symbol[char[]] symtab;

 =A0 private this( string id) {
 =A0 =A0 =A0this.id =3D id;
 =A0 }

 =A0 static Symbol opCall( char[] id) {
 =A0 =A0 =A0Symbol sym =3D symtab[id]; =A0// *** ArrayBoundsError here

 =A0 =A0 =A0if (sym is null) {
 =A0 =A0 =A0 =A0 sym =3D symtab[id] =3D new Symbol( id);
 =A0 =A0 =A0}

 =A0 =A0 =A0return sym;
 =A0 }
 }


 void main() {
 =A0auto sym =3D Symbol( "foo");
 }

 Running gives Error: ArrayBoundsError

 Does symtab need to be initialized in, say, a static if.
 The reference documentation on associative arrays does not suggest that i=

t needs to be.
 Thanks for all help.
 Justin

Justin Johansson <no spam.com> writes:

Bill Baxter Wrote:

 You query presence of a key in an AA using 'in'

Thank you Bill .. esp. the tip to avoid double lookup.

JJ

"Manfred_Nowak" <svv1999 hotmail.com> writes:

Bill Baxter wrote:

   Symbol* pSym = id in symtab;

shouldn't the compiler sort this out?

-manfred

Bill Baxter <wbaxter gmail.com> writes:

On Fri, Oct 16, 2009 at 4:11 PM, Manfred_Nowak <svv1999 hotmail.com> wrote:
 Bill Baxter wrote:

 =A0 Symbol* pSym =3D id in symtab;

 shouldn't the compiler sort this out?

I'm not really sure what you mean, but I think the answer is that
there's a difference between an unset entry and one that's set to
null.

--bb

downs <default_357-line yahoo.de> writes:

Bill Baxter wrote:
 You query presence of a key in an AA using 'in'
 
   if (id in symtab) {
      Symbol sym = symtab[id];
      ...
   } else {
       ..
   }
 
 Or this avoids a double lookup if the symbol is present:
 
   Symbol* pSym = id in symtab;
   if (pSym !is null) {
          Symbol sym = *pSym;
          ...
   } else {
          ...
   }
 
 --bb
 

Slightly shorter form:

if (auto pSym = id in symtab) { ... }

bearophile <bearophileHUGS lycos.com> writes:

Bill Baxter:

Or this avoids a double lookup if the symbol is present:<

In LDC both forms (and several other ones, if the usage of the pointer is
local, and it doesn't get passed away) get simplified to the same single lookup
code :-) (but not in DMD).
So much that you may even think of "in" to return just a boolean.
You can tell I like LDC a lot, it's the best thing happened to D in a lot of
time.

Bye,
bearophile