antlr4 array implementation : getting values of el

Question

I'm trying to implement arrays in antlr4 and I'm lost as to how I can get the multiple elements of the array when it is initialized like so:

int array[] = {1, 2};

I was thinking of placing them in a HashMap like this, the key being the index:

public Map<Integer, Value> array_memory = new HashMap<Integer, Value>();

Below is the grammar I'm following:

grammar GaleugParserNew;

/*
 * PARSER RULES
 */

declare_var 
: INTEGER ID '[' (INT)? ']' (ASSIGN '{' array_init '}')? SCOL 
;

array_init
: INT ',' array_init
| INT 
;


/*
* LEXER RULES
*/

SCOL : ';';

ASSIGN : '=';

INTEGER : 'int';

INT : [0-9]+;

I have a variable that would count how many times declare_var has visited array_init for the index. But I don't know how to visit array_init with multiple elements.

This is my declare_var visitor:

@Override
public Value visitDeclareArray(GaleugParserNewParser.DeclareArrayContext ctx){
    String id = ctx.ID().getText(); //gets array name

    String size = ctx.INT().getText(); //get string version of array size
    int x = Integer.parseInt(size); //convert size(String) to int

    Value elem = this.visit(ctx.array_init());

    return Value.VOID;
}

And this is my array_init visitor:

@Override
public Value visitArray_init(GaleugParserNewParser.Array_initContext ctx){
    index += 1;
    return new Value(Double.valueOf(ctx.getText()));
}

If you have any suggestions as to how I can visit array_init in reference to the number of variables I'd like to hear them. Thank you!

Answer 1

I was thinking of placing them in a HashMap like this, the key being the index:

public Map<Integer, Value> array_memory = new HashMap<Integer, Value>();

Why? A List<Value> would do as well, right? No need to keep track of the index yourself.

You're making thinks more complicated by recursively calling the array_init rule:

array_init
 : INT ',' array_init
 | INT 
 ;

I would do it like this instead:

array_init
 : INT ( ',' INT )*
 ;

You can then do something like this:

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.TerminalNode;

import java.util.ArrayList;
import java.util.List;

public class Main {

  public static void main(String[] args) {

    String source = "int array[] = {1, 2};";

    GaleugParserNewLexer lexer = new GaleugParserNewLexer(CharStreams.fromString(source));
    GaleugParserNewParser parser = new GaleugParserNewParser(new CommonTokenStream(lexer));

    Value value = new EvalVisitor().visit(parser.declare_var());

    System.out.println(value);
  }
}

class Value {

  final Object value;

  public Value(Object value) {
    this.value = value;
  }

  @Override
  public String toString() {
    return String.valueOf(this.value);
  }
}

class EvalVisitor extends GaleugParserNewBaseVisitor<Value> {

  @Override
  public Value visitDeclare_var(GaleugParserNewParser.Declare_varContext ctx) {
    List<Value> numbers = new ArrayList<>();
    if (ctx.array_init() != null) {
      for (TerminalNode tokenNode : ctx.array_init().INT()) {
        numbers.add(new Value(Integer.valueOf(tokenNode.getText())));
      }
    }
    return new Value(numbers);
  }
}

And if you run this Main class, the following will be printed to your console:

[1, 2]

EDIT

But what if I'm not specifically looking for INT, rather I would like to look for any tokens inside a grammar containing all of the data types in my language? what should i use instead of TerminalNode?

Easy as 1-2-3, define you grammar like this:

array_init
 : expr ( ',' expr )*
 ;

expr
 : '(' expr ')'          #nestedExpr
 | lhs=expr '+' rhs=expr #addExpr
 | INT                   #intExpr
 | ID                    #idExpr
 ;

and then do something like this:

import org.antlr.v4.runtime.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Main {

  public static void main(String[] args) {

    String source = "int array[] = { 123456789, (1 + 2), Q };";

    GaleugParserNewLexer lexer = new GaleugParserNewLexer(CharStreams.fromString(source));
    GaleugParserNewParser parser = new GaleugParserNewParser(new CommonTokenStream(lexer));

    Map<String, Value> memory = new HashMap<String, Value>(){{
      put("Q", new Value(42));
    }};

    Value value = new EvalVisitor(memory).visit(parser.declare_var());

    System.out.println(value);
  }
}

class Value {

  final Object value;

  public Value(Object value) {
    this.value = value;
  }

  int asInt() {
    return (Integer) value;
  }

  @Override
  public String toString() {
    return String.valueOf(this.value);
  }
}

class EvalVisitor extends GaleugParserNewBaseVisitor<Value> {

  final Map<String, Value> memory;

  EvalVisitor(Map<String, Value> memory) {
    this.memory = memory;
  }

  @Override
  public Value visitDeclare_var(GaleugParserNewParser.Declare_varContext ctx) {
    List<Value> numbers = new ArrayList<>();
    if (ctx.array_init() != null) {
      for (GaleugParserNewParser.ExprContext expr : ctx.array_init().expr()) {
        numbers.add(super.visit(expr));
      }
    }
    return new Value(numbers);
  }

  @Override
  public Value visitIntExpr(GaleugParserNewParser.IntExprContext ctx) {
    return new Value(Integer.valueOf(ctx.getText()));
  }

  @Override
  public Value visitAddExpr(GaleugParserNewParser.AddExprContext ctx) {
    return new Value(super.visit(ctx.lhs).asInt() + super.visit(ctx.rhs).asInt());
  }

  @Override
  public Value visitNestedExpr(GaleugParserNewParser.NestedExprContext ctx) {
    return super.visit(ctx.expr());
  }

  @Override
  public Value visitIdExpr(GaleugParserNewParser.IdExprContext ctx) {
    return this.memory.get(ctx.getText());
  }
}

which will print:

[123456789, 3, 42]