CODEWITHSUNDEEP
parsingregex-lookaroundsconflictjavacc
Sean
Sean

Reputation: 69

javacc add operation causing issues

I'm trying to create a simple parser but I'm having a bit of trouble when trying to take into account the optional sign before the first element: 

    void expr5() : {}
    {
        [addop()] expr6() (addop() expr6())*
    }

e.g. [-] 5 + 3 - 4

here the issue is with the first optional [addop()] which is defined as:

    void addop() : {}
    {
        <PLUS> | <MINUS>
    }

I'm currently getting a choice conflict and recommended to use a LOOKAHEAD() but there is no choice conflict which javacc recognizes when using a LOOKAHEAD()

Thanks in advance!

Upvotes: 1

Views: 160

Answers (2)

Theodore Norvell
Theodore Norvell

Reputation: 16221

I think more clarification is needed. I get no conflict with the following

void sum() : {}
{
  [ addOp() ]
  term()
  ( addOp()  term() )*
}

void addOp() : {}
{     < PLUS > | < MINUS > }

void term() : {}
{
  element()
  ( ( < MULTIPLY > | < DIVIDE > ) element() )*
}

void element() : {}
{
    < CONSTANT > | "(" sum() ")"
}

Upvotes: 1

Bart Kiers
Bart Kiers

Reputation: 170148

You usually want to give unary-minus and -plus a higher precedence than their binary counterparts. So, I'd imagine you'd want something like this:

// ...

void add() : {}
{
  mul() ( <PLUS> mul() | <MINUS> mul() )*
}

void mul() : {}
{
  unary() ( <MULTIPLY> unary() | <DIVIDE> unary() )*
}

void unary() : {}
{
  <PLUS> atom() | <MINUS> atom() | atom()
}

// ...

which won't need any LOOKAHEAD(...).

Upvotes: 0

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