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example in yacc

Posted on 1998-11-16
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Last Modified: 2010-04-15
Could some one write and example program in yacc that shows how to use the pseudovariables $1,$2,etc.. to get the value of a variable and puts it in a char variable?
Thanks
Will give more points later.
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Question by:laeuchli
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Author Comment

by:laeuchli
ID: 1254320
come on guys, a comment at least.
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Expert Comment

by:mkahlert
ID: 1254321
Here's an example, perhaps this helps:

actualparams
      :S_LB actualparamlist S_RB
            { $$ = $2; }
      |
            { $$ = NULL; }
      ;

The $1 is the first term of the definiton, $2 the second, and so on.
e.g.

term: '(' term ')'
   -> $1 = '('
   -> $2 = term
   -> $3 = ')'
And: $$ is the result (the left side, "term").
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Expert Comment

by:elfie
ID: 1254322


Sorry don't have time to make a small  example  but you cvan check

http://www.cs.um.edu.mt/~hzarb/CSM201/notes/lecture13/node24.html
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by:elfie
ID: 1254323
another exaple and 'manuals'

http://www-iiuf.unifr.ch/idp/applications/bison.html

check here the infix notation calculator: calc (23.0)

ps: remember bison is the GNU equivalent of yacc.
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by:laeuchli
ID: 1254324
sorry, mkahlert,I want more complete sample code.Start from the start of a yacc program.

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Expert Comment

by:BigRat
ID: 1254325
>> Will give more points later.

Should give the point now! (to elfie)
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Expert Comment

by:avigon
ID: 1254326
Very easy, it only recognizes this expression: "1+2", but it works.

Code:

%%

result: '1' '+' '2'
          { $$ = $1 + $3; }

%%

it parses that expression and gives to the "result" ($$), the value of the addition of '1' ($1) and '2' ($3). In this case, $2 is '+'.

You must do now a mini-lex and a main, that can be defined in the yacc-file if you want... Anyway as mkahlert pointed, the idea is that the variables $1,... represent the values of the respective position-field. In the example $$ = value of "result", and if we use a rule like this:

  result : var '+' var

the variables $1 and $3 will have the value of each var, that is is you parse the string: 1+2, "result" will store the value 3 at the end of the parsing.





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by:laeuchli
ID: 1254327
well how do you print the value of $$?

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Accepted Solution

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avigon earned 10 total points
ID: 1254328
You just put:


%%

result: '1' '+' '2'
                { $$ = $1 + $3;
                   printf("%d",$$);  }

%%

For example.
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by:laeuchli
ID: 1254329
Ok,but what if I wanted to do this with a char? and is the one and two the first two of the tokens for that section or for the whole program. Thanks.

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