Re: how to return the position in the input string where the token is found

[Joseph Kraenzle]

As mentioned in <http://www.gnu.org/manual/flex-2.5.4/html_node/flex_14.html>,

"The macro YY_USER_ACTION can be defined to provide an action which is 
always executed prior to the matched rule's action. For example, it could 
be #define'd to call a routine to convert yytext to lower-case. When 
YY_USER_ACTION is invoked, the variable yy_act gives the number of the 
matched rule (rules are numbered starting with 1). Suppose you want to 
profile how often each of your rules is matched. The following would do the 
trick:

#define YY_USER_ACTION ++ctr[yy_act]

where ctr is an array to hold the counts for the different rules. Note that 
the macro YY_NUM_RULES gives the total number of rules (including the 
default rule, even if you use `-s', so a correct declaration for ctr is:

int ctr[YY_NUM_RULES];"

I do a related tracking by making use of the YY_USER_ACTION macro.  You can 
track the number of characters that have been read from the string by lex 
in the current line with a little thought about storing some global 
indices.  And in the YY_USER_ACTION statement, knowing the length of the 
token in the string (accessible by using yyleng), you can subtract it from 
the current position of the lexer in the string, and find the location of 
the token (e.g. "+") in the string.  This can be stored into a global in 
the YY_USER_ACTION macro and accessed in the action part of the lexer.

I am sure there are other ways in the input process to track and mark the 
operator in the expression, and then call a function in the action part to 
grab the globally stored value.

At 01:44 AM 7/4/03 -0700, you wrote:
> Hi,
>
> We are trying to develop a friendly IDE for our own
> grammer. For this purpose we have created a Lexer
> which is taking input from a string buffer.
>
> The user enters the script in a rich edit control. The
> lexer is called after each character.
>
> Once a token is recognized we need to call a function
> in the action part of the lexer, that would return the
> position of the character in the string.
>
> For example if input string is "x+y". Where "+" is the
> token to be identified, the function needs to pass the
> position of "+" in the string i.e. 2.
>
> I am unable to find any means to return this
> position.Please help regarding this
>
> Thanks
>
>
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