野牛中的哈希查找麻烦

Question

I'm working on a sort of the classic bison/flex calculator implementation problem. The issue that I've run is with variable assignment. It seems to work at first; only declared variables can be referenced, it remembers the value that I've assigned... except that all variables seem to share a single value, rather than each having its own. So if I do "x=3; y=5;" then both x and y have the value of 5.

Obviously I'm doing something wrong, but I'm not sure if this is a failure in my understanding of bison, c++, or how the unordered_map data structure works. (Or perhaps, all 3)

I believe I have included all the relevant parts of the code below, but if there is some missing, let me know.

%{

    #include <iostream>
    #include <string>
    #include <unordered_map>
    using namespace std;

    unordered_map<string, double> dict;
%}    

%union {
    double dval;
    char *sval;
}

%token <sval> VARIABLE
%token <dval> NUMBER
%type<dval> expr

%%

assign: 
     VARIABLE '=' expr {dict[$1] = $3; cout << $1<< "="<<dict[$1]<<endl; }
expr: VARIABLE {
    if(dict.find($1) == dict.end())
    {
        yyerror("Bad Var");
    }

Answer 1

You should really show your flex code as well, but I'm guessing that the relevant bit looks something like this:

{id}     { yylval.sval = yytext; return VARIABLE; }

What you need to do is:

{id}     { yylval.sval = strdup(yytext); return VARIABLE; }

And then you need to make sure that you don't leak memory, by freeing the allocated string after you use it in your bison action.

See the flex manual and the bison manual .

Answer 2

So my problem turned out to be not in how I was storing the data, but just in retrieving it. I needed to add the else statement below to actually set the value of expr.

expr: VARIABLE {
    if(dict.find($1) == dict.end())
    {
        yyerror("Bad Var");
    }
    else { //Needed this section
        $$ = dict[$1]
    }
}    

I think I knew I needed to do that and just forgot, but the behavior of expr taking the value of whatever the most recently assigned variable is threw me off.