When I faced issues running a calculator using a YACC compiler, the problem usually came down to syntax errors, missing tokens, or linking issues between YACC and the accompanying lexical analyzer (typically written using Lex or Flex).

First, I made sure my .y file had a proper grammar definition for expressions, numbers, and operators. Then, I confirmed that the tokens used in the YACC file were correctly defined in the Lex file. One common mistake I made early on was mismatching token names—for example, using NUMBER in YACC but defining it as num in Lex.

Another issue I ran into was forgetting to compile and link both files properly. The correct order matters:

lex calc.l

yacc -d calc.y

cc lex.yy.c y.tab.c -o calc -ll

If you skip the -d flag with YACC, the y.tab.h file (which defines tokens) won’t be generated, leading to missing token definition errors during compilation.

Also, make sure you’ve handled precedence and associativity rules for operators. If you get shift/reduce or reduce/reduce warnings, it's often because operator precedence isn't clearly defined.

Debugging with yydebug or inserting print statements helped me trace how tokens were parsed and identify where things were going wrong.

If you’re stuck, feel free to share your .y and .l files—happy to help you debug.