unary minus in shunting yard expression parser

Question

here is my expression parser using shunting-yard algorithm it work well as expected except in one situation , when I use unary minus like in -2*3 it wont work (I think it s

Answer 1

The above option is correct, but it would get very cumbersome and buggy. Consider the case 2*-(1+2)^-(2+5*-(2+4)). As you can see you need to take in account many things. Also whenever you find "*-(" for example you know that you'll substitute that with *(0-(...., which would be coded in a cumbersome recursive function. The best solution is much easier. At the operators, take in account the cases when the operator is "-" and it is preceded by another operator, or preceded by a left parenthesis, or when it is the first character of the input (these cases mean that it is a unary minus rather than binary). In this case, you change it to another character, say 'u' (this was my case), and make its precedence the same as that of '^'.

Also, treating it as part of the number literal has its catch. Imagine a case such as -2^4. In Wolfram Alpha you'd get -16, not 16.

And consider using stacks. They'll make your life easier.

Let me explain what I meant. Consider you are given the input:

2 / - 7 + ( - 9 * 8 ) * 2 ^ - 9 - 5

Making the replacements I suggested, it would become like this:

2 / u 7 + ( u 9 * 8 ) * 2 ^ u 9 - 5

Now your operator precedence switch should be changed to:

switch(c)
{
       case '-' : case '+' :
           return 1 ;
       case '*' : case '/' :
           return 2 ;
       case '^' : case 'u': //note the 'u' operator we added
           return 3 ;
       default :
           return 0 ;
}

And, of course, you need to make changes to support this unary operator.

Answer 2

One option is to put a 0 in front if the first character is '-'. You have to do this also when the - is after a (.

Nicer ones are implementing either the unary minus operator or treating it as part of the number literal.