Precedence Function

Question

I'm trying to write a function, that goes within my main program, that will compare the precedence of operators being input by the user. The function would look for the following operators:

+, -, * , / with *, / having the highest precedence and +, - having the lowest precedence.

The operators with the lowest precedence go on the bottom of a stack while those with the highest precedence go on the top of the stack. So if a user types in +-*/, */+- should be displayed in the terminal.

Any ideas on how I should go about this?

Answer 1

Just sort them using std::sort with a custom predicate.

int precedence(char op)
{
    if (op == '*' || op == '/') return 0;
    return 1;
}

bool comparePrecedence(char a, char b)
{
    return precedence(a) < precedence(b);
}

int main()
{
    char input[] = "+-/";
    std::sort(input, input + 3, &comparePrecedence);
}

Obviously, you can add other operators simply by adding them into the precedence function.

Answer 2

Read all of the operators into a std::string. Write a function with the following type:

bool is_higher_precedence(char lhs, char rhs);

This function should return whether lhs has higher precedence than rhs.

You can then use this function as the functor parameter of std::sort to sort the operators by precedence.