How to associate an operator with a priority value?

Question

I want to implement Dijkstra's Shunting-yard algorithm for the Reverse Polish notation. In order to do that, I have to implement a table which stores priority for operators, as such:

operator	priority
(	0
+, -	1
*, /	2

How do I do that?

Answer 1

Since you have a fixed-number of key-value pairs that are known at compile time, you don't need the dynamic nature and overhead of std::unordered_map. You can use a simple switch statement, which is static and harder to get wrong:

int get_priority(char const op) {
  switch (op) {
    case '(':
      return 0;
    case '+':
    case '-':
      return 1;
    case '*':
    case '/':
      return 2;
    default:
      return -1; // handle the error
  }
}