Arithmetic expression evaluator

Arithmetic expression evaluator based on the Shunting Yard Algorithm

C++ implementation of the Shunting Yard Algorithm by E.W. Dijkstra.

Implementación en C++ del algoritmo Shunting Yard propuesto por E. W. Dijkstra.

Adaptado para aceptar variables, funciones personalizadas y el operador unario de negación "~"

Shunting Yard Algorithm by Edsger Wybe Dijkstra (E. W. Dijkstra)

(Wikipedia Version) https://en.wikipedia.org/wiki/Shunting_yard_algorithm

while there are tokens to be read:
  read a token
  if the token is:
  - a number:
    put it into the output queue
  - a function:
    push it onto the operator stack 
  - an operator o1:
   while (
     there is an operator o2 at the top of the operator stack which is not a left parenthesis, 
     and (o2 has greater precedence than o1 or (o1 and o2 have the same precedence and o1 is left-associative))
    ):
       pop o2 from the operator stack into the output queue
   push o1 onto the operator stack
  - a left parenthesis (i.e. "("):
    push it onto the operator stack
  - a right parenthesis (i.e. ")"):
    while the operator at the top of the operator stack is not a left parenthesis:
      {assert the operator stack is not empty}
       // If the stack runs out without finding a left parenthesis, then there are mismatched parenthesis. 
        pop the operator from the operator stack into the output queue
        {assert there is a left parenthesis at the top of the operator stack}
	pop the left parenthesis from the operator stack and discard it
	if there is a function token at the top of the operator stack, then:
		pop the function from the operator stack into the output queue
	
// After the while loop, pop the remaining items from the operator stack into the output queue. 
while there are tokens on the operator stack:
	// If the operator token on the top of the stack is a parenthesis, then there are mismatched parenthesis. 
    {assert the operator on top of the stack is not a (left) parenthesis}
    pop the operator from the operator stack onto the output queue