Evaluating Infix Expressions

When the user interacts with the calculator and presses certain buttons (0-9, ., +, -, ×, ÷), under the hood, they are actually building up a data structure that represents an infix expression. To evaluate that infix expression I use Dijkstra's Shunting Yard Algorithm to convert it into a postfix expression which is much easier to evaluate. The postfix expression is never explicitly returned since I evaluate it on the fly. As such, my evaluation function makes use of two stacks. Therefore, let's start with an implementation of a stack data structure before moving on to the star of the show.