- Takes an infix from the user in the form of a string
- Checks if it is a valid infix expression
- Converts the infix to postfix and displays the results
- Evaluates the infix and displays the results
A given infix expression is valid if:
- it is not an empty string
- it contains only digits (0-9), operators, and braces.
- it does not begin or end with an operator
- it contains valid number of operands (ie 2 in this case) for every operator