I am using the Shunting-Yard algorithm (https://en.wikipedia.org/wiki/Shunting-yard_algorithm) in a Java program in order to create a calculator. I am almost done, but I still have to implement functions. I have ran into a problem: I want the calculator to automatically multiply variables like x and y when put together - Example: calculator converts xy to x*y. Also, I want the calculator to convert (x)(y) to (x)*(y) and x(y) to x*(y). I have done all of this using the following code:
infix = infix.replaceAll("([a-zA-Z])([a-zA-Z])", "$1*$2");
infix = infix.replaceAll("([a-zA-Z])\\(", "$1*(");
infix = infix.replaceAll("\\)\\(", ")*(");
infix = infix.replaceAll("\\)([a-zA-Z])", ")*$1");
(In my calculator, variable names are always single characters.)
This works great right now, but when I implement functions this will, of course, not work. It will turn "sin(1)" into "s*i*n*(1)". How can I make this code do the multiplication converting only for operators, and not for functions?
Preprocessing the input to parse isn't a good way to implement what you want. The text replacement can't know what the parsing algorithm knows and you also lose the original input, which can be useful for printing helpful error messages.
Instead, you should decide on what to do according to the context. Keep the type of the previously parsed token wth a special type for the beginning of the input.
If the previous token was a value token – a number, a variable name or the closing brace of a subextression – and the current one is a value token, too, emit an extra multiplication operator.
The same logic can be used to decide whether a minus sign is a unary negation or a binary subtraction: It's a subtraction if the minus is found after a value token and a negation otherwise.
Your idea to convert x(y) to x * (y) will, of course, clash with function call syntax.