Three function Calculator 2.0
A couple quick things to make maintenance easier:

It's easier to work out where you're going if you use continue instead of the goto. If you do keep the goto, I'd at least edit the comment to refer to the beginning of the while loop instead of a specific line number. That will change if you have to change something, and you don't want to have to remember to update the comment.

In terms of readability, when checking for order of operations, you could do something like

if (precedence(act2) &gt; precedence(act1))


Then it would be easier if you wanted to add an exponent operator.