前缀Postfix Infix Java表达式求值程序

Question

i have been working on an expression evaluator in java and out of frustration I have also come here to ask about it. I have rewritten it at least 15 times by now and each to no avail.

Basically I need to take a String in either prefix, infix or postfix and evaluate it to an integer. The expression can use any arithmetic operators included in java.

The problems im having: - When i try to parse to an expression tree or parse them all to postfix i have problems with the monomial operators. - I can't get it to identify a multi digit number as one number and subsequently it ruins the whole tree or it switches around the digits. - I can't get the program to figure out where to put down the parentheses correctly and i can't get it to identify the beggining and end of the parentheses using a stack.

So basically the program needs to do this:

For any arbitrary numbers ABCDEF

Evaluate in prefix postfix and infix:

A-- + ++B - --C * D++ / E % F

For any arbitrary bits ABCDEFG and number n

Evaluate in prefix, postfix, and infix: A & B | C ^ ~D

n >> A

n << A

My textbook isn't much help nor are the books i have ordered, I really can't understand how to do this, please help, even just an explanation of how to do it would be good enough for me. Thanks for any help :D

Answer 1

Assuming that you have successfully tokenized all your identifiers, operators, and literals, the next step would be to read a bit about recursive descent parsers. You can probably write one by hand for this case rather than trying to figure out how to use a parser generator.