Hi. I am trying to write a calculator that takes a string from the user and calculates the expression. For example,
55 * 4 + ( 55 - 30 ) + 7
I can use the string tokenizer and seperate the string into individual parts, then use parsedouble for the numbers, but is there a way to parse the operators?i cant figure this out.
You can use the equals method of the String class.
if(token.equals("+")) {
} else if(token.equals("-")) {
etc
And im not really sure how to form a math expression from a stack of strings. I can convert each number to a double but im not sure how to convert each string thats a symbol to a symbol.
If i had 2+4 i would have 3 strings in the stack. "2", "+", and "4"
i guess im just lost on how to turn this into an expression. sry.
You dont have to "turn a symbol to a symbol" It already is one. All you need to do is use the code snippet I provided in reply #1.
Lets say you have the expression 4 + 5 * 6.
Convert it to postfix: 5 6 * 4 +
Now to evaluate you iterate over the values.
If you come to an operand (number) push it onto the stack.
If you come to an operator, pop the two top most operands, perform the operation and push result back onto stack.
At the end, pop your result off the stack. It should be the only value left in the stack.
Ok so i searched google to find out how to convert it to postfix notation, and i cant really find a clear example. Is there a simple way. I saw a 300+ line code that does it. I am not sure if i will be able to use that.
I see this somewhat easy example and it looks like i can just add division and subtraction.Im usually good with my programs and have gotten 100% on all so far... i feel like a deer in headlights... haha
Also, How would this deal with parentheses?
public class Postfix {
public static void main(String[] args) {
Stack<Integer> stack = new Stack<Integer>();
while (!StdIn.isEmpty()) {
String s = StdIn.readString();
if(s.equals("+")) stack.push(stack.pop() + stack.pop());
else if (s.equals("*")) stack.push(stack.pop() * stack.pop());
else stack.push(Integer.parseInt(s));
}
System.out.println(stack.pop());
}
}
> Ok i kind of get that but it looks like it will only
> work in certain situations. what about an expression
> like... 5 + 6 * 3 * 6 * ( 4 + 3 + 2 ) + 12
It's a recursive parsing problem and you have to keep in mind the orders of operation and work from inside to outside.
1. Find the inner most matching parenthesis.
2. Resolve the expression
3. Replace the expression with the resolution of the expression
4. Repeat until you have no enclosing parenthesis
5. Resolve multiplicative terms (* or /)
6. Replace multiplicative terms with resolution of the term
7. Repeat until all multiplicative terms have been removed
8. Resolve additive terms (+ or -)
9. Replace additive terms with the resolution of the term
10. Repeat until no more additive terms remain
11. Should be done at this point.
This may have holes in it, but it's a first pass at how I would approach it, and it is VERY rudimentary with a lot of needed refactoring.
PS.
Thanks a lot for the link and your help. One more thing. I am taking the input as a string. So The main thing is, i can use parseDouble and change the numbers to numbers and send them to the output. What would i do to change the "*" to a * symbol?
Something like
if(str.nextToken() == "*"){
stack.push(*); // This wont work, erro missing term.
}
? So do i push a string "*" onto the stack?... lame final project......
One more thing... the link to http://en.wikipedia.org/wiki/Shunting_yard_algorithm
tells me to add the numbers to an output queue. what format would this be? since i use a stack for the operators , which way will i store the output?
Ok cool. Thanks a lot guys. I am gonna try to work on this some more and see if i can figure it out. Hopefully i will be able to. I appriciate all the help and responses from you all. wish me luck!!
Thanks again guys.
Flounder i didnt ignore what you said i actually appriciate it very much. Just a little confused when i googled it and couldnt find the best way... Thanks for your help and ill check ur new link as well.... just got done golfing so lets see if i can knock this parsing calculator out before its due at midnight :) .. thanks a bunch guys
>Glad to see you have your priorities in order.
hehe. Cant work all the time you know. gotta have some fun or whats the point in living. but thanks again. finished the assignment. dont know how clean my convertToPostfix method is but it should be good enough for full credit. This is just java27C (third class in my first java series). really appriciate all the help.
Time to go hit some balls :)
> >Glad to see you have your priorities in order.
>
> hehe. Cant work all the time you know. gotta have
> some fun or whats the point in living.
Agreed.
For what it's worth, you do appear to be making a genuine effort here. But I also understand why certain respondents feel their comments were ignored. It may be that you were overwhelmed, or didn't understand a given comment until somebody else repeated the same basic idea in different terms. That's fine, but in the future, please provide feedback as to what you didn't understand, disagreed with, etc.
