Parsing an arithmetic expression and building a tree from it in Java

I wanted to respond to Bill K.'s answer, but I lack the reputation to add a comment there (that's really where this answer belongs). You can think of this as a addendum to Bill K.'s answer, because his was a little incomplete. The missing consideration is operator associativity; namely, how to parse expressions like:

49 / 7 / 7

Depending on whether division is left or right associative, the answer is:

49 / (7 / 7) => 49 / 1 => 49

or

(49 / 7) / 7 => 7 / 7 => 1

Typically, division and subtraction are considered to be left associative (i.e. case two, above), while exponentiation is right associative. Thus, when you run into a series of operators with equal precedence, you want to parse them in order if they are left associative or in reverse order if right associative. This just determines whether you are pushing or popping to the stack, so it doesn't overcomplicate the given algorithm, it just adds cases for when successive operators are of equal precedence (i.e. evaluate stack if left associative, push onto stack if right associative).

Assuming this is some kind of homework and you want to do it yourself..

I did this once, you need a stack

So what you do for the example is:

    parse    what to do?                Stack looks like
      (      push it onto the stack     (
      5      push 5                     (, 5
      +      push +                     (, 5, +
      2      push 2                     (, 5, +, 2
      )      evaluate until (           7            
      *      push *                     7, *
      7      push 7                     +7, *, 7
      eof    evaluate until top         49

The symbols like "5" or "+" can just be stored as strings or simple objects, or you could store the + as a +() object without setting the values and set them when you are evaluating.

I assume this also requires an order of precedence, so I'll describe how that works.

in the case of: 5 + 2 * 7

you have to push 5 push + push 2 next op is higher precedence so you push it as well, then push 7. When you encounter either a ) or the end of file or an operator with lower or equal precedence you start calculating the stack to the previous ( or the beginning of the file.

Because your stack now contains 5 + 2 * 7, when you evaluate it you pop the 2 * 7 first and push the resulting *(2,7) node onto the stack, then once more you evaluate the top three things on the stack (5 + *node) so the tree comes out correct.

If it was ordered the other way: 5 * 2 + 7, you would push until you got to a stack with "5 * 2" then you would hit the lower precedence + which means evaluate what you've got now. You'd evaluate the 5 * 2 into a *node and push it, then you'd continue by pushing the + and 3 so you had *node + 7, at which point you'd evaluate that.

This means you have a "highest current precedence" variable that is storing a 1 when you push a +/-, a 2 when you push a * or / and a 3 for "^". This way you can just test the variable to see if your next operator's precedence is < = your current precedence.

if ")" is considered priority 4 you can treat it as other operators except that it removes the matching "(", a lower priority would not.