Question

I have a javascript / jquery code that takes in a formula and numbers from the user and solves the equation. My code in jquery can get the solution down to real numbers, but in the form of a string and I can’t get javascript to solve the equation mathematically.

I have something like

1000 * (200+3928)/2333

However, those numbers are all in string format and forcing each of them into floats or integers and reinserting them only turns them back into a string. Tried using valueOf and other things, but nothing works. Is there a better way or something I am missing?

Thanks

Answer 1

When you have a simple equation in the form of a string, the most practical method is to parse the string into prefix or postfix notation, then evaluate it accordingly…

Assuming the original string is infix (the notation humans use to learn basic math) such as…

1000 * (200+3928)/2333

You then convert it to postfix to obtain…

1000, 200, 3928, +, *, 2333, /

Using this notation, a computer can easily evaluate the expression using a simple loop and stack…

I wont post actual code, because I want to leave the fun parts up to you, but if you’d like a headstart with psuedo code, here goes…

infix to postfix :

  create empty array `postfix` and `temp`

  split the expression up into an array `A` by each operand/operator

  foreach token in `A`:
    if char is operand :
      push to postfix stack
    if char is open parenthesis:
      push to temp stack
    if char is close parenthesis:
      while top of temp stack != '(' :
        push top of temp stack to postfix stack
        pop top of temp stack
      end while
      pop top of temp stack
    if char is operator:
      while temp stack isn't empty *and* top of temp stack != '(' *and* precendence(char) <= precendence(top of temp stack) :
        push top of temp stack to postfix stack
        pop top of temp stack
      end while
      push char to temp stack

  end for loop

  while temp stack is not empty
    push top of temp stack to postfix stack
    pop top of temp stack
  end while

  return postfix stack (will be your postfix stack to evaluate)



evaluate postfix stack:
  create array A
  foreach token in postfix stack:
    if char is operand:
      push to A stack
    if char is operator:
      int x = pop A stack
      int y = pop A stack
      result = x (operation) y
      push result to A stack
  end for loop

  return top of A stack (this will be the final result)