Infix to postfix problem with unary operators

I use the following logic to implement an infix to postfix conversion, to evaluate it later.

• The loop on the infix conversion, and in each iteration, do the following:

  • If space, ignore it.
  • If operator, then keeps popping from the stack and add to the postfix output until the stack is empty or the top of the stack has a lower priority than the current operator. Then, push the current operator.
  • If '(', push it.
  • If '), keep popping and adding to the postfix until you find '('. Then pop the '(' without adding it.
  • Otherwise, then it's a number. Add it directly to the postfix output.

Notes: When I encounter a + or -, I can determine whether it's a binary or unary operator. If it's binary I add it to the stack as '+' or -, but if it's unary I add it as '@' or '$'.

The algorithm works well, except in a case where two unary operators are next to each other.

For example, "--4" becomes "@ 4 @", which is wrong.

What's wrong? What's the correct fix to this issue, that doesn't break other cases?

Looks like you need to change your rules so that you don't pop for successive unary operators. That is, given "--4":

1. You identify the - as a unary operator, and push @
2. You identify the next - as a unary operator, see that the operator on the stack is also a unary operator, and push another @.
3. You see the 4, and output it.
4. At the end of the string, you pop the two unary operators, giving you "4@@".

And of course the unary operators should have higher precedence than any other operator, so that they'll always get popped before any other operator is pushed.