Unreproducible string comparison, forces elsif failure.

Question

In answering this code golf question, I ran across a problem in my answer.

I've been testing this and I cannot even get these two comparisons to work in the code, despite the fact that IRB has the right behavior. I really need some help here.

Here's the code, below that will be an explanation of the problem.

def solve_expression(expr)
  chars = expr.split '' # characters of the expression
  parts = []  # resulting parts
  s,n = '','' # current characters

  while(n = chars.shift)
    if (s + n).match(/^(-?)[.\d]+$/) || (!chars[0].nil? && chars[0] != ' ' && n == '-') # only concatenate when it is part of a valid number
      s += n
    elsif (chars[0] == '(' && n[0] == '-') || n == '(' # begin a sub-expression
      p n # to see what it breaks on, ( or -
      negate = n[0] == '-'
      open = 1
      subExpr = ''
      while(n = chars.shift)
        open += 1 if n == '('
        open -= 1 if n == ')'
        # if the number of open parenthesis equals 0, we've run to the end of the
        # expression.  Make a new expression with the new string, and add it to the
        # stack.
        subExpr += n unless n == ')' && open == 0
        break if open == 0
      end
      parts.push(negate ? -solve_expression(subExpr) : solve_expression(subExpr))
      s = ''
    elsif n.match(/[+\-\/*]/)
      parts.push(n) and s = ''
    else
      parts.push(s) if !s.empty?
      s = ''
    end
  end
  parts.push(s) unless s.empty? # expression exits 1 character too soon.

  # now for some solutions!
  i = 1
  a = parts[0].to_f # left-most value is will become the result
  while i < parts.count
    b,c = parts[i..i+1]
    c = c.to_f
    case b
      when '+': a = a + c
      when '-': a = a - c
      when '*': a = a * c
      when '/': a = a / c
    end
    i += 2
  end
  a
end

The problem occurs in the assignment of negate.

I need negate to be true when the character just before a expression is a dash, but the condition isn't even working. Both n == '-' and n[0] == '-', the form of quotation doesn't matter, wind up FALSE every time. Yet, I've been using this exact comparison and n == '(' works correctly every time!

What is going on? Why doesn't n == '-' work, when n == '(' does? This is encoded in UTF-8 w/o BOM, UNIX linebreaks.

What's wrong with my code?

Answer 1

You have:

if (s + n).match(/^(-?)[.\d]+$/) || (!chars[0].nil? && chars[0] != ' ' && n == '-')
      s += n
elsif (chars[0] == '(' && n[0] == '-') || n == '('

As n is always a one-character string, if (chars[0] == '(' && n[0] == '-')) is true, then the previous condition, (!chars[0].nil? && chars[0] != ' ' && n == '-'), will also be true. Your code will never enter the second part of the if if n[0]=='-'.

If your p n line is outputting a dash, be sure it is exactly the same character you are looking for, not some character that looks like a dash. Unicode has many kinds of dashes, maybe you have a weird unicode dash character in your code or on your input.