from "The AWK Programming Language" (A.V.Aho, B.W.Kernighan, P.J.Weinberger) recursive descendent parser

calc3.rb

# from "The AWK Programming Language" (A.V.Aho, B.W.Kernighan, P.J.Weinberger)
# recursive descendent parser

def expr
  e = term
  while $tokens[$index] == "+" || $tokens[$index] == "-"
    $index += 1
    e = ($tokens[$index - 1] == "+") ? e + term : e - term
  end
  return e
end

def term
  e = factor
  while $tokens[$index] == "*" || $tokens[$index] == "/"
    $index += 1
    e = ($tokens[$index - 1] == "*") ? e * factor : e / factor
  end
  return e
end

def factor
  if $tokens[$index] =~ /^[+-]?(\d+\.?\d*|\.\d+)$/
    $index += 1
    return $tokens[$index - 1].to_f
  elsif $tokens[$index] == "("
    $index += 1
    e = expr
    $index += 1
    if $tokens[$index - 1] != ")"
      $stdout.printf("error: missing ) at %s\n", $tokens[$index])
    end
    return e
  else
    $stdout.printf("error: expected number or ( at %s\n", $tokens[$index])
    return 0
  end
end

while (line = $stdin.gets)
  $index = 0
  $tokens = line.split
  e = expr()
  if $index < $tokens.size
    $stdout.printf("error at %s\n", $tokens[$index])
  else
    $stdout.printf("\5%.8g\n", e)
  end
end