Parsing mathmatical functions

Question

I have finally finished a Ruby calculator project, which is not based on eval. Instead it parses input char by char. The project is hosted on GitHub.

However, I find a specific part of the program very annoying since whenever I have to add something I have to rewrite a portion of it again.

def trig
 case @look
 when 'c'
 match_all('cos')
 case @look
 when 'h'
 match_all('h(')
 value = cosh(calculate)
 when '('
 match('(')
 value = cos(calculate)
 else
 expected('cos() or cosh()')
 end
 when 's'
 match('s')
 if @look == 'q'
 match_all('qrt(')
 value = sqrt(calculate)
 else
 match_all('in')
 case @look
 when 'h'
 match_all('h(')
 value = sinh(calculate)
 when '('
 match('(')
 value = sin(calculate)
 else
 expected('sin() or sinh()')
 end
 end
 when 'r'
 match_all('root')
 base = get_number
 match('(')
 value = calculate ** (1.0/base)
 when 't'
 match_all('tan')
 case @look
 when 'h'
 match_all('h(')
 value = tanh(calculate)
 when '('
 match('(')
 value = tan(calculate)
 else
 expected('tan() or tanh()')
 end
 when 'l'
 match('l')
 case @look
 when 'n'
 match_all('n(')
 value = log(calculate)
 when 'o'
 match_all('og')
 if digit? @look
 base = get_number
 elsif @look == "("
 base = 10
 else
 expected("integer or ( ")
 end
 match('(')
 value = log(calculate, base)
 else
 expected('ln() or log()')
 end
 when 'e'
 match_all('exp(')
 value = exp(calculate)
 when 'a'
 match_all('arc')
 case @look
 when 'c'
 match_all('cos')
 case @look
 when 'h'
 match_all('h(')
 value = acosh(calculate)
 when '('
 match('(')
 value = acos(calculate)
 else
 expected('arccos() or arccosh()')
 end
 when 's'
 match_all('sin')
 case @look
 when 'h'
 match_all('h(')
 value = asinh(calculate)
 when '('
 match('(')
 value = asin(calculate)
 else
 expected('arcsin() or arcsinh()')
 end
 when 't'
 match_all('tan')
 case @look
 when 'h'
 match_all('h(')
 value = atanh(calculate)
 when '('
 match('(')
 value = atan(calculate)
 else
 expected('arctan() or arctanh()')
 end
 end
 else
 raise InvalidInput, "unexpected input: \"#{@look}\""
 end
 match(')')
 value
end

The @look variable will contain the next character. The match_all function will simply match the input character by character. For example, it would match c and if it works it continues to match o and so on, but if it doesn't it will output an error.

Answer 1

Here's a toy program to illustrate the separation of user input collection from the string matching that determines which calc to use, as we discussed in the comments. Note that the class NamedCalculations has no knowledge of IO with the user. All the IO is encapsulated in CmdLineCalculator.

When I find a match, I just evaluate it with the argument pi. In your real application, you'd need to do further process to gather the user's input for the argument.

Also note how if we decide we want to change which calculations we support (one of the most likely changes for the application), it becomes a 1 line change, and the user input processing code doesn't need to be touched.

class NamedCalculations
 # a hash of named calcs
 # eg, {cos: Math.method(:cos), sin: Math.method(:sin) }
 def initialize(calc_definitions)
 @calc_definitions = calc_definitions
 @calc_names = @calc_definitions.keys.map(&:to_s)
 end
 def method_for(calc_name)
 calc_name = calc_name.to_sym
 raise "Invalid Calculation Name" unless @calc_definitions.key? calc_name
 @calc_definitions[calc_name]
 end
 def exact_match?(input)
 @calc_names.any? {|name| input == name}
 end
 def partial_match?(input)
 @calc_names.any? {|name| name.include? input}
 end
end
class CmdLineCalculator
 def initialize(named_calcs)
 @named_calcs = named_calcs
 @input = ''
 end
 def start
 begin
 system("stty raw -echo")
 collect_user_input while not calc_complete?
 show_answer
 ensure
 system("stty -raw echo")
 end
 end
 private
 def collect_user_input
 @input += next_input_char
 validate_input
 echo_last_char
 end
 def validate_input
 raise "Invalid Calculation" unless @named_calcs.partial_match? @input
 end
 def next_input_char
 STDIN.getc
 end
 def echo_last_char
 STDOUT.putc @input[-1]
 end
 def calc_complete?
 @named_calcs.exact_match? @input
 end
 def show_answer
 selected_method = @named_calcs.method_for(@input)
 value_at_pi = selected_method.call(Math::PI)
 STDOUT.puts "\nThe #{@input}(pi) = #{value_at_pi}" 
 end
end
my_calcs = {cos: Math.method(:cos), sin: Math.method(:sin) }
client = CmdLineCalculator.new(NamedCalculations.new(my_calcs))
client.start

Answer 2

Should sqrt, exp, etc. be in your Trig function? This seems to break the S in SOLID. I would abstract out your trig function and write something like:

def trig(input, val)
 case input
 when "sinh" return sinh(val)
 when "sin" return sin(val)
 when "cosh" return cosh(val)
 ... // cos and tans
 end
end

You were right to be concerned about repeating similar pieces of code. That's something that should be avoided at all times. Coding is about being concise and minimal.

• Mohammad: The problem with this approach is that I don't know the next character . For example, if I find an s I don't know if it is sin or sqrt
• Jonah: @Mhmd, How do you not know the next character? Does match_all collect user input 1 character at a time? Can you explain why it works this way?
• Mohammad: @jonah yes it does that, I did this based on the "let's build a compiler" book and then added some functionality.
• Jonah: @Mhmd, In that case the first thing you want to do is to separate the logic for matching a string to a computuation (sin, cos, etc getting mapped to their matching methods) from the user input collection. You should have one method that is collecting user input, and after each charcter is typed, it should check if there is a match yet. Does that make sense. You have the matching method responsible for collecting input. But the two things are totally separate. The matching method should know nothing about input collection.
• Jonah: @Mhmd, sure, but what happens 1) if the user types something that doesn't match any calculation? 2) after the user types the open paren... eg, I type "cos(" -- now what happens? does it now collect the function's arguments? Basically, it would be easier to help with a full answer if I knew the spec of the program itself.