# Parsing mathmatical functions

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.

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?
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

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


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.

• 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 – Mhmd Sep 20 '15 at 8:19
• @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? – Jonah Sep 21 '15 at 3:02
• @jonah yes it does that, I did this based on the "let's build a compiler" book and then added some functionality. – Mhmd Sep 21 '15 at 6:32
• @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 Sep 21 '15 at 18:17
• @Jonah thanks could you write this as an answer. – Mhmd Sep 21 '15 at 18:19