From: Axel Etzold <AEtzold@...>
Date: 2007-06-05T05:03:31+09:00
Subject: Re: Help needed: Dynamically create a Proc from a String and evaluate it

Dear Jason,

thank you for responding.
Here is what I have so far ( I hope the formatting won't
be destroyed):

class String
	def generate_formula
		# left-hand side of equation gives the function
		f=self.split(/\=/)
		formula='def ' + f[0]
		args=f[1].gsub(/[^\+\-\*\/]+\(/,'').gsub(/\)/,'').split(/[\/\*\-\+]/).uniq
		args_new=args.collect{|x|  if x==x.capitalize ; x='_' + x; else x=x; end}
		rhs=f[1]
		lhs=f[0]
		args.each_with_index{|x,i|
			if args_new[i]!=x
				lhs.gsub!(x,args_new[i])
				rhs.gsub!(x,args_new[i])
			end
		}
		formula='def ' + lhs + "\n"
		#formula<<'p ' + lhs
		#formula<<'p ' + rhs
		formula<<"\n return Proc.new{|" + args_new.join(',') + '| ' 
		formula<< rhs  + ' }'
		formula<<"\n end \n"
		name=f[0][0...f[0].index('(')]
		# return the following:
		# 1) formula=String containing Proc method to be evaluated
		# 2) name of the Proc method
		# 3) Array of arguments ( variables and constants occurring
		#	in the original formula
		# 4) arguments from 3) changed to non-uppercase, if needed
		return formula,name,args,args_new
	end
	def eval_formula(hash)
		f,name,vars,new_vars=self.generate_formula
		# do this to change uppercase variables, if needed
		hash2={}
		hash.each{|k,v|
			ind=vars.index(k)
			if v.class!=Array
				v=[v]
			end
			hash2.store(new_vars[ind],v)
		}
		# generate string for Proc to be evaluated
		ind1=f.split(/\n/)[0].index('(')
		ind2=f.split(/\n/)[0].index(')')
		args_left=f.split(/\n/)[0][ind1+1...ind2]
		other_args=new_vars-args_left.split(',')
		eval_fn = name + '(args_left)'
		max_nr_of_vals=[]
		# how many evaluations ?
		# to do: check that number of changed values
		# is either 1 or coincides for all variables
		hash2.values.each{|x| max_nr_of_vals<<x.length }
		# to evaluate Proc (later), substitute constant values
		ev_hash={}
		for k in 0...max_nr_of_vals.max
			ev_hash_text=eval_fn + '.call('
			for my_var in other_args
				if hash2[my_var].length>1
					this_val=hash2[my_var][k] # several values of "constants"
				else
					this_val=hash2[my_var][0]  # just one value
				end
				ev_hash_text<<this_val.to_s + ','
			end
			ev_hash_text.chop!
			ev_hash_text<<')'
			repl=''
			
			# substitute actual function values
			args_left.each{|y| 
				for m  in hash2[y]
					repl<<m.to_s + ','
				end
			}
		
		
			ev_hash_text.sub!('args_left',repl.chop)
			# this is the call to the formula with substituted values
			p ev_hash_text
			eval(ev_hash_text)
		end
	end
end


 
a='f(x)=c*exp(x/k)'
b=a.generate_formula
a.eval_formula({'c',[5.0],'k',[2.0],'x',[1.0]})


Thank you,

Axel 
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