Today I ran (again) into the following question:

- Write a function
**that doesn’t use eval** that calculates input strings with operators +,-,*,/ eg. “5+5*6+4/2″ should output 37

The first time I ran into this question I designed a solution, in Java, which had a SimpleCalculator class, and `enam Operation`

which supported the four basic arithmetic operations: +-*/ each of which had an `apply`

method etc. Very object oriented.

When I read this question today, I figured that it would be a nice exercise to do in Ruby – a few minutes later the result spread, elegantly, over less than 20 lines of code (and I bet that a professional Rubiest can do it in less)!

def is_number? expr
return false if expr.nil?
expr = "#{expr}" # we need this condition in case the expr is a number
expr.match /^(\d+|\d+\.\d+)$/ # since match() is defined only for strings
end
def calc(expr)
return expr.to_i if is_number? expr
expr.gsub!(" ","") # clean the string from whitespaces
# pay attention to the order: + and - should come before * and /
# can you figure out why ?
arr = expr.split /\+/
return arr.inject(0){|x,y| calc(x) + calc(y) } if arr.size > 1
arr = expr.split /\-/
return arr.inject(0){|x,y| calc(x) - calc(y) } if arr.size > 1
arr = expr.split /\*/
return arr.inject(1){|x,y| calc(x) * calc(y) } if arr.size > 1
arr = expr.split /\//
return arr.inject {|x,y| calc(x) / calc(y) } if arr.size > 1
end
puts calc("5+5* 6+4/2.0")
#output 37

Do you have a better/shorter/more elegant solution ?

Please post it in the comments section!