CODEWITHSUNDEEP
rubyeval
Tommy
Tommy

Reputation: 985

Ruby evaluate without eval?

How could I evaluate at mathematical string without using eval?

Example:

mathstring = "3+3"

Anyway that can be evaluated without using eval?

Maybe something with regex..?

Upvotes: 5

Views: 3507

Answers (5)

Anand
Anand

Reputation: 3760

Dentaku seems (I haven't used it yet) like a good solution - it lets you check your (mathematical and logical) expressions, and to evaluate them. 

calculator = Dentaku::Calculator.new
calculator.evaluate('kiwi + 5', kiwi: 2)

Upvotes: 0

mdesantis
mdesantis

Reputation: 8517

You must either or eval it, or parse it; and since you don't want to eval:

mathstring = '3+3'
i, op, j = mathstring.scan(/(\d+)([+\-*\/])(\d+)/)[0] #=> ["3", "+", "3"]
i.to_i.send op, j.to_i #=> 6

If you want to implement more complex stuff you could use RubyParser (as @LBg wrote here - you could look at other answers too)

Upvotes: 7

Hauleth
Hauleth

Reputation: 23556

You have 3 options:

  1. In my honest opinion best - parse it to Reverse Polish Notation and then parse it as equation
  2. As you say use RegExps

  3. Fastest, but dangerous and by calling eval but not Kernel#eval

    RubyVM::InstructionSequence.new(mathstring).eval

Upvotes: 1

user21033168
user21033168

Reputation: 444

I'm assuming you don't want to use eval because of security reasons, and it is indeed very hard to properly sanitize input for eval, but for simple mathematical expressions perhaps you could just check that it only includes mathematical operators and numbers?

mathstring = "3+3"
puts mathstring[/\A[\d+\-*\/=. ]+\z/] ? eval(mathstring) : "Invalid expression"
=> 6

Upvotes: 3

Dan Tao
Dan Tao

Reputation: 128317

Sure--you'd just want to somehow parse the expression using something other than the bare Ruby interpreter.

There appear to be some good options here: https://www.ruby-toolbox.com/search?q=math

Alternatively, it probably wouldn't be that hard to write your own parser. (Not that I've seriously tried--I could be totally full of crap.)

Upvotes: 0

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