How to do user-inputted string templating in Ruby? - ruby

Question

I know writing like
a=23
p "the value of a is #{a}"
it will print: the value of a is 23.
but now I am actually receiving this string as a parameter like
def evaluate string
a=23
puts string
end
calling method pass that string as a parameter
evaluate "the value of a is #{a}"
Is there any way to evaluate this string inside the method? puts string has to interpolate the value a=23.
Edit:
I have to read and execute the program from Excel.
At the first line,
Excel entry is,
"id=something" setvalue a
So now corresponding program will read the value from locator id=something and set it into the instance variable #a.
and user's next excel entry would be
"the value of a is 23" compare "the value of a is #{a}"
Now the program will read "the value of a is 23" and this "the value of a is #{a}" for comparison, but before it compares, it has to replace the value a. That's all I want. I hope now my question is very clear.

Answer 1

For ruby you can change how you "format" your strings in Excel, than you can use "classic" formatting
a = 23
s = 'the value of a is %s'
def evaluate(text, value)
puts text % value
end
You can use different formatting keys, for example %d for integers, %f for float numbers
You can use named arguments
dynamic_text = 'the value of the %<product_name>s is %<product_price>0.2f'
def evaluate(text, args)
puts text % args
end
name = "Product"
price = 78.99
evaluate dynamic_text, product_name: name, product_price: price
Without names, use order of the given values
dynamic_text = 'the value of the %s is %0.2f'
def evaluate(text, args)
puts text % args
end
name = "Product"
price = 78.99
evaluate dynamic_text, [name, price]

Answer 2

You can make a block and then evaluate the string:
def evaluate &block
a=23
block.call(a)
end
evaluate { |a| "the value of a is #{a}" } #=> "the value of a is 23"

Answer 3

It's a very odd thing you're attempting to do. When you have some sort of a pattern with placeholders, you do it like:
def evaluate(string)
a=23
format string, a: a
end
evaluate "the value of a is %{a}"
String interpolation with #{..} is not meant for the case you're describing as the value is evaluated at the time of constructing the string, not later. You could do some regexp matching and replace the #{..} with %{..} as a workaround.

Answer 4

There's a few ways:
"Code" Dynamic
lazy evaluation with lambdas:
def evaluate(str_template)
a = 23
str_template.call(a)
end
user_input = gets
my_lambda = lambda do |str|
user_input.size > 10 ? "dynamic 1 #{str}" : "dynamic 2 #{str}"
end
evaluate(my_lambda)
# => "dynamic 1/2 23"
This is "code dynamic", but not "input dynamic", i.e. you can't receive the string template from the user.
"Input" Dynamic 1
ERB templating:
require 'erb'
user_input_erb = gets
puts user_input_erb # "Hello <%= name %>"
name = gets # also user input, e.g. "World"
ERB.new(user_input_erb).result
# => "Hello World"
Note that in general, getting string templates from the user and evaluating them is a potential security vulnerability. If there's any possibility user input can be adversarial, you'll want to see if you can find a "guaranteed to be safe against all user input" string templating library.
"Input" Dynamic 2
user_input_template = gets
puts user_input_template # "Hello %s"
name = gets # also user input, e.g. "World"
user_input_template % name
# => "Hello World"
"Input" Dynamic 3
Really dangerous, but:
user_input_ruby_code = gets
puts user_input_ruby_code # '"Hello #{name}"'
name = gets # also user input, e.g. "World"
eval user_input_ruby_code # DANGER
# => "Hello World"