The Book of Ruby

eval

The eval method provides a simple way of evaluating a Ruby expression in a string. At first sight, eval may appear to do the same job as the #{ } delimiters in a double-quoted string. These two lines of code produce identical results:

eval.rb

puts( eval("1 + 2" ) )    #=> 3
puts( "#{1 + 2}" )        #=> 3

Sometimes, however, the results may not be what you are expecting. Look at the following, for instance:

eval_string.rb

exp = gets().chomp()    #<= User enters 2*4
puts( eval( exp ))      #=> 8
puts( "#{exp}" )        #=> 2*4

Let’s suppose you enter 2 * 4, and this is assigned to exp. When you evaluate exp with eval, the result is 8, but when you evaluate exp in a double-quoted string, the result is "2*4". This is because anything read in by gets() is a string and "#{exp}" ...