Question

As a novice it’s probably something about Ruby I missed, but for the life of me I don’t understand this result. So I have this simple function:

def crazyfunc(s)
  s.gsub!('a', 'b')
  #return has not purpose here
end

Now I have this simple few sets.

s1 = 'abc'
s2 = s1
s2 = crazyfunc(str2)

puts s1

=> bbc

Why in the world is s1 affected by crazyfunc? So doing this instead:

def crazyfunc(s)
  return s.gsub('a', 'b')
end

doesn’t change str1, so I figure it’s got to do with what the inplace gsub is doing. But I still don’t get the logic of why str1 would be changed.

Answer 1

String assignment in Ruby doesn’t implicitly copy the string. You are simply assigning another reference to it. If you want to copy the string, use clone.

To demonstrate, you can check object IDs:

ree-1.8.7-2010.02 > a = "foo"
 => "foo"
ree-1.8.7-2010.02 > b = a
 => "foo"
ree-1.8.7-2010.02 > a.object_id
 => 81728090
ree-1.8.7-2010.02 > b.object_id
 => 81728090

Since a and b have the same object ID, you know they’re the same object. If you want to modify b as a copy of a, you can either use methods which return a new string (like gsub rather than gsub!), or you can use b = a.clone, and then operate on b.

ree-1.8.7-2010.02 > a = "foo"
 => "foo"
ree-1.8.7-2010.02 > b = a.clone
 => "foo"
ree-1.8.7-2010.02 > a.object_id
 => 81703030
ree-1.8.7-2010.02 > b.object_id
 => 81696040
ree-1.8.7-2010.02 > b.gsub! "f", "e"
 => "eoo"
ree-1.8.7-2010.02 > a
 => "foo"
ree-1.8.7-2010.02 > b
 => "eoo"

Or more simply:

ree-1.8.7-2010.02 > a = "foo"
 => "foo"
ree-1.8.7-2010.02 > b = a.gsub("f", "e")
 => "eoo"
ree-1.8.7-2010.02 > puts a, b
foo
eoo