Are strings in Ruby mutable? [duplicate]

Question

Consider the following code:

$ irb > s = "asd" > s.object_id   # prints 2171223360 > s[0] = ?z     # s is now "zsd" > s.object_id   # prints 2171223360 (same as before) > s += "hello"  # s is now "zsdhello" > s.object_id   # prints 2171224560 (now it's different) 

Seems like individual characters can be changed w/o creating a new string. However appending to the string apparently creates a new string.

Are strings in Ruby mutable?

Answer 1

Yes, strings in Ruby, unlike in Python, are mutable.

s += "hello" is not appending "hello" to s - an entirely new string object gets created. To append to a string 'in place', use <<, like in:

s = "hello" s << "   world" s # hello world 

Answer 2

ruby-1.9.3-p0 :026 > s="foo"  => "foo"  ruby-1.9.3-p0 :027 > s.object_id  => 70120944881780  ruby-1.9.3-p0 :028 > s<<"bar"  => "foobar"  ruby-1.9.3-p0 :029 > s.object_id  => 70120944881780  ruby-1.9.3-p0 :031 > s+="xxx"  => "foobarxxx"  ruby-1.9.3-p0 :032 > s.object_id  => 70120961479860  

so, Strings are mutable, but += operator creates a new String. << keeps old