Ruby “.downcase! ” and “downcase” confusion

Question

I just started Ruby programming. I had read Difference Between downcase and downcase! in Ruby. However I encounter an interesting problem in practice, here is code:

Answer 1

a.downcase! modifies "a" directly -- you do not need to assign the result to "a".

1.9.3p362 :003 > a = "A"
 => "A" 
1.9.3p362 :004 > a.downcase!
 => "a" 
1.9.3p362 :005 > puts a
a
 => nil 
1.9.3p362 :006 > a = "A"
 => "A" 
1.9.3p362 :007 > a.downcase
 => "a" 
1.9.3p362 :008 > puts a
A
 => nil 

The danger of assigning variable = variable.downcase! is that if variable is already downcase then you have just set the variable to nil -- probably not your intent.

1.9.3p362 :001 > variable = 'a'                                                                                                                                                                                                                                               
 => "a"                                                                                                                                                                                                                                                                       
1.9.3p362 :002 > variable = variable.downcase!
 => nil                                                                                                                                                                                                                                                                       
1.9.3p362 :003 > puts variable                                                                                                                                                                                                                                                

 => nil 

Answer 2

downcase! is a method which modifies the string in-place (whereas downcase creates a new string instance).

The return value of downcase! is nil if the string has not been modified, or the new modified string. In the latter case the string in a gets overwritten. The correct way of using downcase! is:

a = "LOWER"
a.downcase! # no assignment to a here
print a # prints "lower", the original "LOWER" is lost

And for downcase:

a = "LOWER"
print a.downcase # a is still "LOWER", but "lower" gets printed

As a general rule of thumb: If a methods ends with !, the method overwrites values or modifies state in your variables.

Additionally in your case:

print "lower".downcase! # prints nil, because "lower" is already written in lower case

Answer 3

downcase! will return nil if no changes were made. You can check it with ri downcase!

Answer 4

downcase! → str or nil

Downcases the contents of str, returning nil if no changes were made. Note: case replacement is effective only in ASCII region.

Documentation: String#downcase!