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

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