Difference Between downcase and downcase! in Ruby

Methods with an exclamation mark at the end are often called bang-methods. A bang method doesn't necessarily modify its receiver as well as there is no guarantee that methods without a exclamation mark doesn't.

It's all very well explained in this blog post. To quote the post:

The ! in method names that end with ! means, “This method is dangerous”—or, more precisely, this method is the “dangerous” version of an otherwise equivalent method, with the same name minus the !. “Danger” is relative; the ! doesn’t mean anything at all unless the method name it’s in corresponds to a similar but bang-less method name.

and

The ! does not mean “This method changes its receiver.” A lot of “dangerous” methods do change their receivers. But some don’t. I repeat: ! does not mean that the method changes its receiver.

The non-bang downcase() method simply returns a new object representing you string downcased.

The bang version modifies your string itself.

my_text = "MY TEXT"my_new_text = my_text.downcaseputs my_new_text # will print out "my text"puts my_text     # will print out "MY TEXT" - the non-bang method doesn't touch itmy_text.downcase!puts my_text # will print out "my text". The bang version has modified the object you're calling the method on