问题
I am just learning Ruby and I don't quite understand the difference between several Ruby methods with and without a '!' at the end. What's the difference? Why would I use one over the other?
回答1:
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.
回答2:
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.downcase
puts my_new_text # will print out "my text"
puts my_text # will print out "MY TEXT" - the non-bang method doesn't touch it
my_text.downcase!
puts my_text # will print out "my text". The bang version has modified the object you're calling the method on
来源：https://stackoverflow.com/questions/709229/difference-between-downcase-and-downcase-in-ruby