Reply to thread

Message: [QUOTE="Stefano Crocco, post: 4616343"] Unless I misunderstood you, you're asking why two different methods (String#sub and String#sub!) work differently. The answer is simple: because they're different. It's like asking why String#upcase and String#downcase work differently. The documentation do speak of this difference: ri String#sub gives: ------------------------------------------------------------- String#sub str.sub(pattern, replacement) => new_str str.sub(pattern) {|match| block } => new_str ------------------------------------------------------------------------ Returns a copy of _str_ with the _first_ occurrence of _pattern_ replaced with either _replacement_ or the value of the block. [...] while ri String#sub! gives: ------------------------------------------------------------ String#sub! str.sub!(pattern, replacement) => str or nil str.sub!(pattern) {|match| block } => str or nil ------------------------------------------------------------------------ Performs the substitutions of +String#sub+ in place, returning _str_, or +nil+ if no substitutions were performed. You don't need to know about the C implementation of class String, of String#sub or of String#sub! to understand how these methods work. The documentation says that sub returns a copy of the string with the replacement done, which means a different object, which has nothing to do with the original. In the case of sub!, instead, the substitution is done in place, that is, the receiver itself (str) is modified, not a copy of it. As for the fact that the difference doesn't show in irb, this is not true. Look at this: irb(main):001:0> str = "this is a test string" => "this is a test string" irb(main):002:0> str1 = str.sub "h", "H" => "tHis is a test string" irb(main):003:0> str => "this is a test string" The above lines show that str is not changed by sub irb(main):004:0> str.sub "k", "K" => "this is a test string" irb(main):005:0> str.sub! "k", "K" => nil This shows the different behavior concerning the return value when there's nothing to replace. sub returns a copy of the string without modifications, while sub! returns nil irb(main):006:0> str.sub! "a", "A" => "this is A test string" irb(main):007:0> str => "this is A test string" irb(main):008:0> Here you can see that sub!, unlike sub, changes the original string. In short, here's the difference between sub and sub!: * sub creates a new string which has the same contents of the original one, but is indipendent from, then replaces the pattern with the replacement text in the copy. The original is not altered in any way. It always returns the copy and you can see whether a replacement has been made by comparing the original and the copy. * sub! performs the replacement on the string itself, thus changing it. Obviously, you can't compare the 'new' and the 'original' string to see whether a replacement has been made (since there's no 'new string' and the original has been changed), so you have to look at the return value: if it is nil, nothing has been changed; if it is the string itself then a replacement has been made. I hope this helps Stefano [/QUOTE]

Verification