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Firstly, the error you are receiving has been explained by= Sam. Now, to the interesting part:

Code in 4.13 defines method_missing= for Person class. This method gets run if you call some undefined method i= n the class. So, for example, if you do Person.blablabla("argument&quo= t;, 2), ruby looks in the Person's class methods (not instance methods,= because blablabla was sent to the class itself, not to the instance of it)= for a method with name "blablabla". It does not find such method= , do then it looks whether the Person class has method_missing method defin= ed. It does, so ruby calls Person.method_missing(:blablabla, "argument= ", 2), passing the parameters we called "blablabla" with to = the method_missing. Ruby puts name method to the "m" variable and= the rest of the arguments - to the args array (because it's marked wit= h asterisk - so called splat method, you can read a little bit about it = here). Then, ruby converts :blablabla symbol to string and checks wheth= er it starts with "all_with_". It doesn't, so ruby calls supe= r, which means "call parent class' method with the same name of th= e current one and pass it the same arguments". Now, let's have a l= ook at what happens if we call Person.all_with_name("Julia"). Met= hod Person.all_with_name is not defined, so ruby goes to method_missing for= help. This time method's name starts with "all_with_", so ru= by extracts the rest of the method's name after "all_with_" t= o attr variable. Now, ruby checks whether Person class has instance method = with the name stored in newly created attr variable (which now contains &qu= ot;name"). Then ruby finds all persons from PEOPLE array, who have nam= e equal to the args[0], which is first argument passed to the Person.all_wi= th_name. "person.send" is another way of calling an instance meth= od, e.g. person.send("haha") is the same as person.haha. Docs for= object.send is=A0here. So, at last ruby finds all persons, whose name includes= "Julia" as a substring. And, as this PEOPLE.find_all is the last= line in the method to be executed, method_missing returns an array of foun= d persons.

P.S. If we called Person.all_with_blablabla, we would get ArgumentErro= r exception, because Person does not have instance method blablabla defined= , which is checked in line 6 of 4.13 gist.

P.P.S. I am myself not= really experienced rubyist, so my explanation may contain some mistakes. B= e sure to correct them if you notice.

P.P.P.S. Also, my English is far from perfect, sorry for that.

2013/7/= 9 Arslan Farooq <lists@ruby-forum.com>

Hi,

I am new to programming and new to Ruby.

I have been studying Ruby using David Black's book, and I need bit of help with this:

Code in listing 4.11, 4.12 and 4.13 needs to be pieced together (I
think) to do something useful. However it's a bit confusing... whether<= br> they are meant to just studied as examples separately, or whether I am
supposed to piece them together. Please have a look at the code:

https://gist.github.com/arslanfarooq/54ac5aa7796385e4a063<= br>
If you could explain a bit what's going on in 4.13 that would be great.=

On my computer I have combined the code in one file "person_class.rb&q= uot;
and I am getting this error:

person_class.rb:17:in `has_friends': undefined method `<<' fo= r
nil:NilClass (NoMethodError) from person_class.rb:43:in `<main>'<= br>
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