# Standards

Blogged by Ujihisa. Standard methods of programming and thoughts including Clojure, Vim, LLVM, Haskell, Ruby and Mathematics written by a Japanese programmer. github/ujihisa

## Wednesday, June 10, 2009

### Default Parameter -- Dynamic Define, Recursive Call

In method parameter, we can write any kind of assignment statements in Ruby.

The most popular default parameter is literals:

``````def f(a, b = :aaa)
...
end
``````

Rarely we can see method calls in default parameter:

``````def f(a, b = Array.new)
...
end

def g(a, b = 1 + 2)
...
end
``````

We can refer the other precedent parameters:

``````def f(a, b = a * 2)
b
end
p f(10) #=> 20
``````

We can write more complicated code there.

``````def g(a, b = (def j; end; 1))
[a, b]
end
``````

This code works tricky. After `g` was called without the second argument, method `j` is defined.

We also can write recursive call there.

``````def fib(n, x = n == 0 ? 0 : n == 1 ? 1 : fib(n-2) + fib(n-1))
x
end

p fib(10) #=> 55
p fib(10, :hehehe) #=> :hehehe
``````