vim function argument

I write a function like this:
function InsertNumber(start, end, step)

let i = a:start

let curr_line = 0

while i <= a:end
if a:step <= 0
echo "Error: step cannot <=0."
break
endif

call append(curr_line, i)

let i += a:step

let curr_line += 1

endwhile
endfunction


when I call this function, I type this:
:echo InsertNumber(8,10,1)
8
9
10


1) How can I give arguement "step" a default value(eg: 1) when define the function?
like a C function:
void C_func(int a, int b_have_default_val = 1)
{

;
}
2)
I want to print number like this, how to do it?
08
09
10

--
--
You received this message from the "vim_use" maillist.
Do not top-post! Type your reply below the text you are replying to.
For more information, visit http://www.vim.org/maillist.php

---
You received this message because you are subscribed to the Google Groups "vim_use" group.
To unsubscribe from this group and stop receiving emails from it, send an email to vim_use+unsubscribe@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.