How to get and use the number of each matched line in a global command

Question

In his (very good) book "Pratical Vim" Drew Neil shows how to collect all the lines containing the word "TODO" into a register to use them latter.

To do so he simply use a global command: :g/TODO/yank A (The capital A allows to happen lines to the named register a).

I think that's a pretty cool trick but I need to improve it: I'm trying to insert the number of the line before it's content. I think the solution would be to get the line number and use it in the last part of the command I mentioned before. The problem is that I don't know how to get this line number.

So my question is: How in a global command can I get the number of the matched line and how can I use this number?

Just to be clear here is an example. Let's consider this file:

1  //TODO: Hey this is a todo
2  int main(void){
3      //TODO: and this is another one
4      printf("Hello world");
5
6      return 0;
7  }

When I type :g/TODO/yank A and I put the content of the register in a file I get:

//TODO: Hey this is a todo
    //TODO: and this is another one

What I would like to get is:

1 //TODO: Hey this is a todo
3    //TODO: and this is another one

Bonus the yank also include the indentation of the line, it would be pretty cool if I could remove it directly from the global command.

Answer 1

One way to do it:

1. clear the register:

   :let @a=''
   

2. append search results in it:

   :g/TODO/let @A = getpos('.')[1] . ' ' . getline('.') . "\n"
   

Re: BONUS remove indentation in the global command:

    :g/TODO/let @A = getpos('.')[1] . ' ' . substitute(getline('.'), '^\s*', '', '') . "\n"