Vim: automate reading line from external file

Question

I am trying to automate the following in vim:

:r! sed -n 10p filename

Where the line number (10) is constantly changing. I was thinking something along the lines of

function! GetLine(line, filename)
    :r! sed -n a:line,p a:filename
endfunction

which I have put in my .vimrc and properly sourced. But when I call with :call GetLine(10, "testfile.txt") it doesn't work.

Now, while I have been using vim for quite a while, I am new to scripting it, so please bear with me if I am missing something obvious.

Answer 1

I would do something like this

function! GetLine(line, filename)
    execute 'r! sed -n ' . a:line . 'p ' . a:filename
endfunction

execute executes the following ex command. We generate the ex command using string concatenation. . concatenates strings and variables

Take a look at :h :execute

Answer 2

A different approach more suitable to a mapping is inserting the line number via <C-r>:

:nmap <Leader>r :r! sed -n <C-r>=variable<CR>p filename<CR>