How to move a line onto the end of some other line

Question

For example suppose i have

1 the cat jumps
2 over the moon

I would like the result

1 the cat jumps over the moon

In general how can I move line X onto the end of line Y. A solution that would alllow something like

1 The fat cat   
2 jumps high
3 over the sky
4 is well behaved

Is there a command that takes line X line Y as input and appends the content of line Y onto line X. In this example if X=1 and Y =4 we get

command(1,4)

1 The fat cat is well behaved
2 jumps high
3 over the sky

Answer 1

For the following, I am assuming that the numbers 1 to 4 indicate the line numbers.

I am not aware of a one-shot command of moving an arbitrary line to the end of another line. J can be used to join the next line to the current line. Else one can do something like this:

1. 4G (move to line 4)
2. dd (delete the line)
3. 1G (move to line 1)
4. pkJ (insert line and join them)

You can create a custom command that does this for you, similar to what you ask:

command! -nargs=* Join call Join(<f-args>)
function! Join(...)
  if a:0 != 2 | return | endif
  execute 'normal!' a:2 . 'Gdd'
  execute 'normal!' (a:2 > a:1 ? a:1 : a:1 - 1) . 'GpkJ'
endfunction

Now you can do things like this:

" Join line 4 to line 1
:Join 1 4

" Join line 2 to line 4
:Join 4 2

Note that the behaviour of J depends on your formatoptions setting, see :h formatoptions and :h fo-table.