Advance to next non-blank line only if current line is not blank

Question

I want to tell a command or mapping to advance to the next non-blank line before executing - but to do this only if the current line is not blank.

Ultimately, I intend to apply this to a command to collapse blank lines in a given range, replacing the blank lines with tabs. This must be done from the first non-blank line to prevent preceding newlines becoming tabs ahead of the first word of the collapsed line.

So I might find the next non-blank line with /^.\+/

I want to apply that to this command that collapses blank lines:

:command -range=0 CollapseLines <line1>,<line2>s/\n/\t/

For example, like this:

:command -range=0 CollapseLines /^.\+/<line1>,<line2>s/\n/\t/

But if I combine the two, it advances to the next non-blank line in disregard of whether the current line is blank or not. So the first line of the range gets missed.

Finally, I intend to use this with a mapping:

:nmap <leader>cl :CollapseLines<CR>j

So the placing of the cursor on the first or current non-blank line might be done in the mapping rather than the command.

Answer 1

Check :h nextnonblank() function:

nextnonblank({lnum})                    *nextnonblank()*
        Return the line number of the first line at or below {lnum}
        that is not blank.  Example: >
            if getline(nextnonblank(1)) =~ "Java"
<       When {lnum} is invalid or there is no non-blank line at or
        below it, zero is returned.
        {lnum} is used like with |getline()|.
        See also |prevnonblank()|.

So

" if current line is non-blank
if getline('.') !~ "^\s*$"
  let nextline = nextnonblank(line('.') + 1)
  " do smth with nextline
  " echo getline(nextline)
endif

PS, I am not sure what exactly you need, but here is smth implemented a bit differently:

func! CollapseWhatever() abort
    if getline('.') !~ '^\s*$' | return | endif
    exe "normal! vip\<esc>"
    '<,'>-1s/\n/\t/g
endfunc

command Collapse :call CollapseWhatever()