How to join every second line in Vim?

i would do this:

1. start recording a macro 'q': qqJjq

2. replay the macro 'q' 500 times: 500@q

(actually it is not a macro called 'q', it is a named register called 'q'. instead of interactively fill that register as in 1., you could also do :let @q = "Jj" and then do 2.)

To do this on every line of the file:

:%normal J

or, shorter:

:%norm J

To do this on just a portion of the file, select the lines with V or get a range some other way:

:'<,'>global/^/normal J

or, shorter:

:'<,'>g/^/norm J

What about this:

:g/$/j  

or

:g/$/j!  

and group every three lines

:g/$/j3