Global macro: How to remove a variable?

[Jonathan Afilalo]

I created a macro of available variable names in my dataset ($vitamins):

global vitamins vita vitb vitc vitd vite

I created another macro of variables names which I would like to exclude ($vitamins_to_exclude):

global vitamins_to_exclude vitd vite

I would like to generate a third macro of variable names which I would like to include in my analyses ($vitamins_to_include), but both of these command don't work:

global vitamins_to_include $vitamins - $vitamins_to_exclude

global vitamins_to_include : list $vitamins - $vitamins_to_exclude

What is wrong with these commands? How can I code this correctly?

[Jonathan Afilalo]

Just to clarify, the end result that I am seeking is $vitamins_to_include to contain:

vita vitb vitc

[Bjarte Aagnes]

Type global(macname) if you wish to refer to a global macro.

global vitamins_to_include : list global(vitamins) - global(vitamins_to_exclude)

[Leonardo Guizzetti]

Bjarte has provided excellent advice.

I offer an example using locals or globals.

local abcde a b c d e
local cde c d e
local ab : list abcde - cde
di "`ab'"

global abcde a b c d e
global cde c d e
global ab : list global(abcde) - global(cde)
di "${ab}"

There are two related Stata tips that may also help provide some broader context.
1) Stata tip 91: Putting unabbreviated lists into local lists
2) Speaking Stata: Problems with lists