;
;  idl structures, as in c, allow you to combine a variety of variables
;  associated with a given data point
;
m31 = create_struct('name','m31','mass',1.5d12,'pos',[1d0,0.1,2.d0])
help,m31,/str

; create another galaxy
mw  = create_struct('name','MW','mass',1.d12,'pos',[0d0,0.1,0.d0])

; combine them
local_group = create_struct('m31',m31,'mw',mw)
help,local_group,/str
;
print,' coordinates of second galaxy: ',local_group.(1).pos

; print all names of the galaxies in the local_group
ngals = n_tags(local_group)
for igal=0,ngals-1 do begin
   print,' name of galaxy ',igal,' in the group is ',local_group.(igal).name
endfor

end