Subchapters:

The structures

You can create a sub-array containing those stars:

      IDL> my_stars = catalog[index]
      IDL> help,my_stars
            MY_STARS        STRUCT    = -> ETOILE Array[33]

You can also sort your catalog according to one of the tags. Let's define first the index table of the sorted my_stars catalog, according to the magnitude:

      IDL> sort_index = sort(my_stars.mag)
      IDL> help,sort_index
            SORT_INDEX      LONG      = Array[33]

Like for the where condition, the returned index is not the sorted table, but the index of the elements. You can now print the sorted sub-catalog using a format similar to the Fortran one:

      IDL> print,my_stars(sort_index), $
      IDL>                     format='(a10," ra=",f7.3," dec=",f7.3," mag=",f 7.3)'
             etoile_15 ra= 30.015 dec= -0.752 mag= 10.722
             etoile_11 ra= 31.163 dec= -9.109 mag= 10.761
             etoile_16 ra= 39.789 dec= -7.716 mag= 11.071
             etoile_14 ra= 35.110 dec= 6.785 mag= 11.176
             etoile_31 ra= 33.530 dec= 9.306 mag= 11.823
             etoile_04 ra= 33.480 dec= 5.568 mag= 11.978
             etoile_00 ra= 31.795 dec= 7.631 mag= 12.055
             etoile_18 ra= 35.884 dec= 8.612 mag= 12.516
             etoile_17 ra= 38.860 dec= 2.498 mag= 12.628
             etoile_19 ra= 37.472 dec= 8.993 mag= 12.722
             etoile_12 ra= 36.151 dec= 7.670 mag= 12.792
             etoile_20 ra= 31.940 dec= 4.497 mag= 13.276