# Extract unmasked elements from Astropy Table **URL:** \ **Category:** Astropy\ **Tags:** astropy, table, question\ **Created:** [September 8, 2021, 3:02am UTC](https://community.openastronomy.org/t/extract-unmasked-elements-from-astropy-table/145 "2021-09-08T03:02:03Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![kelle](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/kelle/32/39_2.png) [@kelle](https://community.openastronomy.org/u/kelle)\ **Post date:** [September 8, 2021, 3:02am UTC](https://community.openastronomy.org/t/extract-unmasked-elements-from-astropy-table/145/1 "2021-09-08T03:02:03Z")
Let’s kick the tires of this forum! I’m trying to extract the non-masked rows from an Astropy Table. From the [Astropy Table documentation](https://docs.astropy.org/en/stable/table/masking.html#mask), I can get the indices of the masked, but how do I get the inverse? > To get the indices of masked elements, use an expression like: > `>>> t['a'].mask.nonzero()` I’ve tried `nans = t['a'].mask.nonzero()` `unmasked_t = t['a'][-nans]` But that didn’t work. `bad operand type for unary -: 'tuple'` Any pointers? Thanks! Kelle ---
**Author:** ![Cadair](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/cadair/32/422_2.png) [@Cadair](https://community.openastronomy.org/u/Cadair)\ **Post date:** [September 8, 2021, 8:56am UTC](https://community.openastronomy.org/t/extract-unmasked-elements-from-astropy-table/145/2 "2021-09-08T08:56:09Z")
I think (without testing it) you could use `np.logical_not` on the mask before the `nonzero` call. I.e. ```auto unmasked = np.logical_not(t['a'].mask).nonzero() ``` ---
**Author:** ![kelle](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/kelle/32/39_2.png) [@kelle](https://community.openastronomy.org/u/kelle)\ **Post date:** [September 8, 2021, 2:09pm UTC](https://community.openastronomy.org/t/extract-unmasked-elements-from-astropy-table/145/3 "2021-09-08T14:09:52Z")
it works! any thoughts on this approach vs using Pandas `.notna()` ? ---
**Author:** ![nstarman](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/nstarman/32/41_2.png) [@nstarman](https://community.openastronomy.org/u/nstarman)\ **Post date:** [September 8, 2021, 7:40pm UTC](https://community.openastronomy.org/t/extract-unmasked-elements-from-astropy-table/145/4 "2021-09-08T19:40:39Z")
Isn’t the mask a NumPy boolean array? then the non-masked elements are just `notmasked = t['a'][~t['a'].mask]` Or do you want the indices of the rows? ---
**Author:** ![nstarman](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/nstarman/32/41_2.png) [@nstarman](https://community.openastronomy.org/u/nstarman)\ **Post date:** [December 15, 2021, 11:09pm UTC](https://community.openastronomy.org/t/extract-unmasked-elements-from-astropy-table/145/5 "2021-12-15T23:09:14Z")
Addendum: `~` can sometimes be finicky. I’ve taken to using `not` instead, even if it’s a Boolean array. ---
**Author:** ![saimn](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/saimn/32/40_2.png) [@saimn](https://community.openastronomy.org/u/saimn)\ **Post date:** [December 16, 2021, 8:51am UTC](https://community.openastronomy.org/t/extract-unmasked-elements-from-astropy-table/145/6 "2021-12-16T08:51:12Z")
I think what you want is `t[~t['a'].mask]` to get a new table where rows with masked elements in `'a'` have been removed: ```auto In [29]: t Out[29]: a b int64 int64 ----- ----- 1 -- -- 4 In [30]: t[~t['a'].mask] Out[30]:
a b int64 int64 ----- ----- 1 -- ``` Pandas has powerful ways to do selection on columns, but I’m not sure there is really a point in reimplementing that in Astropy. In Astropy columns are simple Numpy arrays, so most simple things can be in a slightly more verbose way (compared to Pandas) with simple Numpy selections. And for more advanced usage it’s easy to convert the table to Pandas (`t.to_pandas()`).