Strange ds/head and ds/tail behaviour in v7

[alex314159]

Hi Chris,

some weird behaviour with these functions in v7, see below:

(def x (ds/->dataset (repeat 1000 {:a 1 :b 2})))
=> #'jasmine.var/x

(ds/select-rows (ds/select-rows x (range 10)) [0 1]) ;;expected behaviour
=> _unnamed [2 2]:

| :a | :b |
|---:|---:|
|  1 |  2 |
|  1 |  2 |

(ds/select-rows (ds/tail x 5) [0 1]) ;;unexpected
=> _unnamed [2 2]:

| :a | :b |
|---:|---:|
|  1 |  2 |
|  1 |  2 |
|  1 |  2 |
|  1 |  2 |
|  1 |  2 |

(ds/select-rows (ds/head x 5) [0 1]) ;;unexpected
=> _unnamed [2 2]:

| :a | :b |
|---:|---:|
|  1 |  2 |
|  1 |  2 |
|  1 |  2 |
|  1 |  2 |
|  1 |  2 |

Also note the docstring for ds/head and ds/tail is wrong as it says "Get the first n row of a dataset. Equivalent to `(select-rows ds (range n)). Arguments are reversed, however, so this can be used in ->> operators." (the arguments are not reversed, and it's better that way IMHO!)

Thanks,

[alex314159]

And maybe a better example of the issue:

(def x (ds/->dataset {:a (range 1000) :b (range 1000)}))
=> #'jasmine.var/x
(ds/select-rows (ds/tail x 5) [0 1])
Error printing return value (IndexOutOfBoundsException) at ham_fisted.ChunkedList/sublistCheck (ChunkedList.java:216).
End index out of range: end-index(5), n-elems(2)

[cnuernber]

That is the error I get - index out of bounds - it was because the head/tail operations put print-index-range metadata on the returned dataset and the select-rows operation didn't clear the metadata (which in general it should not). So when you go to print the third, 2 row dataset it has print-index-range of (range 5) from the head operation :-).

[cnuernber]

The fix is really two-fold. First, select-rows should clear specifically print-index-range. Second, printing itself should never cause that exception so it needs to always filter print-index-range to be in the range of the current dataset row indexes.

One thing you may really like - select-rows now accepts negative indexes which will index from the end python-style.

[cnuernber]

New release is up :-)

[alex314159]

thanks!!

[harold]

Wow! Nice!