[Numpy-discussion] relational join
Robert Kern
robert.kern at gmail.com
Wed Feb 2 16:46:36 EST 2011
On Wed, Feb 2, 2011 at 21:42, Ilya Shlyakhter <ilya_shl at alum.mit.edu> wrote:
> Does numpy have a relational join operation for joining recordarrays?
[~]
|1> from numpy.lib import recfunctions
[~]
|2> recfunctions.join_by?
Type: function
Base Class: <type 'function'>
String Form: <function join_by at 0x17bba30>
Namespace: Interactive
File:
/Library/Frameworks/Python.framework/Versions/6.3/lib/python2.6/site-packages/numpy/lib/recfunctions.py
Definition: recfunctions.join_by(key, r1, r2, jointype='inner',
r1postfix='1', r2postfix='2', defaults=None, usemask=True,
asrecarray=False)
Docstring:
Join arrays `r1` and `r2` on key `key`.
The key should be either a string or a sequence of string corresponding
to the fields used to join the array.
An exception is raised if the `key` field cannot be found in the two input
arrays.
Neither `r1` nor `r2` should have any duplicates along `key`: the presence
of duplicates will make the output quite unreliable. Note that duplicates
are not looked for by the algorithm.
Parameters
----------
key : {string, sequence}
A string or a sequence of strings corresponding to the fields used
for comparison.
r1, r2 : arrays
Structured arrays.
jointype : {'inner', 'outer', 'leftouter'}, optional
If 'inner', returns the elements common to both r1 and r2.
If 'outer', returns the common elements as well as the elements of r1
not in r2 and the elements of not in r2.
If 'leftouter', returns the common elements and the elements of r1 not
in r2.
r1postfix : string, optional
String appended to the names of the fields of r1 that are present in r2
but absent of the key.
r2postfix : string, optional
String appended to the names of the fields of r2 that are present in r1
but absent of the key.
defaults : {dictionary}, optional
Dictionary mapping field names to the corresponding default values.
usemask : {True, False}, optional
Whether to return a MaskedArray (or MaskedRecords is `asrecarray==True`)
or a ndarray.
asrecarray : {False, True}, optional
Whether to return a recarray (or MaskedRecords if `usemask==True`) or
just a flexible-type ndarray.
Notes
-----
* The output is sorted along the key.
* A temporary array is formed by dropping the fields not in the key for the
two arrays and concatenating the result. This array is then sorted, and
the common entries selected. The output is constructed by
filling the fields
with the selected entries. Matching is not preserved if there are some
duplicates...
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
-- Umberto Eco
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