On Fri, Nov 28, 2014 at 8:40 AM, Julian Taylor < jtaylor.debian@googlemail.com> wrote:
On 28.11.2014 09:37, Robert Kern wrote:
On Fri, Nov 28, 2014 at 8:22 AM, Julian Taylor <jtaylor.debian@googlemail.com <mailto:jtaylor.debian@googlemail.com>> wrote:
On 28.11.2014 04:15, Alexander Belopolsky wrote:
I probably miss something very basic, but how given two arrays a and
b,
can I find positions in a where elements of b are located? If a were sorted, I could use searchsorted, but I don't want to get valid positions for elements that are not in a. In my case, a has unique elements, but in the general case I would accept the first match. In other words, I am looking for an array analog of list.index() method.
np.where(np.in1d(a, b))
Only if the matching elements in `b` have the same order as they do in `a`.
seems to work also if unordered: In [32]: a = np.arange(1000)
In [33]: b = np.arange(500,550, 3)
In [34]: np.random.shuffle(a)
In [35]: np.random.shuffle(b)
In [36]: np.where(np.in1d(a, b)) Out[36]: (array([ 0, 106, 133, 149, 238, 398, 418, 498, 533, 541, 545, 589, 634, 798, 846, 891, 965]),)
I meant that the OP is asking for something stricter than that. He wants this array of indices to be in the order in which those matching elements appear in `b` so that he can use this information to merge two datasets. -- Robert Kern