How to find a list of elements in an array using python

Question

I have two sets of data which both have values which refer to part of a larger set of data (points in an unstructured mesh).

The two smaller sets of data contain vectors which have the global id which references the point in the larger set of data. Something like:

Large set of data:

Smaller sets of data:

A

B

0 10
4 12
7 60

The first column in the smaller sets of data is a reference to the line number in the larger set of data. The second column in the smaller set of data are just example data.

It is also worth mentioning that the first column of B is always a subset of the first column of A.

What I need is the row indices of A where the point ids match those in B.

In this case this would be:

ind = [0,2,4]

i.e. A[ind,0] = B[:,0]

I have managed to do this previously using a loop, but now the datasets are increasing in size to over 10 million and the loop is far too slow. Can anyone suggest any faster methods?

whats the type of A and B ?

Kasravnd
– Kasravnd

2014-10-15 11:22:36 +00:00
Commented Oct 15, 2014 at 11:22 — Kasravnd
– Kasravnd, Commented Oct 15, 2014 at 11:22
do you mean 0,2,4?

Padraic Cunningham
– Padraic Cunningham

2014-10-15 11:29:01 +00:00
Commented Oct 15, 2014 at 11:29 — Padraic Cunningham
– Padraic Cunningham, Commented Oct 15, 2014 at 11:29
I do, I will edit accordingly, thank you

Bensciens
– Bensciens

2014-10-15 12:47:10 +00:00
Commented Oct 15, 2014 at 12:47 — Bensciens
– Bensciens, Commented Oct 15, 2014 at 12:47
The type of A and B can be either numpy array or lists

Bensciens
– Bensciens

2014-10-15 12:48:25 +00:00
Commented Oct 15, 2014 at 12:48 — Bensciens
– Bensciens, Commented Oct 15, 2014 at 12:48

PM 2Ring · Accepted Answer · 2014-10-15 11:41:02Z

1

Putting the first column data of B into a set should speed things up. Assuming that A and B are lists of tuples (or lists), try this:

>>> A
[('0', '1'), ('3', '5'), ('4', '5'), ('6', '7'), ('7', '2')]
>>> B
[('0', '10'), ('4', '12'), ('7', '60')]
>>> bkeys=set([i[0] for i in B])
>>> [i for i,v in enumerate(A) if v[0] in bkeys]
[0, 2, 4]

answered Oct 15, 2014 at 11:41

PM 2Ring

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1 Comment

Bensciens Over a year ago

This is exactly what I was looking for. This runs in a fraction of the time previously, Thank you.

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