[SciPy-user] Way to get values which are the same on two arrays

[Michael Sorich]

Hi José,

I remember reading your question last week. Coincidently, I have just
written a function to find the intersection of 2 sets (using a histogram
function) for my own work. I'm not sure if it is the most efficient
method, but it seems to work.

from Numeric import *

def histogram(a, bins):
    """counts the number of values in a that fall into each of the bins
in bin"""
    n = searchsorted(sort(a),bins)
    n = concatenate([n,[len(a)]])
    return asarray(n[1:]-n[:-1])


def intersection(a,b):
    """takes two 1D arrays and returns an array of values that are in
both arrays"""
    if a.typecode() != 'l' or b.typecode() != 'l':
        raise ValueError, "arrays must be of type Int"

    #make bins from intersection of ranges
    minab = max([minimum.reduce(a),minimum.reduce(b)]) 
    maxab = min([maximum.reduce(a),maximum.reduce(b)])
    #   -> If either set is empty, or their ranges don't intersect
    if maxab < minab or maxab < 0:
        return None 
    bins = arange(minab,maxab+1)

    return
minab+nonzero(logical_and(histogram(a,bins),histogram(b,bins)))


if __name__ == '__main__':
    a = array([1,2,3,4,6])
    b = array([4,5,6,1,7])
    print intersection(a,b)


Hope this helps,

Michael Sorich
PhD Student
School of Pharmaceutical, Molecular and Biomedical Sciences
University of South Australia
Email: michael.sorich at postgrads.unisa.edu.au
           mike_sorich at hotmail.com


--------------------------------
Hi,
	I seem to remember a Numerical function that could produce the
list of 
values (or array positions) which are common to two arrays. In other 
words:
a1=[1,2,3,9,10]
a2=[1,4,9,10]
	result is [1,9,10].

	Browsing through the Numerical docs doesn't refresh my memory,
so any 
kind souls?

	many thanks,
José
-- 
José L Gómez Dans			PhD student
Tel: +44 114 222 5582			Radar & Communications Group
FAX; +44 870 132 2990			Department of Electronic
Engineering
					University of Sheffield UK


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