sorting and counting

2012 4 7
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I have two arrays 'a' and 'b' of the same length and they may contain repeating numbers. I want to count how many number of pairs exist for a pair a(i),b(j). for e.g. a = [ 1 2 5 7] ; b = [ 9 9 3 4] ; then what I want is the following: (1,9) comes 2 times, (1,3) comes 1 time, (1,4) comes 1 time, (2,9) comes 2 times ......and so on. Can somebody please tell how can this be done ?

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Karan
Karan 2012-4-7
Here is solution I am working on but doesnt seem to give the answer :
l=length(a);
i=1;j=1;
while i<(l+1);
while j<(l+1);;
if (b(i)==b(j) && a(i)==a(j));
m(i,j,i)=1;
else
end
j=j+1
end
i=i+1;
end
here, m is 3d ,matrix where for every pair obtained, it stores a value of 1 in one layer, and finally sum of 1 in each layer gives the number of pairs.
but this code is not working and I dont seem to find a flaw in it.
could somebody please tell the flaw, or an other way to do this problem ?

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Andrei Bobrov
Andrei Bobrov 2012-4-7

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a = [ 1 2 5 7] ; b = [ 9 9 3 4] ;
[aa bb] = ndgrid(a,b);
k = [aa(:) bb(:)];
[n1, n2, n2] = unique(k,'rows');
n3 = histc(n2,1:max(n2));
out = [n1, n3]

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Karan
Karan 2012-4-7
bobrov you are simply amazing ... always glued to help others ... Thank you very much

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