sorting and counting

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 ?

[Karan]

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 ?

Answer 1

[Andrei Bobrov]

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]

[Karan]

bobrov you are simply amazing ... always glued to help others ... Thank you very much