Intersect2

Finds the intersection (common elements) of several (more than two) arrays

Description

In MATLAB, there is a command called "intersect", which finds the set intersection (common elements) of two vectors. However, this command doesn't work if I want to compare more than two vectors and find out their intersection. Therefore, I decided to write this function which is called "intersect2". It can find the common elements of any number of 1D numeric arrays of the same or different sizes and returns an array consisting of only those elements.

INPUT:
The input variable "cell" has to be a cell array, with each cell occupied by an 1D numeric array. For example, if you want to find the intersection (common elements) of the following three arrays
       a = [ 1 3 4 6 8 9 ];
       b = [ 3 1 0 8 6 4 ];
       c = [ 7 8 1 9 3 4 ];,
you need to first put all of them into a cell array, i.e.
       cell = {a, b, c};
Then you could use "cell" as the input variable to this function, i.e.
       result = intersect2(cell);
   
OUTPUT:
The output of this function is simply an array consisting of elements that are common to all the arrays stored in "cell". For the particular case in the example above, the output would be
       result = [ 1 3 4 8 ];
Also note that the numbers in the result are sorted in ascending order.

• MATLAB release: MATLAB 7.4 (R2007a)

[Thierry Dalon]

I would code this using recursivity:
function res=intersectall(c)

if length(c) == 2
    res = intersect(c{1}, c{2});
return
end

res = intersect(c{1},intersectall(c(2:end)));

[Julia]

I've submitted a script that fulfills the same function as this one, using a recursive scheme:
intersect several arrays
http://www.mathworks.com/matlabcentral/fileexchange/24835

There is a similar one for the union of several arrays:
http://www.mathworks.com/matlabcentral/fileexchange/24834

[Ulrik]

Thanks..
but I would change the code a bit:

n = numel(cell);
if n == 2
    result = intersect(cell{1}, cell{2});
elseif (n > 2 && rem(n,2) == 0)
    for i = 1:(n/2)
        intersections{1, i} = intersect(cell{i}, cell{n-i+1});
    end
elseif (n > 2 && rem(n,2) == 1)
    for i = 1:((n-1)/2)
        intersections{1, i} = intersect(cell{i}, cell{n-i+1});
    end
        intersections{1, i+1} = cell{(n+1)/2};
else
    error('You need at least two arrays as inputs');
end

if n > 2
    result = intersect2(intersections);
end