Does something similar to 'intersect' command exists for more than 2 vectors?

Hi,

There are 5 row vectors with different(or same) number of elements. The problem is to pick out the 'intersecting' elements from these 5 vectors.

'intersect(A,B)' works with only 2 vectors. Is there a command/procedure for more than 2 vectors?

Thanks.

Answer 1

[Matt Fig]

You could call INTERSECT multiple times:

A = [1 2 3 4 5];B = [3 7 8 9 5];C = [5 0 22 3 77];
F = intersect(intersect(A,B),C)

Or download this file, or this collection.

[Peeyush]

Yes, That's one way to do it. Thanks.

Perhaps, for more vectors (say 20), it will be easier to write a program that uses intersect.

[Matt Fig]

But you don't have to write a program! The two I pointed you to on the FEX are already written...

[Matt Fig]

Here is an example of another way to do it for the many variables case. Note that if you have variables in the workspace which you do not want to be a part of the intersection, just call SAVE with the 3+ argument format, passing only the variables you wish compared:

clear all % Start with a clean slate.

A = round(rand(1,50)*20);

B = round(rand(1,50)*20);

C = round(rand(1,50)*20);

D = round(rand(1,50)*20);

E = round(rand(1,50)*20);

F = round(rand(1,50)*20);

save 'myvars'

X = load('myvars');

F = fieldnames(X);

H = X.(F{1});

for ii = 2:length(F)

H = intersect(H,X.(F{ii}));

end

H % Show the intersection.

Answer 2

[Morteza Darvish Morshedi]

Hi,

For three input, you can simply do this:

function [Com,ia,ib,ic] = intersect3(A,B,C)
[C1,ia,ib] = intersect(A,B);
[Com,ic1,ic] = intersect(C1,C);
%~ ic is okay
ia = ia(ic1);
ib = ib(ic1);
end

Going from 3 input sets to 5 input sets or more, you would need to follow same procedure, each time on the outputs from the last step. Like (for 4 inputs):

function [Com,ia,ib,ic,id] = intersect4(A,B,C,D)
[C1,ia,ib] = intersect(A,B);
[C2,ic1,ic] = intersect(C1,C);
ia = ia(ic1);
ib = ib(ic1);
% or [C2,ia,ib,ic] = intersect3(A,B,C);

% Now D
[Com,id1,id] = intersect(C2,D);
%~ id is okay
ia = ia(id1);
ib = ib(id1);
ic = ic(id1);
end

[Stephen23]

This can be generalized using a loop:

function [A,ia,varargout] = intersectm(A,varargin)
varargout = cell(size(varargin));
ia = 1:numel(A);
for ii = 1:numel(varargin)
    [A,ixa,varargout{ii}] = intersect(A,varargin{ii});
    ia = ia(ixa);
    for jj = 1:ii-1
        varargout{jj} = varargout{jj}(ixa);
    end
end
end

[Morteza Darvish Morshedi]

Correct. Thanks.

[Cesar Daniel Castro]

Could you please give more details about your code?

[Morteza Darvish Morshedi]

Cesar Daniel Castro Having three sets of A,B and C, you always start with finding intersection of two of them, e.g. intersection of A and B as C1. Next, you take intersection between C and C1, as C2. When it comes to indeces of elemtns of C2 in A, B and C, you can directly have those of C from built-in function 'intersect' as ic. To find 'ia' and 'ib', you take a subset of the first intersection C1 that exist in the second intersection C2 as ia=ia(ic1) and ib = ib(ic1). Generalization of this procedure as one function is what Stephen Cobeldick mentioned.