MATLAB Central Newsreader - compare two vectors

compare two vectors

ann au — Sun, 02 Nov 2008 00:45:03 -0400

Hi,

I have two vectors:

a=[1 1 1 1 1 2 2 3 4 5 6 7]'
b = [1 1 2 2 5 6]'

I want to get the following result:
r = [1 1 1 3 4 7]'

What code can I use?
Thanks a lot!

Ann

Re: compare two vectors

Image Analyst — Sun, 02 Nov 2008 02:38:02 -0500

"ann au" <fannylyy@hkusua.hku.hk> wrote in message <geit6f$8ev$1@fred.mathworks.com>...
> Hi,
>
> I have two vectors:
>
> a=[1 1 1 1 1 2 2 3 4 5 6 7]'
> b = [1 1 2 2 5 6]'
>
> I want to get the following result:
> r = [1 1 1 3 4 7]'
>
> What code can I use?
> Thanks a lot!
>
> Ann
-----------------------------------------------------
Not exactly sure what you're after, or what algorithm you're using, but this will work ;-)

a=[1 1 1 1 1 2 2 3 4 5 6 7]'
b = [1 1 2 2 5 6]'
r = CompareVectors(a, b);

function returnArray=CompareVectors(a, b)
returnArray = [1 1 1 3 4 7];
return;

Re: compare two vectors

Husam Aldahiyat — Sun, 02 Nov 2008 04:16:02 -0500

I think it's a riddle.

Re: compare two vectors

Seth — Sun, 02 Nov 2008 05:48:02 -0500

Maybe you are looking for something like this that returns the dissimilar elements?

a=[1 1 1 1 1 2 2 3 4 5 6 7]';
b = [1 1 2 2 5 6]';
r = zeros(1,length(a)-length(b));
r_c = 1;

for i = 1:max(a)
    k = (sum(a==i) - sum(b==i));
    if k > 0
        for j = 1:k
            r(r_c) = i;
            r_c = r_c + 1;
        end
    end
end

Seth

"Image Analyst" <imageanalyst@mailinator.com> wrote in message <gej3qa$609$1@fred.mathworks.com>...
> "ann au" <fannylyy@hkusua.hku.hk> wrote in message <geit6f$8ev$1@fred.mathworks.com>...
> > Hi,
> >
> > I have two vectors:
> >
> > a=[1 1 1 1 1 2 2 3 4 5 6 7]'
> > b = [1 1 2 2 5 6]'
> >
> > I want to get the following result:
> > r = [1 1 1 3 4 7]'
> >
> > What code can I use?
> > Thanks a lot!
> >
> > Ann
> -----------------------------------------------------
> Not exactly sure what you're after, or what algorithm you're using, but this will work ;-)
>
> a=[1 1 1 1 1 2 2 3 4 5 6 7]'
> b = [1 1 2 2 5 6]'
> r = CompareVectors(a, b);
>
> function returnArray=CompareVectors(a, b)
> returnArray = [1 1 1 3 4 7];
> return;

Re: compare two vectors

Walter Roberson — Sun, 02 Nov 2008 06:04:48 -0500

Husam Aldahiyat wrote:
> I think it's a riddle.

No, it's just the multi-set difference.

--
.signature note: I am now avoiding replying to unclear or ambiguous postings.
Please review questions before posting them. Be specific. Use examples of what you mean,
of what you don't mean. Specify boundary conditions, and data classes and value
relationships -- what if we scrambled your data or used -Inf, NaN, or complex(rand,rand)?

Re: compare two vectors

Roger Stafford — Sun, 02 Nov 2008 08:14:02 -0500

"ann au" <fannylyy@hkusua.hku.hk> wrote in message <geit6f$8ev$1@fred.mathworks.com>...
> Hi,
>
> I have two vectors:
>
> a=[1 1 1 1 1 2 2 3 4 5 6 7]'
> b = [1 1 2 2 5 6]'
>
> I want to get the following result:
> r = [1 1 1 3 4 7]'
>
> What code can I use?
> Thanks a lot!
>
> Ann

  I assume that for each element present in 'a' in greater quantity than in 'b' you wish 'r' to contain as many copies of that element as this difference. That is, in your example 'r' has three 1's presumably because 'a' contains three more 1's than does 'b'.

  If that is what you intend, try this:

e = union(a,b); % List all unique elements of combined a and b
n = max(histc(a,e)-histc(b,e),0); % How many of each e element
p = accumarray(cumsum(n)+1,1); % Proceed to generate r
p(1) = p(1)+1;
r = e(cumsum(p(1:end-1)));

  The vector 'n' above contains the desired number of elements for 'r' of each of the corresponding elements of 'e'. The remaining three lines are devoted to generating just such a vector 'r'.

Roger Stafford

Re: compare two vectors

Bruno Luong — Sun, 02 Nov 2008 12:43:02 -0500

"Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in message <gejnga$3p9$1@fred.mathworks.com>...
> The remaining three lines are devoted to generating just such a vector 'r'.
>

These "remaining" three lines are quite clever.

I would suggest a more naive alternative for these three lines:

r=arrayfun(@(ek,nk) repmat(ek,nk,1),e,n,...
'UniformOutput',false)
r = cat(1,r{:})

And here is another approach:

e=union(a,b);
[trast ia]=ismember(a,e);
[trast ib]=ismember(b,e);
count = @(i) {1:length(i)};
ca = accumarray(ia(:,1), (1:length(a))', [], count);
cb = accumarray(ib(:,1), (1:length(b))', [], count);
ia(:,2)=cat(2,ca{:});
ib(:,2)=cat(2,cb{:});

r = a(~ismember(ia,ib,'rows'))

% Bruno