How to combine vectors based on value?

Question

Chameleon17 2015-10-14

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评论： Chameleon17 2015-10-16

Good Afternoon,

Another quick question. Say I have a matrix,

   B = [732145      732151      732168           0           0           0           0
         284         281         297           0           0           0           0]

and

   C = 
    [ 0   281   284     0     0   297     0
     0     0     0     0     0     0     0]

I am trying to get it so that I have:

   D = [0    281      284    0   0   297     0
        0  732151    732145  0   0  732168   0].

Should I be using the [r c] kind of terminology. I keep almost getting there, I think, then I get confused.

Any thoughts would be greatly appreciated! or reading!

Thank you very much for any time!

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Answer 1

Guillaume 2015-10-16

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Regarding your latest comment, a much simpler way of obtaining your D result, assumming that elements of B and C are uniques is:

assert(numel(unique(C)) == numel(C) && numel(unique(B)) == numel(B)); %does not work if elements are not unique
[values, ic, ib] = intersect(C, B);
D = zeros(numel(C), numel(B));
D(sub2ind(size(D), ic, ib)) = values;

If the values of B and C are not unique, you need an additional level of indirection through unique.

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Chameleon17 2015-10-16

Thank you for that, i've not used the intersect before, that's good to know!

The values of B are all unique - but I may have repeats of the C values at certain times.

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Answer 2

Andrei Bobrov 2015-10-16

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    B = [4 5 6 7; 0 0 0 0; 0 0 0 0; 0 0 0 0];
    C = [0; 5; 4];
    D = B;
    [ll,idx] = ismember(B(1,:),C);
    D(sub2ind(size(D),1+idx(ll),find(ll))) = C(idx(ll));

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Answer 3

Geoff Hayes 2015-10-14

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Do you wish to simply iterate over each element in the first row of C and, for those that are non-zero, check to see if it exists in the second row of matrix B and copy the the value from its first row into D? If so, try the following

 D = C;
 for k=1:size(C,2)
     if C(1,k) ~= 0
         [idx] = find(B(2,:)==C(1,k));
         if ~isempty(idx)
             D(2,k) = B(1,idx);
         end
     end
 end

In the above, we just iterate over each column in the first row of C. If the element is non-zero then we use find to find that element of the second row of B that matches the value in C. If the returned index, idx, is non-empty then we set this value to the corresponding element of D.