dot product and indexing

Question

kalana agampodi 2021-10-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1569113-dot-product-and-indexing

评论： De Silva 2021-10-24

I am reviewing this code and I do not undestand what this code means and how the dot operation work.

Can you please explain with some example ?

NCol = [
    1     3     1     2     4     3     5     2     3     1     2     5    ]    
NRow = [
    1     1     2     2     2     3     3     4     4     5     5     5    ]
c=(NCol(1:k-1) == NRow(k)) .* (NRow(1:k-1) == NCol(k)) 

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Answer 1

Sulaymon Eshkabilov 2021-10-21

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https://ww2.mathworks.cn/matlabcentral/answers/1569113-dot-product-and-indexing#answer_813788

The explantion is as follows:

% Step 1. Comparing every individual value (element of) of NCol row vector (row matrix) 
% with the k-th (the last one) element of NRow (i.e. 5) whether their are equal or not.
% If equal then, the result will be 1 (true); otherwise, 0 (false)
(NCol(1:k-1) == NRow(k)) 
% Step 2. Similar to Step 1, every individual value (element of) of 
% NRow row vector (row matrix) with the kth (the last one) element of NCol (row vector) 
% whether their are equal or not.
% If equal then, the result will be 1 (true); otherwise, 0 (false)
(NRow(1:k-1) == NCol(k)) 
% Step 3. Elementwise computation of the comparisons from Step 1 and Step 2
% simultaneously.
(NCol(1:k-1) == NRow(k)) .*(NRow(1:k-1) == NCol(k)) 
% Finally, the computation results from the two logical vectors from Step 1 and 2 are there

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kalana agampodi 2021-10-23

Sorry still not clear,

This is the whole code below and I am trying to undestand it.

I have addd some code but i do not undestad fully whats going on. Can you please explain or write the same code to in a simpler way ?

Thank you

nz=length(NRow);
Degrees=zeros(1,n);
for k=1:nz
    if NRow(k)~=NCol(k)  % Don't count the diagonal elements in degree.
        c=(NCol(1:k-1) == NRow(k)) .* (NRow(1:k-1) == NCol(k));
        % The 'c' is to find if the degree has been counted.
        if not(max(c))
            %if the degree have been counted, don't get in 'if'.
            Degrees(NRow(k))=Degrees(NRow(k))+1;
            Degrees(NCol(k))=Degrees(NCol(k))+1;
        end
    end
end

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Answer 2

Walter Roberson 2021-10-21

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https://ww2.mathworks.cn/matlabcentral/answers/1569113-dot-product-and-indexing#answer_813793

The 1:k-1 can only work properly if k is a scalar.

NRow(1:k-1) == NCol(k)

That part tests whether each value in NRow before index k, is equal to the NCol entry at index k. The result is a logical vector of length k-1 . For example if k were 4, then NCol(4) is 2, and you would be testing NRow(1:3) == 2 which would give you the logical vector [false, false, true]

(NCol(1:k-1) == NRow(k))

That part tests whether each value in NCol before index k, is equal to the NRow entry at index k. The result is a logical vector of length k-1 . For example if k were 4, then NRow(4) is 2, and you would be testing NCol(1:3) == 2 which would give you the logical vector [false, false, false]

So you have two logical vectors of length k-1 and the .* multiplies the corresponding entries. Multiplication of corresponding logical values is the same as "and" of the entries.

The code could have been written as

c=(NCol(1:k-1) == NRow(k)) & (NRow(1:k-1) == NCol(k))

and the only difference would have been as to whether c would end up being a double precision vector (original code) or a logical vector (suggested code)

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Walter Roberson 2021-10-24

@kalana agampodi

No, I don't think I can explain that. I would have to work backwards and figure out what kind of problem might exist that could lead to that code being considered a solution. Some kind of graph theory algorithm. It might perhaps be calculating the degree of each vertex in a graph.

De Silva 2021-10-24