Sum function on part of an array

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Robert Demyanovich 2021-6-8

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https://ww2.mathworks.cn/matlabcentral/answers/851160-sum-function-on-part-of-an-array

评论： Walter Roberson 2021-6-8

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

I don't understand what the following sum function does.

mA=zeros(M,1);     
mA(1)=sum(cA(1,:))*dx; 

mA is a column vector, so what does mA(1) mean?

The documentation for the sum function indicates that taking the sum of a vector just results in one value which is the sum of all of the values in the vector. Is that also what happens when you take the sum of a single row in a matrix (sum(cA(1,:)))? So is mA(1) equal to a single value? I can't seem to find out what mA(1) means for a vector.

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Walter Roberson 2021-6-8

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mA(1) in this context is designating a single array element as the destination for the assignment. You are putting the results into the first box of the stack of cubby-holes.

Is that also what happens when you take the sum of a single row in a matrix (sum(cA(1,:)))

Yes.

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Robert Demyanovich 2021-6-8

I see. So in a 5x5 array, the first cell would have an index number of 1 and the last cell would have an index number of 25. Thanks for the explanation.

Walter Roberson 2021-6-8

在 MATLAB Online 中打开

Right.

reshape(1:25,5,5)
ans = 5×5
   6    11    16    21
   7    12    17    22
   8    13    18    23
   9    14    19    24
  10    15    20    25

reshape() changes the information about dimensions without changing the order in memory, so the values you see above in this particular case also happen to correspond to linear index.

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