How can I find the sum of the perimeter of a matrix?

Question

Alexandra Huff 2016-8-5

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回答： Ibrahim Abouemira 2019-5-19

采纳的回答： dpb

Hi, I am wondering if I had a matrix [1,2,3;1,2,3;1,2,3] how I could find the sum of the perimeter. Thanks an advance.

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Answer 1

dpb 2016-8-5

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One way amongst any number...

>> M=[1,2,3;1,2,3;1,2,3];
>> I=ones(size(M)); I(2:end-1,2:end-1)=0;
>> S=sum(reshape(M.*I,1,[]))
S =
  16
>>

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Answer 2

Stephen23 2016-8-5

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编辑：Stephen23 2016-8-5

在 MATLAB Online 中打开

In one line is easy and very fast:

>> M = [1,2,3;1,2,3;1,2,3];
>> sum(sum([M([1,end],2:end-1),M(1:end,[1,end]).']))
ans =  16

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dpb 2018-2-10

[ ... Ricardo Florez Answer moved to comment since was a follow-up question on earlier Answer syntax ... dpb]

Hi. Why is it necessary to write two "sum" in the code: sum(sum(M(...)? Please see below:

the sum(M(:))-sum(sum(M(2:end-1,2:end-1)))

Thank you.

dpb 2018-2-10

编辑：dpb 2018-2-11

Because sum(M(2:end-1,2:end-1)) is a row vector of the sums of the columns in the submatrix; to get the total have to then add those values. This is taken care of in the first sum by the use of the special syntax (:) that returns all elements of an array/matrix in a column so sum only needs to add the one column vector. There is no syntax in Matlab to apply the secondary indexing operation to the result of the first function call return; there've been many suggest the enhancement.

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Answer 3

Sanjay Zamindar 2018-6-28

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https://ww2.mathworks.cn/matlabcentral/answers/298528-how-can-i-find-the-sum-of-the-perimeter-of-a-matrix#answer_326658

编辑：dpb 2018-6-28

在 MATLAB Online 中打开

function final_sum = peri_sum(A)
size_row = size(A,1)
size_col=size(A,2)
Sum_of_first_row = sum (A(1, [1:1:end])); %base 
Sum_of_first_Colmn = sum(A([2:1:size_row],1))
%sum_col= sum(A([2:1:size_col], [1:size_col:size_col]));
Sum_of_last_column= sum(A([2:1:size_row],size_col))
sum_of_last_row = sum(A(size_row,[2:1:size_col-1]))
final_sum = sum_of_last_row+ Sum_of_last_column+Sum_of_first_Colmn+Sum_of_first_row
end