How to store " first_element " values?

Question

parag gupta 2020-2-21

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https://ww2.mathworks.cn/matlabcentral/answers/506804-how-to-store-first_element-values

评论： madhan ravi 2020-2-21

Could you please tell how to store the "first_element" values in a matrix?

%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%code
first_element = [];
end_element = [];
A = [ 1 1 1 1 2 3 4 4 4 ; 1 2 3 4 5 5 5 5 5];
 for this_row = A.'
V = this_row'; 
X = diff(V)~=0;
B = find([true,X]);
E = find([X,true]);
D = 1+E-B ;
first_element = D(1,1)
end_element = D(end);
end
%%%%%%%%%%%%%%%%%%%%

output of the following code is :

first_element =
     4
first_element =
    1

But I want output to be :

first_element = [ 4 ; 1]
%%%%%%%%%%%%%%%%%%%%%%%%%

Thanks

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Answer 1

madhan ravi 2020-2-21

1
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https://ww2.mathworks.cn/matlabcentral/answers/506804-how-to-store-first_element-values#answer_416673

A = [ 1 1 1 1 2 3 4 4 4 ; 1 2 3 4 5 5 5 5 5];
[first_element,end_element] = deal(zeros(size(A,1),1)); % preallocate
for k = 1:size(A,1)
    V = A(k,:);
    X = diff(V)~=0;
    B = find([true,X]);
    E = find([X,true]);
    D = 1+E-B ;
    first_element(k) = D(1,1);
    end_element(k) = D(end);
end

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parag gupta 2020-2-21

Thanks Madhan :)

madhan ravi 2020-2-21

Thank you Stephen :) , you're solution of accumarray() indeed is impressive as always! It's been a while now since I am indulged with MATLAB so couldn't get my head around that function.

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Answer 2

Stephen23 2020-2-21

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https://ww2.mathworks.cn/matlabcentral/answers/506804-how-to-store-first_element-values#answer_416674

编辑：Stephen23 2020-2-21

No loop needed, here using accumarray:

>> A = [1,1,1,1,2,3,4,4,4;1,2,3,4,5,5,5,5,5]
A =
   1   1   1   1   2   3   4   4   4
   1   2   3   4   5   5   5   5   5
>> D = diff(A(:,[1,1:end,end]),1,2)~=0;
>> D(:,[1,end]) = true;
>> [C,R] = find(D.');
>> first = accumarray(R,C,[],@(v)v(2)-v(1))
first =
   4
   1
>> last = accumarray(R,C,[],@(v)v(end)-v(end-1))
last =
   3
   5