Find the first appearance of a number in a row.

Question

SChow 2023-9-28

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2026914-find-the-first-appearance-of-a-number-in-a-row

评论： Voss 2023-9-28

Hi I have a matrix A from which I need to return a vector which contain the index of first occurence of a number say 2030 such that we get the matrix below

A = [0   0   0  2095    3030
 0   2030   2030 2030 2030
 0   0   2095   2055    2065
 2065   2055    2050    2030    2030]
B=[5 2 0 4]

I tried using

    for i=1:4
        d=A(i,:);
        targetval = 2030;
        num_wanted = 2030;
        xx=(find(d==targetval, num_wanted, 'first'))
            
    end
    

which apparently does the job (inefficiently) , but I am confused on how to store the indices in a vector and if the value (2030) is not there in a row - then return 0.

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Answer 1

Voss 2023-9-28

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2026914-find-the-first-appearance-of-a-number-in-a-row#answer_1321154

编辑：Voss 2023-9-28

在 MATLAB Online 中打开

A = [0   0   0  2095    2030
 0   2030   2030 2030 2030
 0   0   2095   2055    2065
 2065   2055    2050    2030    2030]
A = 4×5
           0           0           0        2095        2030
           0        2030        2030        2030        2030
           0           0        2095        2055        2065
        2065        2055        2050        2030        2030
targetval = 2030;
n_rows = size(A,1);
B = zeros(1,n_rows);                      % pre-allocate vector B with a zero for each row of A.
for i = 1:n_rows
    d = A(i,:);
    idx = find(d==targetval, 1, 'first'); % find the first occurrence of targetval in row i.
    if ~isempty(idx)                      % if there is one, store it as element i of B;
        B(i) = idx;                       % (otherwise element i of B remains 0).
    end
end
disp(B);
     5     2     0     4