Find the first appearance of a number in a row.

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.

Answer 1

[Voss]

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

[Voss]

Note that the second argument of find() is the number of occurrences you want to find, not the number you want to find, so it's 1 here because you want to find only one (the first).

[Voss]

Here's another way, still using find(), but without the loop, applying find() to the entire matrix A at once.

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;
B = zeros(1,size(A,1));
[rows,cols] = find(A == targetval);
[rr,ii] = unique(rows);
B(rr) = cols(ii);

disp(B);

     5     2     0     4

[SChow]

Thanks, much much appreciated.

[Voss]

You're welcome!