# Find the first appearance of a number in a row.

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SChow il 28 Set 2023
Commentato: Voss il 28 Set 2023
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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### Risposta accettata

Voss il 28 Set 2023
Modificato: Voss il 28 Set 2023
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
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SChow il 28 Set 2023
Thanks, much much appreciated.
Voss il 28 Set 2023
You're welcome!

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