The use of strfind here is not correct:
% Read the DNA sequence from the file
dna_sequence = strfind(fileID, '%s');
It looks like you meant to use fscanf:
% Read the DNA sequence from the file
dna_sequence = fscanf(fileID, '%s');
With that change, the code appears to work (but don't forget to fclose!!!!). (Note that the sequence 'TACG' is found in files 2-1 and 3-1 but not 1-1.)
% Open the file
fileID = fopen('cDNA2-1.txt', 'r');
% Read the DNA sequence from the file
dna_sequence = fscanf(fileID, '%s');
% Close the file
fclose(fileID);
%
DNAsequence=string(dna_sequence)
% Define the primer sequence
primer_sequence = 'TACG';
% Find the location of the primer sequence in the DNA sequence
primer_location = strfind(DNAsequence, primer_sequence)
% Display the location of the primer sequence
if isempty(primer_location)
disp('Primer sequence not found in the DNA sequence.');
else
disp(['Primer sequence found at position(s): ', num2str(primer_location)]);
end
