Numeric arrays can never contain strings or characters. It takes the ASCII value of that character if you try to replace an element in numerical matrix with a “char”. You can use cell arrays to do this. As an example, refer the following code:
new_seq1 = [45 16 20 42]; % let
new_cell = num2cell(new_seq1); % save numerical array in cell format
new_cell {1} = 'I'; % replace
Refer the following document for more details on Cell Arrays: