I think you should try to work with string untill the end since when you convert the string to a number:
firstPart = str2double(stringParts(1));
Matlab does automatically remove the not usefull 0 since 1.25690 = 1.2569
something like this should work (yep it is not very elegant, but you can base on this code to solve your problem)
x = 1.25690e-15;
%Note that I had to change .8 to .5!!!
string = sprintf('%.5e', x); % Convert number to a scientific notation string with 8 decimal places of precision
stringParts = strsplit(string,'e'); % Split the string where 'e' is
firstPart = stringParts(1); % Get the 1st part of stringParts which is the first part of standard form.
%split the number into two parts: 1.25690 -> 1 and 25690
strSplit = split((firstPart{1}),'.');
%extract the decimal
decimals = strSplit(2);
%Reshape the decimal
for i = 1:length(decimals{1})
k(i) = str2double(decimals{1}(i));
end
