findluhndig(num)

function dig = findluhndig(num)
% dig = findluhndig(num). If num is a string of numbers, with a
% missing digit substituted with 'X', then dig will supply the missing
% digit according to the Luhn algorithm.  This function is dependent on
% luhncheck.
x_index = strfind(num, 'X');
for i = 0:9
    num(x_index) = num2str(i);
    if luhncheck(num)
        dig = num(x_index);
    end
end