LOWEST MULTIPLE WITH 9s and 0s

% Lowest multiple with 9s and 0s
clc; clear;
N = input('Enter a positive whole number N: ');
cnt=1;
while true
    cnt = cnt+1;
    M = N*cnt;
    while M >= 1
        remainder = mod(M,10);
        if remainder == 0 && remainder == 9
            continue;
        else
            M = floor(M/10);     
        end         
    end
    if M < 1
        break;
    end
end
fprintf('Lowest multiple with only 9s and 0s: %d \n', (N*cnt));  

I am trying to find the lowest multiple of a number which is only with 9s and 0s. When I input 5 the program outputs 10 and I can't see where my mistake is.

Answer 1

[Geoff Hayes]

Spiro - consider the line of code

if remainder == 0 && remainder == 9

Your remainder cannot be both 0 and 9...it should be one or the other. Try changing this to

if remainder == 0 || remainder == 9

to see what happens. Also, I think that if the remainder is zero or nine then this is telling you that your right-most digit is either zero or nine. If this is true, then you want to consider the next right-most digit which I think that you are trying to obtain with the line

M = floor(M/10); 

So you want reduce your M if and only if the remainder is 0 or 9. If the remainder is something else, then you want to "start again" and so increment cnt and then obtain a new M.

Also, you may want to initialize cnt to zero (instead of one) to handle the case where the input is 9 (no reason to consider multiples of this number).