Counting number of digits in a double C++

I'm using a while loop to count the number of digits in my input.

So my input was 1.525

length = 0;
num = num - int(num);
while ( num >= .0001 ) {
    num = num * 10;
    length = length + 1;
    num = num - int(num); }

When i do

cout << "\n\nLength: " << length << "\n";

The answear I get is 51 and other numbers give me an asnwear of 49 or something that is obviously wrong. Is it the way c++ works or is it just my mistake. Thank you.