Another problem about decimal numbers

hi,
i have a code and inside my code, i declared a variable called 'subtotal' as a double. This variable is to hold the amount of money to be paid for a particular type of item. The part of code is as below:
double subtotal = 0.00, price;
int quantity;
(statements)
subtotal = price * quantity;Then i tested my code. But i found out that, if i entered a price, for example, $2.3, and 3 for quantity, instead of having a value of $6.9 for my subtotal, the value became $6.8888888889. Why is that so? How to solve this problem?
Thanks for help!

@OP: as you might know by now: not every decimal finite fraction can be represented
exactly by a finite binary fraction. Computers represent fractional numbers as a
finite series of negative powers of two, i.e. 1/2, 1/4, 1/8, 1/16 and so on. It only
can store a finite number of those negative powers of two in a 'double' value.
Think of it this way: there are negative powers of two on the table (1/2, 1/4, 1/8 ...)
and you've given the opportunity to pick any combination of them so you can add
them up and equal 0.1 (decimal 1/10). You're only allowed to pick up any number
at most once; here we go:
You pick 1/16 because 1/8 is too big and 1/32 doesn't make sense. You now
have to reach the number 1/10-1/16= 3/80 in order to close the gap.
You pick 1/32 because 1/16 is too big and 1/64 doesn't make sense. You now
have to reach the number 3/80-1/32=1/160 in order to close the gap.
You pick 1/256 because 1/128 is too big and 1/512 doesn't make sense. You now
have to reach the number 1/160-1/256=3/1280 in order to close the gap.
You pick 1/512 because 1/256 is too big and 1/1024 doesn't make sense. You now
have to reach the number 3/1280-1/512=1/2560 to close the gap.
etc.etc. etc.
You end up picking the numbers sum(i= 1, ...)(1/2^(4*i)+1/2^(4*i+1)) and you
have to stop after, say, 64 tries and you never reach 1/10 (decimal) exactly.
This is what that FPU has to do and it can't reach the number 1/10 exactly
either, i.e. it picks the number closest to it.
There are an infinit amount of decimal fractions that can not be represented
as a finite sum of negative powers of two. The opposite is true also.
Learn to live with that truth and let a DecimalFormat pick the correct rounding
value for you, it makes life much easier, because you only want to represent
the 'correct' values (mind the quotes).
kind regards,
Jos

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