Question

I tried to assign a very small number to a double value, like so:

double verySmall = 0.000000001;

9 fractional digits. For some reason, when I multiplicate this value by 10, I get something like 0.000000007. I slighly remember there were problems writing big numbers like this in plain text into source code. Do I have to wrap it in some function or a directive in order to feed it correctly to the compiler? Or is it fine to type in such small numbers in text?

Answer 1

Standard floating point numbers are not stored in a perfect format, they’re stored in a format that’s fairly compact and fairly easy to perform math on. They are imprecise at surprisingly small precision levels. But fast. More here.

If you’re dealing with very small numbers, you’ll want to see if Objective-C or Cocoa provides something analagous to the java.math.BigDecimal class in Java. This is precisely for dealing with numbers where precision is more important than speed. If there isn’t one, you may need to port it (the source to BigDecimal is available and fairly straightforward).

EDIT: iKenndac points out the NSDecimalNumber class, which is the analogue for java.math.BigDecimal. No port required.