format fractions (i.e. 0.25 = 1/4) how?

I don't think there is a built-in mechanism in Cocoa for the rational numbers (there is one for decimal numbers).

Your problem is a very difficult mathematic problem in the general case. For example, is 0.3333… to be represented as 1/3 or 33333…/100000… ?

If you want to represent only decimal numbers as fraction, it's recommended to use the subclass NSDecimalNumber (you can specify the rouding mode, the rouding scale, …), and you'll have to write a specific Formatter.

If you want to represent any rational number as a fraction, you may create a NSNumber subclass to maange the numerator and denominator.

Like JP said you'll have to write your own formatter.

http://devworld.apple.com/documentation/Cocoa/Conceptual/DataFormatting/Articles /CreatingACustomFormatter.html

Now for the hard part here's some code that might help you with the conversion.

http://www.ics.uci.edu/~eppstein/numth/frap.c

Eric

A really simple way to do it would be to just do the math.

If you have 0.25 for example just divide 1/0.25 = 4...then format your fraction with the result as the denominator.

Depending on your fractions though changing the format may not look any more sane.
eg) 1/0.333 = 3.003003003003... :-/ Irrational numbers are not your friend here.