[OreoCookie]

So I started learning objective C and started with the Currency Converter tutorial.

After writing the easy code, I got the followin issue.

When I enter a number like 0,9812 for the exchange rate, and I enter 100 in the next field, my result is

98,1199951171875

If 98,11999999999 would be the answer, I remember hearing about that stuff. But there are many figures off, what happened?

[Detrius]

The problem is that many numbers that are easily representable in the base 10 number system are not easily representable in the base 2 number system. As such, there is rounding error. One way to limit the funkiness of such an effect is to have the system round values that are monetary. This can be set up rather easily in IB.

An example of something that doesn't convert well to base 10:

1/3 in base 3 is 0.1. In base 10, that comes out to about .3333333. In base 2, that comes out to about .010101010101...

If you need more precision, you can use double instead of float. However, for monetary values, you can just round at 2 decimal places.

[OreoCookie]

Originally posted by Detrius:
The problem is that many numbers that are easily representable in the base 10 number system are not easily representable in the base 2 number system. As such, there is rounding error. One way to limit the funkiness of such an effect is to have the system round values that are monetary. This can be set up rather easily in IB.

An example of something that doesn't convert well to base 10:

1/3 in base 3 is 0.1. In base 10, that comes out to about .3333333. In base 2, that comes out to about .010101010101...

If you need more precision, you can use double instead of float. However, for monetary values, you can just round at 2 decimal places.

I was aware of that, but thought just the last digit is prone to error, because the internal precision is higher.

But thanks.

[Rickster]

Another good way for dealing with number representation issues is to use Cocoa's NSDecimalNumber class, which offers several methods for performing arithmetic and can munge the underlying representation as needed to deliver correct results.

[clarkgoble]

There are also lots of math classes available on the web for doing arbitrary precision arithmetic. Whether they'd meet your needs depends upon what you are doing.

While I've never used it, gcc (Apple's compiler) comes with libg++ which purportedly has this.

Here's a nice list of other free libraries.

http://www.oonumerics.org/oon/#libraries

[OreoCookie]

Thanks for all the stuff.

Another quick question: is the only way to get a power of another number pow(x,y) (= x^y)?

[parallax]

Originally posted by OreoCookie:
Thanks for all the stuff.

Another quick question: is the only way to get a power of another number pow(x,y) (= x^y)?

There are a couple other specific functions (constant^x) like scalbn() and exp() but if you just want x^y, I think pow is the way to go.

[OreoCookie]

Originally posted by parallax:


There are a couple other specific functions (constant^x) like scalbn() and exp() but if you just want x^y, I think pow is the way to go.

Thanks. Wasn't sure, because every other programming language that I knew so far could calculate powers directly.

[phidauex]

This is drifting off topic, but uh, i don't care

The reason C doesn't have a built-in power function is because the language is kept intentionally trim. C is actually one of the simplest programming languages out there, with less than 30 keywords (i don't remember the exact number though, and my C book is all the way on the other side of the room ). For instance, C doesn't even include standard input and output, no special math functions (even common ones like pow() and sqrt()), etc. Everything is meant to be added on through libraries and modules. In C, nearly everything you write will have a few common includes, stdlib.h, stdio.h, math.h, etc. Because those aren't built into the language, its easier to keep C portable. Since objectiveC is built straight off of C, it too is reasonably simple, with barely enough additions to the language to support OO.

Peace,
phidauex