[MajorMatt]

Hello,
I began tinkering with x-code last night. I am trying to do some math from some fields and display a result in another field. I got it to display one field plus another, but making a more complex math expression yields only operand errors

I tried without success:

[ddisplay setFloatValue:[avalue floatValue] * [bvalue floatValue] + [avalue floatvalue] * [cvalue floatValue] * [cvalue floatValue]];

Anyone help would be appreciated!

[johnMG]

Looks like an operator precedence issue. How about adding some parenthesis?

Code:
[ddisplay setFloatValue:
    ( [avalue floatValue] * [bvalue floatValue] +
      [avalue floatvalue] * [cvalue floatValue] * [cvalue floatValue] )
    ];

[Ghoser777]

Originally posted by MajorMatt:

[ddisplay setFloatValue:[avalue floatValue] * [bvalue floatValue] + [avalue floatvalue] * [cvalue floatValue] * [cvalue floatValue]];

Notice in the bold code, you didn't capitalize your floatValue call. I'm guessing that you also got an error about being unable to find the method and another about returning id.

Hope this helps,
Matt Fahrenbacher

[00101001]

A nitpick, but no reason you shouldn't know this:

a*(b+c) is faster to compute than a*b + a*c
(1 mul, 1 add vs 2 mul, 1 add)

so in your case, factor out the [avalue floatValue] -- you won't notice a speed difference, but if anything, it's shorter to write

[MajorMatt]

Thanks, guys.


I made a build, it works but not as standalone as there is this 15 meg build folder?

I notice the compile option is grayed out and I suspect that is what I need.

[Juggle5]

Originally posted by 00101001:
A nitpick, but no reason you shouldn't know this:

a*(b+c) is faster to compute than a*b + a*c
(1 mul, 1 add vs 2 mul, 1 add)

so in your case, factor out the [avalue floatValue] -- you won't notice a speed difference, but if anything, it's shorter to write

To extend this, any time you have a polynomial expression, it's nest to factor it like this, both for speed and numerical stability. I.e., rather than:
y = 3*x*x*x*x + 5*x*x*x + 2*x*x + 6*x + 7
instead write
y = (((3*x + 5)*x + 2)*x + 6)*x + 7

This is known as Horner's method, I believe. I once sped a (math-intensive fractal) program up by about 10x using this technique.