How do I create a function at runtime in Objective-C

Question

So it's late here, and my google skills seem to be failing me. I've found some great responses on SO before (time and time again), I thought you guys could help.

I have a neural network I'm trying to run in native objective-c. It works, but it's too slow. These networks are not recurrent. Each network I run about 20,000 times ( 128x80 times, or around that). The problem is these networks really just boil down to math functions (each network is a 4 dimensional function, taking x,y,dist(x,y),and bias as inputs, and outputting 3 values).

What I want to do is convert each network (only once) into a function call, or a block of code at runtime in objective-c.

How do I do this? I could make a big string of the math operations that need to be performed, but how do I go about executing that string, or converting the string into a block of code for execution?

Again, my late night search failed me, so sorry if this has already been answered. Any help is greatly appreciated.

-Paul

Edit: Aha! Great success! Nearly 24 hours later, I have working code to turn a neural network with up to 4 inputs into a single 4 dimensional function. I used the block method suggested by Dave DeLong in the answers.

For anybody who ever wants to follow what I've done in the future, here is a (quick) breakdown of what I did (excuse me if this is incorrect etiquette on stackoverflow): First, I made a few typedef's for the different block functions:

typedef CGFloat (^oneDFunction)(CGFloat x);
typedef CGFloat (^twoDFunction)(CGFloat x, CGFloat y);
typedef CGFloat (^threeDFunction)(CGFloat x, CGFloat y, CGFloat z);
typedef CGFloat (^fourDFunction)(CGFloat x, CGFloat y, CGFloat z, CGFloat w);

A oneDFunction takes the form of f(x), twoD is f(x,y), etc. Then I made functions to combine two fourDFunction blocks (and 2 oneD, 2 twoD, etc, although these were not necessary).

fourDFunction (^combineFourD) (fourDFunction f1, fourDFunction f2) =
  ^(fourDFunction f1,     fourDFunction f2){
    fourDFunction blockToCopy = ^(CGFloat x, CGFloat y, CGFloat z, CGFloat w){
        return f1(x,y,z,w) + f2(x,y,z,w);
    };
    fourDFunction act = [blockToCopy copy];
    [f1 release];
    [f2 release];
    //Need to release act at some point
    return act;            
};

And, of course, I needed to apply the activation function to the fourD function for every node, and for each node, I would need to multiply by the weight connecting it:

//for applying the activation function
fourDFunction (^applyOneToFourD)( oneDFunction f1, fourDFunction f2) = 
^(oneDFunction f1, fourDFunction f2){
    fourDFunction blockToCopy = ^(CGFloat x, CGFloat y, CGFloat z, CGFloat w){
        return f1(f2(x,y,z,w));
    };    

    fourDFunction act = [blockToCopy copy];
    [f1 release];
    [f2 release];

    //Need to release act at some point
    return act; 

};

//For applying the weight to the function
fourDFunction (^weightCombineFour) (CGFloat x, fourDFunction f1) =
 ^(CGFloat weight, fourDFunction f1)
{
    fourDFunction blockToCopy = ^(CGFloat x, CGFloat y, CGFloat z, CGFloat w){

        return weight*f1(x,y,z,w);
    };

    fourDFunction act = [blockToCopy copy];
    [f1 release];
    //[act release];
    //Need to release act at some point
   return act;

};

Then, for each node in the network, I simply applied the activation function to the sum of the fourD functions from the source neurons multiplied by their connection weight. After composing all those blocks, I took the final functions from each output. Therefore, my outputs are separate 4D functions of the inputs.

Thanks for the help, this was very cool.

Answer 1

You can do this with blocks. Something like:

//specify some parameters
int parameter1 = 42;
int parameter2 = 54;
//create your block
int (^myBlock)(int) = ^(int parameter3){
  return parameter1 * parameter2 * parameter3;
};
//copy the block off the stack
myBlock = [myBlock copy];
//stash the block somewhere so that you can pull it out later
[self saveBlockOffSomewhereElse:myBlock underName:@"myBlock"];
//balance the call to -copy
[myBlock release];

And then elsewhere...

int (^retrievedBlock)(int) = [self retrieveBlockWithName:@"myBlock"];
int theAnswer = retrievedBlock(2);  //theAnswer is 4536

If you have a string representing some math to evaluate, you could check out GCMathParser (fast but not extensible) or my own DDMathParser (slower but extensible).