C++ Dynamically Define Function

Question

I am on visual c++ working on a console calculator, I am creating a way to let the user define a custom linear function. Here is where I am stumped: Once I get the users desired name of the function, the slope, and the y-intercept, I need to use that data to create a callable function that I can pass to muParser.

In muParser, you define custom functions like this:

double func(double x)
{
    return 5*x + 7; // return m*x + b;
}

MyParser.DefineFun("f", func);
MyParser.SetExpr("f(9.5) - pi");
double dResult = MyParser.Eval();

How could I dynamically create a function like this based on the users input for the values 'm' and 'b' and pass that to the 'DefineFun()' method? This is what I have so far:

void cb_SetFunc(void)
{
    string FuncName, sM, sB;
    double dM, dB;
    bool GettingName = true;
    bool GettingM = true;
    bool GettingB = true;
    regex NumPattern("[+-]?(?:0|[1-9]\\d*)(?:\\.\\d*)?(?:[eE][+\\-]?\\d+)?");

    EchoLn(">>> First, enter the functions name. (Enter 'cancel' to abort)");
    EchoLn(">>> Only letters, numbers, and underscores can be used.");

    try
    {
        do // Get the function name
        {
            Echo(">>> Enter name: ");
            FuncName = GetLn();
            if (UserCanceled(FuncName)) return;

            if (!ValidVarName(FuncName))
            {
                EchoLn(">>> Please only use letters, numbers, and underscores.");
                continue;
            }
            GettingName = false;

        } while (GettingName);

        do // Get the function slope
        {
            Echo(">>> Enter slope (m): ");
            sM = GetLn();
            if (UserCanceled(sM)) return;

            if (!regex_match(sM, NumPattern))
            {
                EchoLn(">>> Please enter any constant number.");
                continue;
            }
            dM = atof(sM.c_str());
            GettingM = false;

        } while (GettingM);

        do // Get the function y-intercept
        {
            Echo(">>> Enter y-intercept (b): ");
            sB = GetLn();
            if (UserCanceled(sB)) return;

            if (!regex_match(sB, NumPattern))
            {
                EchoLn(">>> Please enter any constant number.");
                continue;
            }
            dB = atof(sB.c_str());
            GettingB = false;

        } while (GettingB);

            // ------------
            // TODO: Create function from dM (slope) and
            // dB (y-intercept) and pass to 'DefineFun()'
            // ------------
    }
    catch (...)
    {
        ErrMsg("An unexpected error occured while trying to set the function.");
    }
}

I was thinking that there isn't a way to define an individual method for each user-defined-function. Would I need to make a vector<pair<double, double>> FuncArgs; to keep track of the appropriate slopes and y-intercepts then call them dynamically from the function? How would I specify which pair to use when I pass it to DefineFun(FuncStrName, FuncMethod)?

Morwenn · Accepted Answer · 2015-04-30 12:08:55Z

4

What you need (in addition to a script language interpreter) is called a "trampoline". There is no standard solution to create those, in particular since it involves creating code at runtime.

Of course, if you accept a fixed number of trampolines, you can create them at compile time. And if they're all linear, this might be even easier:

const int N = 20; // Arbitrary
int m[N] = { 0 };
int b[N] = { 0 };
template<int I> double f(double x) { return m[I] * x + b; }

This defines a set of 20 functions f<0>...f<19> which use m[0]...m[19] respectively.

Edit:

// Helper class template to instantiate all trampoline functions.
double (*fptr_array[N])(double) = { 0 };
template<int I> struct init_fptr<int I> {
  static const double (*fptr)(double) = fptr_array[I] = &f<I>;
  typedef init_fptr<I-1> recurse;
};
template<> struct init_fptr<-1> { };

edited Apr 30, 2015 at 12:08

Morwenn

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answered Sep 13, 2012 at 11:25

MSalters

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12 Comments

Brandon Miller Over a year ago

Which would be great, except that when muParser calls the function, it will not pass the I template parameter to the function to get the values at the specified index in m and b. I do not control how the function is called, muParser independently does that when it finds "f(x)" in the set expression string. Not to mention I would be passing a template<int> double(double) to a double(double) argument.

MSalters Over a year ago

@BrandonMiller: You use it as MyParser.DefineFun("foo", f<0>); MyParser.DefineFun("bar", f<1>); . The C++ compiler will generate up to 20 unique addresses, and muParser will pass one of those addresses - not I.

Brandon Miller Over a year ago

Hmm! I think this is exactly what I need! It seems pretty simple and straight forward. boost::function's with all that 'bind', 'reference' and 'delegate' jargon confuses me. Going to try this right away. And this will be done at runtime, so instead of two arrays I'm thinking I should use a vector<pair<double, double>> FuncArgs and access them like return FuncArgs.at(I).first * x + FuncArgs.at(I).second;

MSalters Over a year ago

@BrandonMiller: That would work as well, just keep in mind that you can't instantiate templates at runtime. If you mention f<0>, f<1>, f<3> at compile time, only those 3 functions would be instantiated. Editing...

Brandon Miller Over a year ago

I've hit a brick wall. I need to dynamically get the index of the function, but I can only use a constant value for indexes:

pair<double,double> argPair;  		argPair.first = dM;  		argPair.second = dB;  		LinFuncArgs.push_back(argPair);  		int index = NCast<int,size_t>(LinFuncArgs.size());  		Calc->DefineFun(FuncName, LinFunc<index>);

And at the last line I get the error: "Expression must have a constant expression". So I have to manually enter the index number, which totally defeats the purpose.

|

Kerrek SB · Accepted Answer · 2012-09-13 10:51:52Z

1

I would keep it simple:

#include <functional>

std::function<double(double)> f;   // this is your dynamic function

int slope, yintercept;             // populate from user input

f = [=](double x) -> double { return slope * x + yintercept; };

Now you can pass the object f to your parser, which can then call f(x) at its own leisure. The function object packages the captured values of slope and yintercept.

answered Sep 13, 2012 at 10:51

Kerrek SB

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11 Comments

PiotrNycz Over a year ago

I believe OP wants to "dynamically create a function ... based on the users input". However maybe we shall ask if dynamically means during runtime or before compilation...

Stefan Majewsky Over a year ago

That will probably cause problems since the external math library wants a function pointer, which std::function is not.

Brandon Miller Over a year ago

The problem is there will be many of these functions with different slopes and y-intercepts. With only those two variables keeping track of them, once the user defines another function he will no longer be able to use the previous functions. There needs to be a list of these functions with different M's and B's that the user can call by typing func(7) + anotherFunc(9.53)

Brandon Miller Over a year ago

@StefanMajewsky Yes, I believe you are correct. I need there to be (I guess a vector) of these function pointers all with different slopes and intercepts in them

Kerrek SB Over a year ago

@BrandonMiller: You could trivially make a std::vector<std::function<double(double)>> and put all your different functions in there. The question is whether you're able to change the interface of the parser to accept std::function objects as its core primitive.

|

Leonid Volnitsky · Accepted Answer · 2012-09-13 13:05:30Z

1

GiNaC is C++ lib which can parse and evaluate math expressions.

answered Sep 13, 2012 at 13:05

Leonid Volnitsky

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1 Comment

Brandon Miller Over a year ago

Thanks, but that part is all under control. My calculator is fully functional in terms of evaluating expressions, I'm just seeking to enable the user to define their own linear functions for use in expressions.

Pete · Accepted Answer · 2012-09-13 12:49:27Z

Generating a fixed array of functions bindable to boost function.

Someone else already said about a similar method, but since I'd taken the time to write the code, here it is anyway.

#include <boost/function.hpp>

enum {
    MAX_FUNC_SLOTS = 255
};

struct FuncSlot
{
    double (*f_)(double);
    boost::function<double(double)> closure_;
};

FuncSlot s_func_slots_[MAX_FUNC_SLOTS];

template <int Slot>
struct FuncSlotFunc
{
    static void init() {
        FuncSlotFunc<Slot-1>::init();
        s_func_slots_[Slot - 1].f_ = &FuncSlotFunc<Slot>::call;
    }
    static double call(double v) {
        return s_func_slots_[Slot - 1].closure_(v);
    }
};
template <> struct FuncSlotFunc<0> {
    static void init() {}
};

struct LinearTransform
{
    double m_;
    double c_;
    LinearTransform(double m, double c)
        : m_(m)
        , c_(c)
    {}
    double operator()(double v) const {
        return (v * m_) + c_;
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    FuncSlotFunc<MAX_FUNC_SLOTS>::init();

    s_func_slots_[0].closure_ = LinearTransform(1, 0);
    s_func_slots_[1].closure_ = LinearTransform(5, 1);

    std::cout << s_func_slots_[0].f_(1.0) << std::endl; // should print 1
    std::cout << s_func_slots_[1].f_(1.0) << std::endl; // should print 6

    system("pause");
    return 0;
}

So, you can get the function pointer with: s_func_slots_[xxx].f_ And set your action with s_func_slots_[xxx].closure_

halfer · Accepted Answer · 2018-10-01 10:59:18Z

0

Try to embed to your application some script language. Years ago I was using Tcl for similar purpose - but I do not know what is the current time best choice.

Either you can start from Tcl or search yourself for something better:

See: Adding Tcl/Tk to a C application

edited Oct 1, 2018 at 10:59

halfer

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answered Sep 13, 2012 at 10:43

PiotrNycz

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12 Comments

Some programmer dude Over a year ago

Lua is popular these days, and very easy to embed. It also has a very small footprint.

piokuc Over a year ago

Another candidate: 'Chibi-Scheme is a very small library intended for use as an extension and scripting language in C programs.' code.google.com/p/chibi-scheme

piokuc Over a year ago

Also, Squirrel is a very nice language suitable for embedding in C/C++ apps, similar to Lua, but even better: squirrel-lang.org

Brandon Miller Over a year ago

That's funny because originally to actually parse the mathematical equations I tried embedding TCL in my program, but I ran into problems with missing headers which I then tried to find online to no avail. So if I embed one of those languages I can simply create a string which defines my function and pass it to the embedded language parser and then pass the newly created function to the DefineFun() method? What about scope issues? I need the function to exist throughout my program so that my muParser instance can call it. When the function returns wont my new method be destroyed?

PiotrNycz Over a year ago

@Brandon: I believe many (or all) of these script languages provide functionality to "compile" script function once and then use it many times. Just look at the doc.

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