Passing a function with array parameter as a parameter to another function-CodePudding

I am trying to calculate the Jacobian matrix (numerical, at a given point) of a given equation. Now, I don't know the dimension of the equation beforehand, so I can't just do something like

static double f(double x1, x2)
{
    return x1 * x1 - 2 * x1 * x2;
}

so instead, I am getting the input values as an array like so


static double f(double xArray[]) 
{
    return xArray[0] * xArray[0] - 2 * xArray[1] * xArray[0];
}

void jacobian(double xArray[], double jacob_matrix, size_t size,
        double f(double xArray[], size_t)) 
{
    // calculations
}

However, when I try to call the function from main like

int main(void)
{
    double x_input[4] = {1., 1., 3., 4.};
    double jacob_matrix[4]; 
    
    jacobian(x_input, jacob_matrix, 4, f(x_input, 4));
    return 0;
}

I get incompatible type for argument 4 of 'jacobian' I imagine this has to do with my array being casted into a pointer, but I can't figure out how to fix this.

CodePudding user response：

You need to pass the function pointer to f, not the result of calling f. Try:

jacobian(x_input, jacob_matrix, 4, f);

CodePudding user response：

For the function signature:

void jacobian(double xArray[], double jacob_matrix, size_t size,
    double f(double xArray[], size_t))

For the second argument, if you're going to pass an array you need the argument to be a pointer, double* jacob_matrix or double jacob_matrix[].

For the 4th argument, the passed function arguments should match the caller signature, so in your case you are passing a function which is of type double (*)(double *), but the caller expects a function of type double (*)(double *, size_t), so one of them should be changed to match, either add a second argument to the called function, or remove the second argument of the caller function signature.

#include <stdio.h>

static double f(double xArray[]) 
{
    return xArray[0] * xArray[0] - 2 * xArray[1] * xArray[0];
}

void jacobian(double xArray[], double* jacob_matrix, size_t size,
        double f(double xArray[])) 
{   
    printf("%f", f(xArray)); // for example
}

int main(void)
{
    double x_input[4] = {1., 1., 3., 4.};
    double jacob_matrix[4]; 
    
    jacobian(x_input, jacob_matrix, 4, f);
    return 0;
}

Note that my approach only addresses the code correctness and not the mathematical part, i.e. the jacobian calculation.