C change behavior of function on behalf of input

my program in C has some functionality (#obviously). The program gets input from the user, this user can then choose different implementations, e.g. myProgram -V1, or myProgram -V2 …

This specification of -V1, -V2, … decides how the function performs a particular calculation.

E.g..

for (int i = 0; i < len; i++) {
  i += myFunctionWhichChangesBehaviorOnUserInput(arr[i]);
} 

Now I don’t want to create a separate function for each V and then change the respective myFunctionWhichChangesBehaviorOnUserInput() function there.

I know you can do it this way in java.

interface Compute {
  double compute(double n);
}

... some imple of Compute 

class Task {
  Compute compute;

  void setCompute(Compute c) {this.compute = c}

  double doMyStuff(double[] arr) {
    double n = 0;
    for (int i = 0; i < arr.length; i++) {
      n += compute.compute(arr[i]);
    }
    return n;
  }
}

How can you implement this in C, must be possible somehow.

>Solution :

It sounds like you want function pointers.

#include <stddef.h>
#include <stdio.h>

typedef void (*compute_t)(double);
void printing_a(double);
void printing_b(double);

double arr[5] = {1.0, 2.0, 3.0, 3.5, 4.0};
size_t len = 5;

int main(int argc, char *argv[]) {
    compute_t myfunction;
    if (argc > 2) {
        myfunction = printing_a;
    } else {
        myfunction = printing_b;
    }

    for (size_t i = 0; i < len; i++) {
        myfunction(arr[i]);
    }
}

void printing_a(double n) {
    printf("printing_a(%f)", n);
}

void printing_b(double n) {
    printf("printing_b(%f/2)", n*2.0);
}

You can set the function pointer, and then call the function pointed to by the pointer. This is analogous to your Java code.