What does this statement mean in C? [closed]

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I am trying to understand the piece of code written in C and not sure I understand it fully.

Here is the function written in C:

int
gsl_multimin_diff (const gsl_multimin_function * f,
                   const gsl_vector * x, gsl_vector * g)
{
  size_t i, n = f->n;

  double h = GSL_SQRT_DBL_EPSILON;


  gsl_vector * x1 = gsl_vector_alloc (n);  /* FIXME: pass as argument */

  gsl_vector_memcpy (x1, x);

  for (i = 0; i < n; i++)
    {
      double fl, fh;

      double xi = gsl_vector_get (x, i);
      double dx = fabs(xi) * h;

      if (dx == 0.0) dx = h;    

     (x1, i, xi + dx);
      fh = GSL_MULTIMIN_FN_EVAL(f, x1)开发者_运维百科;

      gsl_vector_set (x1, i, xi - dx);
      fl = GSL_MULTIMIN_FN_EVAL(f, x1);

      gsl_vector_set (x1, i, xi);
      gsl_vector_set (g, i, (fh - fl) / (2.0 * dx));
    }

  gsl_vector_free (x1);

  return GSL_SUCCESS;
}

There is a line 14 in this code, which has this: (x1, i, xi + dx) What does it do? For referenc: x1 is the pointer to the function that allocates memory for a newly created vector. i - loop iterator xi - returning an element from the vector at position i dx is just a value. Thanks for your help!

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It looks like there's something missing there. It should be function arguments. Otherwise it's a no-op - (x1, i, xi + dx) is a valid expression in C, but one that doesn't do anything. It just mentions x1, then mentions i, then mentions the sum of xi and dx.