code: public void mapInPlace(Function<Double, Double> fn) {
      for (int i = 0; i < pointers.length; i++) {
        if (pointers[i] == null) continue;
  
        if (copyOnWrite[i]) {
          copyOnWrite[i] = false;
          pointers[i] = pointers[i].clone();
        }
  
        if (sparse[i]) {
          for (int j = 0; j < pointers[i].length / 2; j++) {
            pointers[i][(j * 2) + 1] = fn.apply(pointers[i][(j * 2) + 1]);
          }
        } else {
          for (int j = 0; j < pointers[i].length; j++) {
            pointers[i][j] = fn.apply(pointers[i][j]);
          }
        }
      }
    }
docs: Apply a function to every element of every component of this vector, and replace with the result.
  
  @param fn the function to apply to every element of every component.