ND_MAG(dx[0],dx[1],dx[2]);

Please let me know the above statement ND_MAG.

it is equivalent with:

sqrt(pow(dx[0],2.0)+pow(dx[1],2.0)+pow(dx[2],2.0)).

Should be used with care in 2D version since dx[2] is not always 0 as expected.

in 2D it is automatically set to zero. you need not bother about dx[2]. Its a macro that calculates the MAGnigitude of a Vector quantity