question: public static void toPolarCoordinates ( double [ ] x , double [ ] y ) { if ( x . length != y . length ) { throw new IllegalArgumentException ( "Arrays must be equal length" ) ; } for ( int i = 0 ; i < x . length ; i ++ ) { double r = Math . sqrt ( x [ i ] * x [ i ] + y [ i ] * y [ i ] ) ; double phi = Math . atan2 ( y [ i ] , x [ i ] ) ; x [ i ] = r ; y [ i ] = phi ; } }
target:  Convert a pair of arrays from cartesian (x, y) coordinates to polar (r, phi) coordinates. Phi will be in radians, i.e. a full circle is two pi.