question: public static double sumSquaredError ( double [ ] a , double [ ] b ) { if ( a . length != b . length ) { throw new IllegalArgumentException ( "Arrays must be equal length" ) ; } double sum = 0 ; for ( int i = 0 ; i < a . length ; i ++ ) { double delta = a [ i ] - b [ i ] ; if ( ! Double . isNaN ( delta ) ) { sum += delta * delta ; } } return sum ; }
target:  Build the sum of the squared difference of all elements with the same index numbers in the arrays. Any NaN values in either a or b are ignored in computing the error.