question: public void registerOutput ( String varName ) throws DMLRuntimeException { if ( ! ( DMLScript . rtplatform == RUNTIME_PLATFORM . SPARK || DMLScript . rtplatform == RUNTIME_PLATFORM . HYBRID_SPARK ) ) { throw new DMLRuntimeException ( "The registerOutput functionality only supported for spark runtime. Please use MLContext(sc) instead of default constructor." ) ; } if ( _outVarnames == null ) _outVarnames = new ArrayList < String > ( ) ; _outVarnames . add ( varName ) ; if ( _variables == null ) _variables = new LocalVariableMap ( ) ; }
target:  Marks the variable in the DML script as output variable. Note that this expects a "write(varName, ...)" statement in the DML script which through non-MLContext invocation would have written the matrix to HDFS.