question: protected int selectOperator ( ) { lastUpdate ++ ; if ( ( lastUpdate >= UPDATE_WINDOW ) || ( probabilities == null ) ) { lastUpdate = 0 ; probabilities = getOperatorProbabilities ( ) ; } double rand = PRNG . nextDouble ( ) ; double sum = 0.0 ; for ( int i = 0 ; i < operators . size ( ) ; i ++ ) { sum += probabilities [ i ] ; if ( sum > rand ) { return i ; } } throw new IllegalStateException ( ) ; }
target:  Returns the index of one of the available operators randomly selected using the probabilities.