Computers today are used to forecast the weather, to operate machines, to cut shapes out of sheet metal and even to guide spacecraft to the moon. Computers are necessary in printing book and news papers, in diagnosing disease, in looking of obscure documents in archives and elusive criminals etc. Travel agents all round the world can know whether a seat or a bed will be available in an aero plane or in hotel, either today or a year from now. Companies use them for accounting, invoicing, stock control and pay rolls. Computer helps greatly in medical science, particularly in surgery and pathology.