29. Channel's information characteristics: information transfer speed,capacity.

The information channel can also be characterized by the three parameters: the time of channel use Tk bandwidth, channel skip, Fk, and a dynamic range of channel Dk characterizing its ability to transmit different signal levels. Value is called the capacity of the channel. Undistorted signal transmission is possible only provided that the signal on the volume "fit" in the channel capacity. For information channel concepts are: the speed of data entry, data transfer rate and bandwidth of the channel.  The data transfer speed I (Z, Y) - average amount of information transmitted on the channel at a time. It depends on the statistical properties of the transmitted signal and the channel properties. Under the input speed (the flow of information) I (X) mean the average amount of information entered in the source message data channel per unit time. This characteristic is determined by the source of the messages and only the statistical properties of messages. Capacity C - maximum theoretically achievable for the channel data rate. This reference channel and is not dependent on the signal statistics. The capacity of the information channel is defined by two parameters: word length and frequency. It is proportional to their product. The bit is called the maximum amount of information that can be simultaneously placed in the channel. The frequency indicates how many times information can be placed in the channel during a time unit.

30. Hash Codes: codes for efficient information retrieval.

A hashCode is a way of computing a small (32-bit) digest numeric key from a long String or even an arbitrary clump of bytes. The numeric key itself is meaningless and the hashCode functions for computing them can look a bit insane. However, when you go to look for something, you can do the same digest calculation on the long alphabetic key you are looking for, and no matter how bizarre an algorithm you used, you will calculate the same hashCode, and will be able to look up numerically with it. Of course there is always the possibility two different Strings will have the same digest hashCode. However, even then, all is not lost; it greatly narrows down the search, hence speeding it up. A Hashtable goes a step further, scrunching down the hashCode even further to an even smaller number that it can use to directly index an array, usually by dividing it by some (ideally prime) number and taking the remainder. A hash table is called a data structure that can store pairs of the form (key, hash), and supports the search operation, insert, and delete an item. Object hash table search is accelerating, for example, text entry fields in the database can be calculated from their hash code and data may be placed in the section corresponding to the hash code. Then when searching for data will have to first calculate the hash of the text, and once it becomes known, in which section they should look for, that is, the search should not be all over the base, and only one of its section (it is much faster to find.) Domestic analogue hash in this case can serve as a space of words in the dictionary alphabetically. The first letter of a word is its hash code, and when we look at not finding the whole dictionary, but only the desired letter.