Файл: ИВб-14-2 Информатика для ИВб.pdf

When expressions contain operators from more than one category, arithmetic operators are evaluated first, comparison operators are evaluated next, and logical operators are evaluated last. Comparison operators all have equal precedence; that is, they are evaluated in the left- to-right order in which they appear. Arithmetic and logical operators are evaluated in the following order of precedence.

When multiplication and division occur together in an expression, each operation is evaluated as it occurs from left to right. Likewise, when addition and subtraction occur together in an expression, each operation is evaluated in order of appearance from left to right.

The string concatenation (&) operator is not an arithmetic operator, but in precedence it does fall after all arithmetic operators and before all comparison operators. The Is operator is an object reference comparison operator. It does not compare objects or their values; it checks only to determine if two object references refer to the same object.

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Digital Computers

There are two fundamentally different types of computers: analog and digital. In current usage "computer" usually refers to high speed digital computers. These computers are playing an increasing role in all branches of the economy. Digital computers are based on manipulating discrete binary digits ( I s and Os). They are generally more effective than analog computers for four principal reasons: they are faster: they are not so susceptible to signal interference; they can transfer huge data bases more accurately; and their coded binary data are easier to store and retrieve than the analog signals. For all their apparent complexity, digital computers are considered lo be simple machines. Digital computers are able lo recognize only two states in each of its millions of switches, "on" or "off", or high voltage or low voltage. By assigning binary numbers to these states 1 for "on" and 0 for "off", and linking many switches together, a computer can represent any type of data from numbers lo letters and musical notes. It is this process of recognizing signals that is known as digitization. The real power of a computer depends on the speed with which it checks switches per second. The more switches a computer checks in each cycle, the more data it can recognize at one time and the faster it can operate, each switch being called a binary digit or bit. A digital computer is a complex system of four functionally different elements: 1) the central processing unit (CPU). 2) input devices. 3) memory-storage devices called disk drives, 4) output devices. These physical parts and all their physical components are called hardware.

The central processing unit is the heart of a computer. In addition to performing arithmetic and logic operations on data, it controls the rest of the system. Sometimes the CPU consists of several linked microchips, each performing a separate task, but most computers require only a single microchip as the CPU.

Input devices let users enter commands, data, or programs for processing by the CPU. Information typed at the computer keyboard, which is much like typewriter, is translated into a series of binary numbers the CPU can manipulate. The mouse is another widely used mechanical input device to move the cursor on the display screen, the user moves the mouse, selects operations and activates commands on the screen by pressing buttons on the top of the mouse. The power of computers greatly depends on the characteristics of memory-storage devices.

Most digital computers store data both internally in what is called mam memory, and externally, on auxiliary storage units. As a computer processes data and instructions, it temporary stores information internally on special memory microchips. Auxiliary storage units supplement the main memory when programmes are too large and they also offer a more reliable method for storing data. There exist different kinds of auxiliary storage devices removable magnetic disks being the most widely used. They can store up to 100 megabytes of data on one disk, a byte being known as the basic unit of data storage.

Output devices let the user see the results of the computer's data processing. Being the most commonly used output device, the monitor accepts video signals from a computer and shows different kinds of information such as texts, formulas and graphics on its screen. With the help of various printers information stored in one of the computer's memory systems can be easily printed on paper in a desired number of copies.

Programmes, also called software, are detailed sequences of instructions that direct the computer hardware to perform useful operations. Due to a computer's operating system hardware and software systems can work simultaneously. An operating system consists of a number of programmes coordinating operations, translating the data from different input and output devices, regulating data storage in memory, transferring tasks to different processors and functions that help programmers to write software. In large corporations software is often written by groups of experienced programmers each person focusing on a specific aspect of the total project. For this reason scientific and industrial software sometimes costs much more than the computers on which the programmes run.

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Error Messages by Numbers

1. File does not exist

3.File is in use

4.End of file encountered

5.Record is out of range

6.Too many files open

7.File already exists

9.Data type mismatch

10.Syntax error

11.Function argument value, type, or count is invalid-

12.Variable "variable" is not found

13.Alias is not found

16.Unrecognized command verb

17.Table number is invalid

18.Line is too long

19.Index file does not match table

20.Record is not in index

21.Length of variable strings exceeds amount of memory-

22.Too many variables

23.Index expression exceeds maximum length

24.Alias name is already in use

26.Table has no index order set-

27.Not a numeric expression

30.Row or column position is off the screen

31.Invalid subscript reference

34. Operation is invalid for a Memo, General, or Picture field

36.Command contains unrecognized phrase/keyword

37.You must use a logical expression with FILTER-

38.Beginning of file encountered

39.Numeric overflow. Data was lost

41.Memo file "name" is missing or is invalid

42.The LOCATE command must be issued before the CONTINUE command-

43.There is not enough memory to complete this operation

45.Not a character expression-

46.Expression evaluated to an illegal value

47.No fields found to process

50. Report file "name" is invalid

52. No table is open in the current work area

54.Label file is invalid

55.Memory file is invalid

56.There is not enough disk space for "name

62.Cannot access characters beyond string

67. Expression evaluator failed

91. File was not placed in memory using the LOAD command

94.Must specify additional parameters

95.Statement is not allowed in interactive mode

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102.Cannot create file "file

103.Allowed DO nesting level exceeded

104.Unknown function key

107 Operator/operand type mismatch

108.File is in use by another user

109.Record is in use by another user

110.File must be opened exclusively

111.Can not update the file

112.Invalid key length

114.Index does not match the table. Delete the index file and recreate the index.

115.DIF file header is invalid

116.DIF vector is invalid DBF field

117.DIF type indicator is invalid

119.SYLK file header is invalid

120.SYLK file dimension bounds are

121.SYLK file format is invalid

124.Printer redirection is invalid

125.Printer is not ready

127. View file is invalid

130. Record is not locked

138. No fields were found to copy

152. Missing expression

164.Menu title has not been defined with DEFINE PAD

165.Menu has not been defined with DEFINE POPUP

166.No menu items have been defined for this menu

167.Menu item position must be a positive number

168.Menu has not been defined with DEFINE

169.Can not define menu

170.Can not release menu item

174.Can not redefine a menu that is in use

175.Can not redefine popup that is in use

176.Can not clear menu that is in use

177.Can not clear popup that is in

178.Menu has not been activated with ACTIVATE MENU

179.Menu has not been activated with ACTIVATE POPUP

181.Menu specified in ACTIVATE MENU is already in use

182.Menu specified in ACTIVATE POPUP is already in use

202. Invalid path or file name

214.Window has not been defined with DEFINE WINDOW

215.Window has not been activated with ACTIVATE WINDOW

216.Display mode is not available

221.Left margin including indent must be less than the right margin

222.Line number must be less than page length

223.Column number must be between 0 and 255

225." name "is not a variable

226." name" is not a file variable

227.Box dimensions are invalid

228.Tab stops must be in ascending order

230.Array dimensions are invalid

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