ACCESS A SEQUENTIAL FILE

ACCESS A SEQUENTIAL FILE

Data stored in a sequential file can read in order and assigned to variables with the following steps:

1.      Choose a number from 1 through 255 to be the reference number of the file. This number is not necessary to be the same number that was used when the file was recorded.

2.      Execute the statement

OPEN filename FOR INPUT AS #n

Where n is the reference number. This procedure is referred to as opening a file for input. It establishes a communication line between the computer and the disk drive for reading data from the diskette.

3.      Read data from the file with the INPUT* statement. INPUT* statement assigns data from file to variable.

INPUT #n. var1, var2, ……

4.      After the desired items have been found or all the data has been read from the file, close the file with the statement CLOSE #n.

5.      Basic function EOF; it tells us if we have reached the end of a file. For example the condition EOF (n) will be true if the end of file n has been reached and false otherwise.

EXAMPLE: Write a program to display a table showing the ages in 1991 of the people in the sequential file YOB.DAT.

Solution

REM Process data from YOB.DAT file to find ages in 1991

CLS

OPEN “YOB.DAT” FOR INPUT AS #5

PRINT “NAME”, “Age in 1991”

DO WHILE NOT EOF (5)

REM Process the entire file

INPUT #5, name$, year

PRINT name$, 1991 – year

REM Display name and age in 1991

LOOP

CLOSE #5

END

[run]

Name                     Age in 1991

Barbra                   49

Ringo                    51

Sylvester                45

Johnny                   65

PROTECTING FILES

Many modern computer systems provide methods for protecting files against accidental and deliberate damage. Computer with multi users implement file permissions to control who may or may not modify, delete or create files and folders. A user may be given permission to modify a file or folder, but not to delete while some may be permitted to only read the contents and not permitted to modify or delete the content.

FILE INSECURITY

File security is a feature of your file system which controls which users can access which files, and places limitations on what users can do to files.

Effect of file insecurity

(a) Data loss: Computer users and many experts often loss data permanently destroyed, with no hope of recovery.

Causes of Data loss

Causes of Data Loss	Customer Perception
Human error	40%
Computer viruses	15%
Natural Disasters	3%
Hardware or System Problem	28%
Software Corruption or Program Problem	12%

   

(b) Overwriting

This is a process of writing a binary set of data on a memory. Data that has been overwritten is generally considered to be unrecovered.

FILE SECURITY METHODS

(i) BACK UP: This is refers to making copies of data so that these additional copies are used to restore the original after loss event. It is also a method of making copies of the file in a separate location so that they can be restored if something happen to the computer. This can be done by using removable media such as rewritable CD, memory card, flash etc.

(ii) VIRUS: This is a self – replicating program that copies itself and that can infect other programs by modifying them or their environment such that a call to an infected program implies a call to a virus.

ANTIVIRUS: This is software to protect your computer from viruses that may try to infect your computer or might have done so.

(iii) PASSWORD PROTECTION: It can prevent people accessing computer system, account files or parts of files by requiring a user to enter password.

(iv) STORAGE DEVICE LABELING: You should label your storage devices like floppies, CDs, DVDs, Pen drivers etc. So that you know what is exactly stored in them and so as not to accidentally delete of format them.

ADVANTAGES OF COMPUTERIZED FILES

(i) Computer can form calculations quickly and efficiently.

(ii) Data can be retrieved quickly and easily.

(iii) Documents that are lost can often be retrieved.

(iv) Security is tight and hard to break into.

(v) Makes work easier.

(vi) Quicker to find things and sort things.

(vii) Transactions, accounts can be handled more properly by computers than manually.

LIMITATIONS OF COMPUTERISED FILES

(i) Computerized filing system is expensive to set up.

(ii) Not effective where there is irregular electric supply.

(iii) Skilled labor with proficiency in computers is required.