Data File Hierarchy/Terminology

© 1999, by Que Education and Training, Chapter 9, pages 447-483 of Introduction to Computer Programming with Visual Basic 6: A Problem-Solving Approach

Data File Hierarchy/Terminology Folder or Directory

A container for 0 or more files accessible from the same location. File

A collection of related records stored as a unit on external media. Record or Row

A collection of related fields such as many pieces of information about a person or thing.

Field A single (meaningful) piece of information about a person or thing.

Byte Normally a collection of one or more characters that comprise a

field (character ~ byte) Bit

Commonly written as 0 or 1 as the smallest unit of computer data. Generally 8 bits = 1 byte, yielding 256 combinations--ASCII code


Data File Hierarchy Example Folder or Directory

For class files called Courses File

The file CPT250Grades.txt inside the Courses folder holds all class information for one course.

Record or Row Each record in the grades file contains Student Name, Student ID,

three test grades and ten project grades. Field

Student ID is comprised of the last 5 digits of the social security number.

Byte Each character of the ID requires one byte of storage.

Bit The character “2” of the student ID has the ASCII code 50, which is

written as 00110010, where each symbol is a bit.


Physical Details of Files Before you can write programs that use files,

you need to get answers to the following: Can records be accessed sequentially? Can records be accessed arbitrarily? Do all of the records have the same length? How are the fields ordered on each record? What is the data type of each field? Is the length of each field the same? How are the fields separated?


Different Types of Files Random-access data files

Files designed to facilitate future retrieval of individual records in an arbitrary order.

Retrieving a record requires determining its location Hashing Index (ISAM)

Thus, it is logically used like an array but data resides on disk

Binary data files A machine-readable format that can contain almost anything Unreadable if loaded in a word processor or text editor Program files

Special type of binary file that the OS can interpret with the right OCXs, DLLs, EXEs.


Different Types of Files Sequential data files

Data elements stored and retrieved one after another, in order

Examples Text files (Report-Record or Display-Formatted format)

– Simple documents such as README files created with ASCII text editor like Notepad

– Designed to be read, so there are no field delimiters– Carriage return and line feed mark the end of each line

Comma-separated value (CSV) files– Common means for transferring data across applications– Contains variable length records separated by commas– Certain types of data are surrounded by special characters– Carriage return and line feed mark the end of each line/record

Fixed-width data files– Commonly created from/for COBOL programs


General usage of Files1) Assign a file number

To refer to file in subsequent file access statements

2) Open the file Identify file name & path, purpose, reference number, and

record length if random file

3) Read/Write the file (possibly within a loop) Specify reference number, action, and data elements

Read retrieves data from the file into specified memory locations (variables)

Write copies data in specified memory locations (variables) to current location in file

4) Close the file Specify reference number to release the number and the file


Assigning a File Number Two choices

Use a literal constant (between 1-511) Use FreeFile function to have VB automatically assign an available reference

number Use caution when working with multiple files simultaneously

Incorrect

Dim InNbr As Integer

Dim OtNbr As Integer

InNbr = FreeFile ' Returns 1.

OtNbr = FreeFile ' Returns 1.

Open "Input.dat" For Input As #InNbr

Open "Output.dat" For Output As #OtNbr

Correct

Dim InNbr As Integer

Dim OtNbr As Integer

InNbr = FreeFile ' Returns 1.

Open "Input.dat" For Input As #InNbr

OtNbr = FreeFile ' Returns 2.

Open "Output.dat" For Output As #OtNbr


Opening a File Provide information about the file before any accessOpen pathname For mode {Access access} {lock} _

As #filenumber {Len = recordlength}

Elements of Open statement pathname: a string that defines the file name with full path mode: (purpose: input, output, append, random, binary)

Establishes record pointer at start of file for all modes but append which starts at end of file

access: specifies allowable operations (needed with random)

lock: specifies allowable operations by other processes (n/a) filenumber: reference number used in subsequent read/write

operations recordlength: number of bytes in a record of a random

access file or buffer size for sequential files


Reading from a File CSV file

Input #filenumber, VariableList Data read from current location of file pointer into

corresponding variables Each comma or end-of-line delimits each field, unless

surrounded in quotations Normally, records were created using the Write

statement


Reading from a File Report (display-formatted) file

Line Input #filenumber, VariableName Read the next line from current location of file pointer into

specified variable Normally, records were created using the Print statement

Fixed record-length fileGet #filenumber, {recordnumber}, VariableName

Read the next record from current location of file pointer into specified variable

Needed by fixed-record length files since there is no end-of-line character

Normally, records were created using the Put statement


Detecting End-of-File Use EOF function

Special function that detects when the end of file has been reached

EOF(filenumber) Use Data sentinel

Record that contains specific, fixed values, which, when read, signal the last record has been processed

Use unique first record that defines the number of records that follow Atypical approach that allows counting loop to be used to

read the fixed number of records

Error handler to detect read past end of file error


Writing to a File CSV file

Write #filenumber, VariableList Data in variable list written to current file location Comma inserted between fields, carriage-return and line

feed inserted at end of record, quotations placed around strings and # around Booleans and dates

Normally, records will be read using Input statement


Writing to a File Report (display-formatted) file

Print #filenumber, OutputList Print the output list to next line in file unless previous

Print ended in comma or semicolon– May contain expressions, Spc(n), Tab(n), separated by

comma or semicolon Normally, records will be read using Line Input statement

Fixed record-length filePut #filenumber, {recordnumber}, VariableName

Write the data in VariableName to current file location Normally, records will be retrieved using the Get

statement


Closing a File Notifies the operating system that you are

done with the fileClose {{#}filenumberlist}

Examples Close #EmpFile, #TaxFile Close

For output files, the buffers are flushed to insure that everything gets written to the file.

When reading the same file more than once in a single session, you must open and close each time.


‘ use static arrays and literal RefNbr

Open “file” For Input As #1

Input #1, EndVal

For Count = 1 To EndVal Step 1

Input #1, Array(Count)

‘ steps to process repeatedly

Next Count

Close #1

Reading Files with pre-data When pre-data is included at

start of data file or the program knows the number of records, use For/Next counting loop

Loop condition compares loop counter against the number of records read so far

False

True

Count > EndVal?

Set Count = 1

Count = Count + 1

Open file for input (#1)

Input #1, EndVal


Other steps to process in loop

Close #1


False

True

TempVar = Sentinel data

Set Count = 0

Count = Count + 1


Input #1, TempVar


Close #1

Input #1, TempVar

Array(Count) = TempVar

Reading Files with post-data dataPre-test loop to check for sentinel data



Input #1, TempVar

Count = 0

Do Until TempVar = SentinelData

Count = Count + 1

Array(Count) = TempVar


Input #1, TempVar

Loop

Close #1


Reading Files with no pre/post dataPre-conditional loop to check EOF



Count = 0

Do Until EOF(1)

Count = Count + 1



Loop

Close #1

**This is the most likely approach when files are used

False

True

EOF(1)

Set Count = 0

Count = Count + 1




Close #1


How does reading a CSV file work? Open statement

Record pointer set to start of file (before first record)

Each time an input statement is processed For each variable listed

Type of data in field must be compatible with the data type of the corresponding variable where it will be stored

Data at file’s record pointer is read and stored in corresponding, named field (variable)

Record pointer moves to the start of the next field (whether on same line or next line)

Close statement to release file to OS


VB Statements to Read CSV File' Use dynamic array and FreeFile function

Dim EmpCount As Integer

Dim CsvInFile As Integer

CsvInFile = FreeFile ' Returns 1.

Open “a:\sample.csv” For Input As #CsvInFile

Do Until EOF(CsvInFile)

EmpCount = UBound(fEmp) + 1

ReDim Preserve fEmp(0 To EmpCount) As EmpType

Input #CsvInFile, fEmp(EmpCount).ID, _

fEmp(EmpCount).Name, fEmp(EmpCount).Rate, _

fEmp(EmpCount).Hours

' optional code to process employee’s data

Loop

Close #CsvInFile


How does writing a CSV file work? Open statement

New file created in stated path with given name Output mode

Record pointer is set to the start of the file Append mode

Record pointer is set to the end of the file

Each time a write statement is processed For each variable listed

Data in memory (variable) is written at record pointer– Strings enclosed in double quotes & Boolean and dates

enclosed in #– Comma separator used unless last variable in list

Carriage return-line feed mark the end of the line



How does reading a report file work? Open statement

same as writing CSV file Each time a print statement is processed

Start on a new line unless the previous print ended in a comma “,” or semicolon “;”

For each expression listed Data is written at current file pointer

– Expressions separated by semicolons “;” keep file pointer where it left off

– Expressions separated by commas “,” move pointer to next print zone



VB Statements to Write CSV File' Use dynamic array and FreeFile function

Dim Index As Integer

Dim EmpCount As Integer

Dim CsvOutFile As Integer

CsvOutFile = FreeFile ' Returns 1.

Open “a:\sample.csv” For Output As #CsvOutFile

EmpCount = UBound(fEmp)

For Index = 1 To EmpCount Step 1

Write #CsvOutFile, fEmp(EmpCount).ID, _

fEmp(EmpCount).Name, fEmp(EmpCount).Rate, _

fEmp(EmpCount).Hours

Next Index

Close #CsvOutFile


Before using Sequential Files You must determine or plan its structure

Number of records To determine type of loop processing & size of array(s) needed

Number, order, and type of fields per record To determine the fields (generally on one line)

– if different types: for the user-defined array

– if same type: number of columns for 2-d array Pre-data

Generally the number of following records (counting loop) Post-data (Sentinel data)

If present, to flag when the last good data set has been reached If no pre-data or post-data, may use EOF function or write

error handler to determine when end of file reached

Data File Hierarchy/Terminology

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