Records

The data that is stored in a file is frequently organized in records.

A record is a complete set of data about an item, and a field is an individual piece of data within a record.

When data is written to a file, it is often organized into records and fields.

A record is a complete set of data that describes one item, and a field is a single piece of data within a record.

Record Types

Allows us to combine data of different types to obtain a single complex type that packages diverse subelements into a single (compound) value

Managing Records

Applications that store records in a file typically require more capabilities than simply writing and reading records.

Component Variable

Component of Records Variable item.name

Records from file to array Reading arrays of records from a file into an array // Define the record, and array of records // Open the file // priming read // Loop until EOF or MaxSize of array: input data into record array // Close file

Constant Integer MAX_EMP = 10 Record Employee String name // spelling/syntax of field names is same as variables Integer id Real rate, hours End Record Declare Employee emp, emps[MAX_EMP] Declare InputFile inFile Declare Integer index = 0 Open inFile "records.txt" Read inFile emp While NOT eof(inFile) AND index < MAX_EMP emps[index] = emp Display "name: " Display emp.name Display "id: " Display emp.id Display "rate: " Display emp.rate Display "hours: " Display emp.hours Read inFile emp index = index + 1 // determines next location in array End While // If not end of file, there are still more to read in If NOT eof(inFile) Then Display "Ooops, there are still records left, not read in to array" End If Close inFil

// Variables for the fields Declare String name Declare Integer idNumber Declare String department // Declare an input file. Declare InputFile employeeFile // Open a file named employees.dat. Open employeeFile "employees.dat

Display "Here are the employee records." Read employeeFile name, idNumber, department // Display the records in the file. While NOT eof(employeeFile) // Read a record from the file. Read employeeFile name, idNumber, department // Display the record. Display "Name: ", name Display "ID Number: ", idNumber Display "Department: ", department // Display a blank line. Display End While // Close the file. Close employeeFile

Field Identifier

Each field has a name of identifier chosen by the programmer when the record type is declared

// Variables for the fields Declare String name Declare Integer idNumber Declare String department // A variable for the number of employee records. Declare Integer numEmployees // A counter variable for the loop Declare Integer counter // Declare an output file. Declare OutputFile employeeFile // Get the number of employees. Display "How many employee records do ", "you want to create?" Input numEmployees // Open a file named employees.dat. Open employeeFile "employees.dat" // Get each employee's data and write it // to the file

For counter = 1 To numEmployees // Get the employee name. Display "Enter the name of employee #", counter Input name // Get the employee ID number. Display "Enter the employee's ID number." Input idNumber // Get the employee's department. Display "Enter the employee's department." Input department // Write the record to the file. Write employeeFile name, idNumber, department // Display a blank line. Display End For // Close the file. Close employeeFile Display "Employee records written to employees.dat."

In pseudocode, we will write an entire record using a single Write statement.

For example, assume the variables name, idNumber, and department contain data about an employee, and employeeFile is the file we will write the data to.

Input from user to record array Reading data for multiple records from user into array // Define the record // Define an array of that record // Loop: prompt and input data into the next record element // Can also later output the data from the arrays record Constant Integer MAX_EMP = 10 Record Employee String name Integer id Real rate, hours End Record Declare Employee emps[MAX_EMP] Declare Integer index

For index = 0 to MAX_EMP - 1 Display "Enter Employee info: " Display "name: " Input emps[index].name Display "id: " Input emps[index].id Display "rate: " Input emps[index].rate Display "hours: " Input emps[index].hours End For // Now output data: Display "which record do you want?" Input index Display "Employee info for index " index Display "name: " Display emps[index].name Display "id: " Display emps[index].id Display "rate: " Display emps[index].rate Display "hours: " Display emps[index].hours

In the case of an array the name is an index. The list must all be the same type

In the case of records the name is a field identifier. The component value of a records maybe a different type

We can write the contents of these variables to the file with the following statement: Write employeeFile name, idNumber, department

In the statement we have simply listed the variables, separated by commas, after the file's internal name.

Read/Write records from/to file Outputting/inputting single records to files Define the record Prompt and input data into the record Open the file Output the single record to the file and close it Open the file, then read the single record. Record Employee String name Integer id Real rate, hours End Record // Declare two employees Declare Employee emp1, emp2

Initialize a record: Set emp.name = "Joe" Set emp2.name = "Mertha" Set emp.id = 4 Set emp.rate = 10.98 Set emp.hours = 40 Write employOut emp Close employOut Open employIn "emp.dat" Read employIn emp2 Display "Employee info: " emp2.name emp2.id emp2.rate emp2.hours

Set Types

Let us combine items of the same data type, but, unlike arrays, they do not order items in any way

Simple record types are seldom used by themselves

More often records are grouped into larger units such as arrays of records

Deleting Records 1 // Variables for the fields 2 Declare String description 3 Declare Real quantity 4 5 // A variable to hold the search value. 6 Declare String searchValue 7 8 // Declare an input file. 9 Declare InputFile coffeeFile 10 11 // Declare an output file to copy the original 12 // file to. 13 Declare OutputFile tempFile 14 15 // Open the files. 16 Open coffeeFile "coffee.dat" 17 Open tempFile "temp.dat" 18 19 // Get the value to search for. 20 Display "Enter the coffee you wish to delete." 21 Input searchValue

Read coffeeFile description, quantity 23 While NOT eof(coffeeFile) 24 // Read a record from the file. 25 Read coffeeFile description, quantity 26 27 // If this is not the record to delete, then 28 // write it to the temporary file. 29 If description != searchValue Then 30 Write tempFile description, quantity 31 End If 32 End While 33 34 // Close the two files. 35 Close coffeeFile 36 Close tempFile 37 38 // Delete the original file. 39 Delete "coffee.dat" 40 41 // Rename the temporary file. 42 Rename "temp.dat", "coffee.dat" 43 44 Display "The file has been updated."

Searching for a Record // Variables for the fields Declare String description Declare Real quantity // A variable to hold the search value. Declare String searchValue // A Flag to indicate whether the value was found. Declare Boolean found = False // Declare an input file. Declare InputFile coffeeFile // Get the value to search for. Display "Enter a value to search for." Input searchValue // Open the file. Open coffeeFile "coffee.dat"

Read coffeeFile description, quantity While NOT eof(coffeeFile) // Read a record from the file. Read coffeeFile description, quantity // If the record contains the search value, // then display it. If contains(description, searchValue) Then // Display the record. Display "Description: ", description, "Quantity: ", quantity, " pounds" // Set the found flag to true. Set found = True End If End While // If the value was not found in the file, // display a message indicating so. If NOT found Then Display searchValue, " was not found." End If // Close the file. Close coffeeFile

Modifying Records // Variables for the fields Declare String description Declare Real quantity // A variable to hold the search value. Declare String searchValue // A variable to hold the new quantity. Declare Real newQuantity // A Flag to indicate whether the value was found. Declare Boolean found = False // Declare an input file. Declare InputFile coffeeFile // Declare an output file to copy the original // file to. Declare OutputFile tempFile // Open the original file. Open coffeeFile "coffee.dat" // Open the temporary file. Open tempFile "temp.dat" // Get the value to search for. Display "Enter the coffee you wish to update." Input searchValue // Get the new quantity. Display "Enter the new quantity." Input newQuantity

Read coffeeFile description, quantity While NOT eof(coffeeFile) // Read a record from the file. Read coffeeFile description, quantity // Write either this record to the temporary // file, or the new record if this is the // one that is to be changed. If description == searchValue Then Write tempFile description, newQuantity Set found = True Else Write tempFile description, quantity End If End While // Close the original file. Close coffeeFile // Close the temporary file. Close tempFile // Delete the original file. Delete "coffee.dat" // Rename the temporary file. Rename "temp.dat", "coffee.dat" // Indicate whether the operation was successful. If found Then Display "The record was updated." Else Display searchValue, " was not found in the file." End If

Reading arrays of records from a file // Define the record // Open the file // priming read // Loop until EOF: input data into record and display // Close file

Record Employee String name // spelling/syntax of field names is same as variables Integer id Real rate, hours End Record Declare Employee emp Declare InputFile inFile Open inFile "records.txt" Read inFile emp While NOT eof(inFile) Display "name: " Display emp.name Display "id: " Display emp.id Display "rate: " Display emp.rate Display "hours: " Display emp.hours Read inFile emp End While Close inFile

Records

Structured data type which is conceptually a unit, the components are different data types

In pseudocode we will read an entire record from a file using a single Read statement.

The following statement shows how we can read three values from employeeFile into the name, idNumber, and department variables: Read employeeFile name, idNumber, department

Component Fields

The individual components of a records

A value of a record type is composed of a group of values of component types specified in the declaration of that record type

This group can be sometimes can be treated as a single structured value

Your next job is to design a program that displays all of the records in the inventory file. // Variables for the fields Declare String description Declare Real quantity // Declare an input file. Declare InputFile coffeeFile // Open the file. Open coffeeFile "coffee.dat"

While NOT eof(coffeeFile) // Read a record from the file. Read coffeeFile description, quantity // Display the record. Display "Description: ", description, "Quantity: ", quantity, " pounds" End While // Close the file. Close coffeeFile

// Variables for the fields Declare String description Declare Real quantity // A variable to control the loop. Declare String another = "Y" // Declare an output file in append mode. Declare OutputFile AppendMode coffeeFile // Open the file. Open coffeeFile "coffee.dat"

While toUpper(another) == "Y" // Get the description. Display "Enter the description." Input description // Get the quantity on hand. Display "Enter the quantity on hand ", "(in pounds)." Input quantity // Append the record to the file. Write coffeeFile description, quantity // Determine whether the user wants to enter // another record. Display "Do you want to enter another record? ", Display "(Enter Y for yes, or anything else for no.)" Input another // Display a blank line. Display End While // Close the file. Close coffeeFile Display "Data appended to coffee.dat."

A file specification document

describes the fields that are stored in a particular file, including their data types

When records are created in a file,

some systems write one type of delimiter after each field and another type of delimiter after each record.