Delphi Basics : Read command

DelphiBasics

Read
Procedure

Read data from a binary or text file

System unit

1	procedure Read(var Value1 {,var Value2 ...};);
2	procedure Read (var FileHandle TextFile; var Value1 {,var Value2 ...};;
3	procedure Read (var FileHandle File; var Value1 {,var Value2 ...};;

Description

The Read procedure reads a single line of data from a file or the console.

Version 1

Is used to read text values from the console.

Version 2

Is used to read text values from a text file with the given FileHandle.

Version 3

Is used to read data from a binary file with the given FileHandle.

You must use AssignFile to assign a file to the FileHandle and open the file with Reset before using Read.

For text files, each line of text is parsed into the given variables. These variables may be text or number types.

For strings, the whole line is read, unless it exceeds the string variable capacity - only that amount of text that fits is passed.

When parsing for numbers, white space characters and line ends are seen as separator values. If a number value exceeds the capacity of the variable, it is cast to the variable without raising an exception.

When reading for strings or characters, a ReadLn must be performed when Eoln (end of the line) is reached. Otherwise, subsequent Read calls repeatedly return a null value.

For binary files, data values Value1, Value2 etc, are read from the data the file. If the file given by FileHandle is a typed file (one defined to contained defined records), then these values must be of the same type (record).

Notes

You cannot use Read to read from an untyped binary file (one declared as File with no following of type).

To read from an untyped binary file, use BlockRead.

Read does not use buffering - BlockRead is more efficient.

Read is also a Delphi directive. It is used with the Property keyword.

Related commands

AssignFile		Assigns a file handle to a binary or text file
CloseFile		Closes an open file
Eof		Returns true if a file opened with Reset is at the end
Eoln		Returns true if the current text file is pointing at a line end
File		Defines a typed or untyped file
ReadLn		Read a complete line of data from a text file
Reset		Open a text file for reading, or binary file for read/write
TextFile		Declares a file type for storing lines of text
Write		Write data to a binary or text file
WriteLn		Write a complete line of data to a text file

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Example code : Reading single characters at a time from a text file

var
  myFile : TextFile;
  letter : char;
  text   : string;

begin
  // Try to open the Test.txt file for writing to
  AssignFile(myFile, 'Test.txt');
  ReWrite(myFile);

  // Write lines of text to the file
  WriteLn(myFile, 'Hello');
  WriteLn(myFile, 'To you');

  // Close the file
  CloseFile(myFile);

  // Reopen the file for reading only
  FileMode := fmOpenRead;
  Reset(myFile);

  // Display the file contents
  while not Eof(myFile) do
  begin
    // Proces one line at a time
    ShowMessage('Start of a new line :');
    while not Eoln(myFile) do
    begin
      Read(myFile, letter);  // Read and display one letter at a time
      ShowMessage(letter);
    end;
    ReadLn(myFile, text);
  end;

  // Close the file for the last time
  CloseFile(myFile);
end;

Show full unit code

  Start of a new line :
  H
  e
  l
  l
  o
  Start of a new line :
  T
  o

  y
  o
  u

Example code : Reading word data from a typed binary file

var
  myWord, myWord1, myWord2 : Word;
  myFile : File of Word;

begin
  // Try to open the Test.cus binary file for writing to
  AssignFile(myFile, 'Test.cus');
  ReWrite(myFile);

  // Write a couple of lines of Word data to the file
  myWord1 := 234;
  myWord2 := 567;
  Write(myFile, myWord1, myWord2);

  // Close the file
  CloseFile(myFile);

  // Reopen the file in read only mode
  Reset(myFile);

  // Display the file contents
  while not Eof(myFile) do
  begin
    Read(myFile, myWord);
    ShowMessage(IntToStr(myWord));
  end;

  // Close the file for the last time
  CloseFile(myFile);
end;

Show full unit code

234
567