Delphi Basics : ReadLn command

DelphiBasics

ReadLn
Procedure

Read a complete line of data from a text file

System unit

1	procedure ReadLn(var Variable1 {,var Variable2...});
2	procedure ReadLn (var FileHandle TextFile; var Variable1 {,var Variable2...};

Description

The ReadLn procedure reads a complete line of data from a text file or to the console.

Version 1

Is used to read from the console.

Version 2

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

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

The current file text line is parsed into the given variable values.

After parsing, any remaining text in the current file line is ignored, and the file positioned to the next line (or Eof set true if none left).

Notes

ReadLn does not buffer records, so performance is degraded. BlockRead is more efficient, but is geared to writing to binary files.

Related commands

AssignFile		Assigns a file handle to a binary or text file
BlockRead		Reads a block of data records from an untyped binary file
BlockWrite		Writes a block of data records to an untyped binary file
Eof		Returns true if a file opened with Reset is at the end
File		Defines a typed or untyped file
Read		Read data from a binary or text 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

Download this web site as a Windows program.

Example code : Reading each text line as a set of 4 numbers

var
  myFile : TextFile;
  n1, n2, n3 : Integer;

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

  // Write numbers in a string
  WriteLn(myFile, '1 2 3 4');  // Note that the 4 will be ignored

  // Write numbers as separate parameters
  WriteLn(myFile, 5, ' ', 6, ' ', 7);

  // Write numbers as separate parameters
  WriteLn(myFile, '8 9');      // Missing numbers will be seen as 0

  // Close the file
  CloseFile(myFile);

  // Reopen the file for reading
  Reset(myFile);

  // Display the file contents
  while not Eof(myFile) do
  begin
    ReadLn(myFile, n1, n2, n3);
    ShowMessage(IntToStr(n1)+' '+
                IntToStr(n2)+' '+
                IntToStr(n3));
  end;

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

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