asked on

How To Open A File?

Hi there,

I am new to Delphi and hasn't been three weeks since I started learning it. I am currently involved in a program in which I have to open a file and read certain bytes out of it one at a time. Like first, 78 bytes and then 100 bytes and so on till the end of file.

But the problem is I don't know much about the syntaxes used to open files. I tried using help and used Assign method. But the problem is when I open the file and use the showmessage it only shows the first two characters and the rest are all blank. Like spaces. But they are not blank. They do contain characters.

Is it the way I am opening the file becuase someone said that I might have to open the file in binary mode or something to fully get al the characters exactly. So can you experts help me out in this situation...

The following is the code that I have been trying to use. And the type of file that I am reading is a file that comes from an equipment. Hope you guys can help me out and kind of explain the amendments made too.

unit StatFileDecoder;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, Buttons, OleServer, CmAdmCtl, NMUUE;

type
TMain = class(TForm)
cmd_Process: TBitBtn;
OD1: TOpenDialog;
procedure cmd_ProcessClick(Sender: TObject);
procedure FileProcess(FileName: String);

private
{ Private declarations }
public
{ Public declarations }
end;

var
Main: TMain;

implementation

{$R *.dfm}

procedure TMain.cmd_ProcessClick(Sender: TObject);
var
FileNumber: Integer;
begin
If OD1.Execute then
begin
with OD1.Files do
for FileNumber:= 0 to Count -1 do
FileProcess(Strings[FileNumber]);

end;
end;

procedure TMain.FileProcess(FileName: String);
var
File_Content: String;
File_Handle: TextFile;
begin
AssignFile(File_Handle,FileName);
Reset(File_Handle);
Readln(File_Handle,File_Content);
showmessage(Copy(File_Content,5,5));
CloseFile(File_Handle);
end;

end.

Stuart_Johnson

The way I'd do it is with FileStreams. You can open the file in a stream, then seek to the position you need, and read X characters. For instance:

procedure ReadFile(Filename: String; CharsToCopy: Integer; ReadPosition: Word);
var
FS: TFileStream;
Buff: Array of Char;
begin
FS := TFileStream.Create(Filename, fmOpenRead);
try
FS.Seek(ReadPosition, soFromBeginning);
SetLength(Buff, CharsToCopy);
FS.Read(Buff, CharsToCopy);
finally
FS.Free;
end;
end;

I haven't tested the above code, but it should work.

If you need any further help, please feel free to ask.

Stu.

snehanshu

warheat001,
Your code would work if the file is a text file. If it is a binary file, you would need to do one of the following:
1) Define an appropriate record and read it record-wise.
2) Read byte by byte

For (1), I need to know the exact file structure, and here's the solution for (2):
The code reads the entire file into a dynamic array of bytes, "MyContents".
You can do whatever processing you want with this array once its read.
Hope this helps. Let me know if you need any clarifications.
...Snehanshu

procedure TForm1.Button2Click(Sender: TObject);
Var
MyFile : File of Byte;
MyFileName, MyStr : String;
MyByte: Byte;
MyContents: Array of byte;
begin
//give your file name here or pass it as a parameter
MyFileName := 'c:\abc.txt';
setlength(MyContents, 0);
If FileExists(MyFileName) Then
Begin
try
Begin
AssignFile(MyFile, MyFileName);
Reset(MyFile);
MyStr := '';
While Not EOF(MyFile) Do
Begin
Read(MyFile, MyByte);
//Just to show some output
MyStr := MyStr + intToStr(MyByte)+',';

//Dynamically increment length of the array
setlength(MyContents, Length(MyContents)+1);
MyContents[High(MyContents)] := MyByte;

End;
CloseFile(MyFile);
ShowMessage(MyStr);
//write code to process MyContents here

//Free array after processing
setlength(MyContents, 0);
end
except on e:Exception do
begin
Raise e;
end;
end;

End
Else
ShowMessage('File does not exist');

end;

snehanshu

Here's a slightly modified code that shows how to process the array :)

...Shu

procedure TForm1.Button2Click(Sender: TObject);
Var
MyFile : File of Byte;
MyFileName, MyStr : String;
MyByte: Byte;
MyContents: Array of byte;
MyCtr: Integer;
begin
//give your file name here or pass it as a parameter
MyFileName := 'c:\abc.txt';
setlength(MyContents, 0);
If FileExists(MyFileName) Then
Begin
try
Begin
AssignFile(MyFile, MyFileName);
Reset(MyFile);
MyStr := '';
While Not EOF(MyFile) Do
Begin
Read(MyFile, MyByte);

//Dynamically increment length of the array
setlength(MyContents, Length(MyContents)+1);
MyContents[High(MyContents)] := MyByte;

End;
CloseFile(MyFile);

//write code to process MyContents here
//here's come sample code to show how to access the array
For MyCtr := low(MyContents) to High(MyContents) Do
MyStr := MyStr + IntToStr(MyContents[MyCtr])+',';

ShowMessage(MyStr);
//Free array after processing
setlength(MyContents, 0);
end
except on e:Exception do
begin
Raise e;
end;
end;

End
Else
ShowMessage('File does not exist');

end;

warheat001

ASKER

Thankyou very much Stuart Johnson for your quick reply.

But could you tell me how I can put that <FS.Read(Buff, CharsToCopy);> into a variable or how I can read it into a variable or put into a database. Just the putting part only.

Thankyou Very Much

Stuart_Johnson

Hi warheat001,

After testing the code I posted above, it appears it doesn't work with dynamic arrays. Unfortunately I've just run short of time and I can't code anything at the moment for you, but I will definately do something for you ASAP.

Sorry not to be able to help immediately.

Stuart.

BJThomsen

procedure ReadFromFile;
var
FileStream: TFileStream;
MyString: String;
begin
FileStream := TFileStream.Create(<filename>, fmOpenRead or fmShareDenyWrite);
try
SetLength(MyString,<length you want to read>);
FileStream1.Read(MyString[1], <length you want to read>);
finally
FileStream.Free;
end;
ShowMessage(MyString);
end;

ASKER CERTIFIED SOLUTION

swift99

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