how to find my error(delphi)

Question

i'm getting an I/o 998 error, my task is to rewrite numbers from file to array, and find max and min values. What i'm doing wrong ?

implementation

var
  f2: file of Real;
  m: array of Real;

procedure TForm1.Button1Click(Sender: TObject);
var 
  f: Real;
  max, min: Real;
  i, j: Integer;
begin
  AssignFile(F2, 'test3.dat');
  Rewrite(f2);

  for i := 1 to 50 do
  begin
    f := RandomRange(-100, 100);
    Randomize;
    Write(f2, f);
  end;

  CloseFile(f2);

  i := 0;

  Reset(f2);

  while not Eof(f2) do
  begin
    SetLength(m, i);
    Read(f2, m[i]);
    Inc(i);
  end;

  CloseFile(f2);

  max := m[1];
  min := m[1];

  for j := 1 to i do
    if m[j] > max then
      max := m[j]
    else 
    if m[j] < min then
      min := m[i];

Not posting all your code. Calling Randomise every time round the loop will destroy your randomness. Pascal I/O? Why? Out of bounds array access cannot help. Incorrect indexing on min/Max loop also bad news. Frankly, this code is a disaster. — David Heffernan, Dec 18 '13 at 17:49
You should initialize i := 1 and not i:=0 otherwise when you do Setlength(m, i) you will always have an array 1 element smaller than what you need. — Giacomo Degli Esposti, Dec 18 '13 at 17:50
possible duplicate of [block read error](http://stackoverflow.com/questions/9493522/block-read-error) — Im0rtality, Dec 18 '13 at 17:52
On the very first iteration you are specifically unsetting your dynarray by doing `SetLength(m, 0)` (equivalent to `m := nil {!!}`). This is the cause of your I/O error. And ignore complains about standard Pascal I/O as nonsense. — Free Consulting, Dec 18 '13 at 19:49
What is "Real" type here ? Is it "double" or "Real48" ? And what does the file consist of, "Real48" or "double" ? In Delphi there si nop real "real" type, only a stub, whic hcan be connected to different actual types. — Arioch 'The, Dec 19 '13 at 10:03

LU RD · Answer 1 · 2013-12-18T18:24:14.647

2

Many errors, see comments in code.

Randomize should be called once at program start.
Dynamic arrays has start index 0.
CloseFile releases file handle
Define length of the dynamic array before the loop, otherwise you will get i/O error.
High(m) will get the max index of the dynamic array.
Index variable for assigning the min value is j.

implementation

var
  f2: file of Real;
  m: array of Real;

procedure TForm1.Button1Click(Sender: TObject);
var 
  f: Real;
  max, min: Real;
  i, j: Integer;
begin
  AssignFile(F2, 'test3.dat');
  Rewrite(f2);

  for i := 1 to 50 do
  begin
    f := RandomRange(-100, 100);
    //Randomize;  <-- Call this once at program start
    Write(f2, f);
  end;

  //CloseFile(f2); <-- Don't close yet.

  Reset(f2);
  SetLength(m, 50);  // <-- Define length of dynamic array
  i := 0;
  while not Eof(f2) do
  begin
    // SetLength(m, i); // <-- Moved to before while loop, or use SetLength(m,i+1);
    Read(f2, m[i]);
    Inc(i);
  end;

  CloseFile(f2);

  max := m[0];  // <-- Dynamic arrays start with index 0
  min := m[0];  // <-- Dynamic arrays start with index 0

  for j := 1 to High(m) do // <- Max index
    if m[j] > max then
      max := m[j]
    else 
    if m[j] < min then
      min := m[j]; // <-- j is correct index variable

edited Dec 18 '13 at 18:24

answered Dec 18 '13 at 18:01

LU RD

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`CloseFile` is not an error at all, just a redundant call. `CloseFile` releases previously acquired write-only handle. If there would no `CloseFile`, subsequent `Reset` would do the same before acquiring read-write handle. – Free Consulting Dec 18 '13 at 20:29
@FreeConsulting, documentation says that `CloseFile` terminates the association between the file variable and the disk file. Furthermore: `The external file associated with F is completely updated and then closed, freeing the file handle for reuse.` So the fact that `Reset()` internally closes the file and swaps to read-write mode is considered implementation detail and best practice here is to play by the rules given by the documentation. – LU RD Dec 18 '13 at 23:21
Then the most correct way (to ensure what external file is completely updated) would be to bring [`CloseFile`](http://www.freepascal.org/docs-html/rtl/system/close.html) call back and not to rely in described `Reset` behaviour, right? – Free Consulting Dec 18 '13 at 23:55
@FreeConsulting, certainly not. Why rely on a file variable that has been detached from the file? Then you need to call `AssignFile()` again according to the documented behavior. – LU RD Dec 19 '13 at 07:43
Exactly. To avoid all of implementation specific stuff, you need to `Close` (completely update and tear down W/O handle association and name association) and then `Assign` back (bind filename) and finally `Reset` (acquire R/W handle). – Free Consulting Dec 19 '13 at 08:09
@FreeConsulting, then I don't understand you at all. It is perfectly legal to call `Reset()` on an open file. – LU RD Dec 19 '13 at 08:36
Its perfect legal to call `Reset` on `Close`'d file, too. In Delphi implementation `Reset`/`Rewrite` does an implicit `Close` as well as `Close` call does not forget filename. – Free Consulting Dec 19 '13 at 08:51
@FreeConsulting, show where this is documented. The file association is lost after a call to `CloseFile()`. Abusing the fact that `File` is a record that may contain valid information after a call to `CloseFile()` is not a recommended practice. – LU RD Dec 19 '13 at 09:07
I posted [fpc doc link](http://www.freepascal.org/docs-html/rtl/system/close.html) above. Delphi behaves the same way. So, `CloseFile` call is no more than redundant statement. – Free Consulting Dec 19 '13 at 09:30

Sertac Akyuz · Answer 2 · 2013-12-18T18:26:50.657

2

  i := 0;

  Reset(f2);

  while not Eof(f2) do
  begin
    SetLength(m, i);
    Read(f2, m[i]);
    Inc(i);
  end;

The above code sets the length of a dynamic array to 0 (i) and tries to read into its non-existing element. This causes the RTL to pass an invalid buffer to ReadFile api. The OS returns '0' indicating the function failed and sets the last error to '998' - that's ERROR_NOACCESS. RTL sets the in/out error code and raises it.

As for the answer, use the debugger. Break when the debugger raises an exception. On the next run, put a breakpoint on the faulting statement then trace into code (RTL in this case). Additionally, should you have 'range checking' on in compiler options, you'd get a range check error instead of an I/O error, in which case you would probably see the mistake quickly.

edited Dec 18 '13 at 18:26

answered Dec 18 '13 at 18:10

Sertac Akyuz

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what is the problem of `i+1`? – ALZ Dec 18 '13 at 18:45
@ALZ - I don't understand your question. – Sertac Akyuz Dec 18 '13 at 18:46
@SertacAkyuz, in previous comments (removed already) You wrote to @TLama that `i+1` is self-destructing comment - or I understood it wrongly. IMHO here really should be i+1, since 1st time `i:=0` and You set length to 0, then try to red into that array (0-length) – ALZ Dec 18 '13 at 18:49
@ALZ - You got it wrong. I asked TLama to not to remove his comment as I intended to remain in the boundary of the question. I also voted for his comment. – Sertac Akyuz Dec 18 '13 at 18:53
@SertacAkyuz, OK, sorry for misunderstanding. Thanks – ALZ Dec 19 '13 at 08:30

how to find my error(delphi)

2 Answers2