my homework (on crack)

**misplaced** · 10-11-2004

well, i was assigned my first project....but it's rather dull, so i decided to go WAY overboard on it.....

i created a random access file to store events for a calendar program..events contains the date and a description of it...

i'll try to post a little code as i can....i'm not asking you to find the bug...am i asking how i would find the bug...the biggest problem i'm facing is that i can't read the file (as a human)....

the output and description of the error is below the code.

Code:

int FileManager::addRecord(Record &srcRec)
{
    srand(56);     //really out of place, just haven't decided where
                         //to put it yet..i should probably use the time too
    
    srcRec.setKey(uniqueKey());
    int openRecord = firstAvailable();
    ofstream fout(filename, ios::binary);
    
    fout.seekp(openRecord * sizeof(Record));
    fout.write(reinterpret_cast<const char *>(&srcRec),
                sizeof(Record));
    
    fout.close();    
}   


// i'm not planning on keeping this function, it's just for testing
void FileManager::print(void)
{
   
    ifstream fin;
    fin.open(filename, ios::binary);
    while(fin && !fin.eof())
    {
        fin.read(reinterpret_cast<char *>(&cur_rec),
                   sizeof(Record));
        cout << endl << endl;
        cout << "Key: " << cur_rec.getKey() << endl;
        cout << "Date: " << cur_rec.getMonth()<<"/"<<
                        cur_rec.getDay()<<"/"<< cur_rec.getYear() << endl;
        cout << "Label: " << cur_rec.getLabel() << endl;
        cout << "Desc: " << cur_rec.getDesc() << endl;
        
    }    
    fin.close();
}

Code:

the only record i added was the one with the key 221....
and for some reason it prints twice...i can't tell if this
is a looping error in the print, or in the write...

Key: 0
Date: 0/0/0
Label:
Desc:


Key: 221
Date: 1/13/1999
Label: xxxx
Desc: nnnn


Key: 221
Date: 1/13/1999
Label: xxxx
Desc: nnnn

**jlou** · 10-11-2004

Don't use eof() to control your loop in that way. The eof() returns true after you attempt to read past the end of the file. The check for eof() should therefore be immediately after the attempt to read.

**misplaced** · 10-11-2004

it's a fixed-size file. the file is sizeof(Record) * numberOfEntries...granted the file is not corrupt, reading it that way will always take me right to the end and eof() becomes true.

say it has 4 entries, 100 bytes a piece...file is 400 bytes....
loop 1: not eof() { read 0-100 bytes; //cursor is at 100 }
loop 2: not eof() { read 100-200 //cursor is at 200 }
loop 3: not eof() { read 200-300 //cursor is at 300 }
loop 4: not eof() { read 300-400 //cursor is at 400 (eof) }
loop 5: eof() is true;

am i wrong?

**misplaced** · 10-11-2004

oh.....i am right....but you're saying that i WANT to try to read passed eof()???

**elad** · 10-11-2004

loop 4: not EOF { read 300-400 //cursor is at 400 (eof) }
loop 5: at but not past EOF so rest of loop input is undefined
loop 6: past EOF so eof() is true;

**misplaced** · 10-11-2004

well i'll be damned.....

sorry...i guess i should have read the FAQ....

i just disregarded anything like that because i figured trying to read past the end of file would cause a segmentation error or something....

so before i go changing 5 or 6 different (mostly) working functions, would you guys agree that if i read once before the loop it would solve my problem?

(that and putting the 'loop read' after the printing)

**jlou** · 10-11-2004

No, that wouldn't solve the problem by itself. To solve the problem you must run the check for eof() immediately after the call to read. There are a bunch of different ways to do that that may or may not include reading once before the loop.

**Hunter2** · 10-11-2004

>>To solve the problem you must run the check for eof() immediately after the call to read.
My favourite method goes as follows:

Code:

read some stuff();
while(!file.eof())
{
process input();
read some stuff();
}

It may not look very intuitive (because processing goes before reading in the loop), but it works and doesn't require a break or extra test condition.

**EDIT**
Just make sure that you're not doing any processing in between reads.. otherwise you'll still need the check-and-break method.

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