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Dec 18th, 2006

What errors can occur while reading a file?

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I want to read a file which contains a list of integers. The first 4 bytes( or whatever sizeof(int) is ) indicates how many numbers the file has.
Suppose I check the file size before reading so that I know I won't encounter EOF. Do I still need to check for file read errors? Is yes, why and what would be the correct way to do it (i.e. check after each read or once at the end )

C Syntax (Toggle Plain Text)
 
istream fin;
fin = open( "input.num", ios::in );
if( !fin )
... // Error
 
fin.seekg( 0, ios::end )	// Find file size
fileSize = fin.tellg( );
 
if( fileSize < sizeof( int ))	// Need atleast one int
...// Error	
 
fin.seekg( 0, ios::beg );
 
fin<<count;	// First int tells the number of ints in the file
// if( fin.bad( ))         // Check here too?
 
if( fileSize != (count+1)*sizeof(int)) // Verify file size
...// Error
 
for( i = 0; i < count; i++ )
{
	fin<<numbers[i];
//	if( fin.bad( ))         // Or shoud I use fin.fail( )? It wont be EOF
//		           // Should this be checked each time?
	...// Error		
}
 
// if( fin.fail( ))	// Or check error only once after reading?
	... // Error
 
 	...
istream fin;
fin = open( "input.num", ios::in );
if( !fin )
... // Error

fin.seekg( 0, ios::end )	// Find file size
fileSize = fin.tellg( );

if( fileSize < sizeof( int ))	// Need atleast one int
...// Error	

fin.seekg( 0, ios::beg );

fin<<count;	// First int tells the number of ints in the file
// if( fin.bad( ))         // Check here too?

if( fileSize != (count+1)*sizeof(int)) // Verify file size
...// Error
	
for( i = 0; i < count; i++ )
{
	fin<<numbers[i];
//	if( fin.bad( ))         // Or shoud I use fin.fail( )? It wont be EOF
//		           // Should this be checked each time?
	...// Error		
}

// if( fin.fail( ))	// Or check error only once after reading?
	... // Error

 	...

Does the same thing apply to writing to a file?

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vicky_dev

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Dec 18th, 2006

Re: What errors can occur while reading a file?

C Syntax (Toggle Plain Text)
fin.seekg( 0, ios::end )	// Find file size
fileSize = fin.tellg( );
 
if( fileSize < sizeof( int ))	// Need atleast one in
fin.seekg( 0, ios::end )	// Find file size
fileSize = fin.tellg( );

if( fileSize < sizeof( int ))	// Need atleast one in

Is the file a binary file or text file? If you view it in Notepad or some other text editor are the numbers readable? If they are, then more than likely it is a text file which means the test you are using to see if the file contains an integer will not work. Why? because the number '1' is only one byte long, not four bytes. Use Notepad or some other text editor to write the number '1' to a file then check the file size with Explorer or some other command-line program (depending on the os you are using). The file size will be from 1 to 3 depending on whether it contains the CR, or CR/LF line terminators.

>>fin<<count; // First int tells the number of ints in the file
this is wrong. should be
fin >> count;

And the extraction operator >> is for text files, not binary files. If the file is truely binary then use this
fin.read((char *)&count, sizeof(int));

>>fin<<numbers[i];
again, you used the wrong operator
fin >> numbers[i];

C Syntax (Toggle Plain Text)
for( i = 0; i < count; i++ )
{
	fin<<numbers[i];
//	if( fin.bad( ))         // Or shoud I use fin.fail( )? It wont be EOF
//		           // Should this be checked each time?
	...// Error		
}
for( i = 0; i < count; i++ )
{
	fin<<numbers[i];
//	if( fin.bad( ))         // Or shoud I use fin.fail( )? It wont be EOF
//		           // Should this be checked each time?
	...// Error		
}

this is how I would have coded it

C Syntax (Toggle Plain Text)
i = 0;
while( i < count && fin >> numbers[i] )
{
   ++i;
}
i = 0;
while( i < count && fin >> numbers[i] )
{
   ++i;
}

Last edited by Ancient Dragon; Dec 18th, 2006 at 12:26 pm.

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Dec 19th, 2006

Re: What errors can occur while reading a file?

>> Is the file a binary file or text file?

It is a text file, because I am creating the file in some other program.

>>this is how I would have coded it

C Syntax (Toggle Plain Text)
i = 0;
while( i < count && fin >> numbers[i] )
{
   ++i;
}
i = 0;
while( i < count && fin >> numbers[i] )
{
   ++i;
}

Then how do you check if the file was read correctly, i.e. if all the numbers were read correctly or not?

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Dec 19th, 2006

Re: What errors can occur while reading a file?

Quote originally posted by vicky_dev ...

The first 4 bytes( or whatever sizeof(int) is ) indicates how many numbers the file has.

Quote originally posted by vicky_dev ...

Then how do you check if the file was read correctly, i.e. if all the numbers were read correctly or not?

You could read the first integer into a variabel (for example x), then jump in the loop which you already have and when you reach EOF compare 'i ' with 'x'. If the numbers match, the file was read correctly.

Regards Niek

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Dec 19th, 2006

Re: What errors can occur while reading a file?

But yiu have to put the integer values out ther in the file in such a way that you can guarantee a read, ie, put it on a separate line with demarcation so you can see it is not part of the list (in case you forgot to write it in the first place, for example):

C Syntax (Toggle Plain Text)
!5
1
2
3
4
5
!5
1
2
3
4
5

I just picked an "!" at random. If humans are going to read the file as well, try something like "linecount:5" so it doesn't confuse readers.

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Dec 19th, 2006

Re: What errors can occur while reading a file?

Click to Expand / Collapse Quote originally posted by vicky_dev ...

Then how do you check if the file was read correctly, i.e. if all the numbers were read correctly or not?

fin will fail and return null if it encounters an error or on end-of-file.

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