End of File in C++

Question

I have a n X 2 matrix stored in a text file as it is. I try to read it in C++

nb_try=0; fin>>c_tmp>>gamma_tmp; while (!fin.eof( )) //if not at end of file, continue reading numbers { // store cs_bit.push_back(c_tmp); gammas_bit.push_back(gamma_tmp); nb_try++; // read fin>>c_tmp; assert(!fin.fail( )); // fail at the nb_try=n if(fin.eof( ))break; fin>>gamma_tmp; // get first number from the file (priming the input statement) assert(!fin.fail( )); }

The first assert failes, i.e. fin.fail( ) is true, when nb_try==n, which happens when it tries to read the first number which does not exist. But how come fin.eof( ) is not true after reading the last number? Does it mean it becomes true only when reading the first number that does ot exist? Also is it true that fin.fail( ) and fin.eof( ) are becoming true at the same time?

Thanks and regards!

CS Student · Accepted Answer · 2024-07-09 22:07:09Z

This is the wrong way to read a file:

while (!fin.eof( )) { // readLine; // Do Stuff }

The standard pattern is:

while(getlineOrValues) { // Do Stuff }

So looking at your code quickly I think it would be easier to write it as:

while(fin>>c_tmp>>gamma_tmp) { // loop only entered if both c_tmp AND gamma_tmp // can be retrieved from the file. cs_bit.push_back(c_tmp); gammas_bit.push_back(gamma_tmp); nb_try++; }

The problem is that EOF is only true AFTER you try and read past it. Having no character left in the file to read is not the same as EOF being true. So you read the last line and get values and there is nothing left to read, but EOF is still false so the code re-enter the loop. When it tries and reads the c_tmp then EOF gets triggered and your asserts go pear shaped.

The solution is to put the read as the while condition. The result of doing the read is the stream. But when a stream is used in a boolean context (such as a while condition) it is converted into a type that can be used like a bool (Technically it is a void* but thats not important).

EOF is true when you try and read past the EOF. Reading upto the end of file does not change the state. So EOF becomes true when you read the first value that does not exist past the end of file.
ifstream::fail() doesn't imply eof, see here for an explanation of the state flags: cplusplus.com/reference/iostream/ios_base/iostate
The fail bit is set if the last operation failed. In this case the fail and eof will become true at the same time. But this does not imply that fail and eof are the same thing. Note the fail will be set if you try and read a non numeric string into an int variable with operator >> etc. At this point the fail bit is true but eof is still false.
@sftrabbit: Your first comment is wrong The EOF flag is not set until you read past the end of file. The last successful read will read upto but not past the EOF. Try it with a file with no \n on the last line. Your second comment is true. But as a result people will try while(!fin.bad()) which then runs into the same problem as above.
@sftrabbit: Opps. OK. I see the same behavior now. But my original statement holds. The EOF flag is not set until you read past the end of file. You can read the whole file (but if you don't read passed the end EOF is not set). The difference here is the formatted input operator>> is actually reading passed the end of file to find the end of the input. For both the string example and my number example behave as described above.

bambams · Accepted Answer · 2009-09-29 19:16:02Z

IIRC, the eofbit doesn't get set until you actually try to read past the end of the file. That is, once you reach the end of the file, you have to read one more time before that flag will be set.

Andre · Accepted Answer · 2009-09-30 00:04:58Z

If the text file contains this sequence, without quotes, "12345 67890", then #3 will return false, but #4 will return true because there is no whitespace after the last number:

int i; bool b; fin >> i; b = fin.fail(); // 1 b = fin.eof(); // 2 fin >> i; b = fin.fail(); // 3 b = fin.eof(); // 4 fin >> i; b = fin.fail(); // 5 b = fin.eof(); // 6

However, if the sequence is "12345 6789 " (note the space after the last number), then #3 and #4 will both return false, but #5 and #6 will return true.

You should check for both, eof() and fail() and if both are true, you have no more data. If fail() is true, but eof() is false, there's a problem with the file.

I tried the first case, and it is not correct that "#4 will return true".

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End of File in C++

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