reading from a file

[mickey]

hi, I need to put in 'buffer' ALL content of a file. Is it possibile. Is this faster way to manage a file?
This below should reads 1000 char but I need to read until the EOF. Is there a way?

char* buffer=0;
	while ( ifile.get(buffer,1000) ) {
		cout << " buffer " << buffer << endl;
	}

char* buffer=0;
	while ( ifile.get(buffer,1000) ) {
		cout << " buffer " << buffer << endl;
	}

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[mickey]

EDIT: (faster way?) I have a very large text file where every line is very long! thanks

[jacek]

This below should reads 1000 char but I need to read until the EOF. Is there a way?

What is the type of that ifile variable? You can check the file size and allocate a buffer of appropriate size.

[mickey]

std::fstream file;
std::fstream ifile = file.open(file_name.c_str();

std::fstream file;
std::fstream ifile = file.open(file_name.c_str();

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1.uhm, but anyway the 'get' cause a crash at runtime....
2.But is there a way to assign in one "hit" all text file to a char*? (I thought use a vector<string> too, but If I have a very large file and large lines maybe vector goes slow...)
thanks

[jacek]

the 'get' cause a crash at runtime....

How did you allocate the buffer?

But is there a way to assign in one "hit" all text file to a char*? (I thought use a vector<string> too, but If I have a very large file and large lines maybe vector goes slow...)

You could use seekg() and tellg() to check the file size first and then use a large enough buffer, but it might lead to race condition if some other process is modifying that file.

[mickey]

thanks for reply;
I just see for the tellp/g solution and it seems ok; But could I put file into vector<string> too? (I mean: is it efficient a vector? eg: my file is 2GB size large)

[jacek]

is it efficient a vector?

If you are going to use push_front/push_back methods it won't be. On the other hand, if you are going to allocate it in one go, it won't differ much from reading data line by line.

eg: my file is 2GB size large

How much RAM do you have in your machine? You certainly don't want your machine to start swapping.

If you want to optimize something, first use a profiler to check where the bottleneck is.

Remember: "premature optimization is the root of all evil".

[mickey]

Hi I have 512mb. and with this below my machine swapping! So haven't I hoping to use file larger than my Ram?

ifile.read (memblock, size);

ifile.read (memblock, size);

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Sorry: are you saying that working on "memblock" is much less efficient than using push_back to put the file in a vector?

thanks

[jacek]

Hi I have 512mb. and with this below my machine swapping! So haven't I hoping to use file larger than my Ram?

You can use files larger than your RAM, but in parts that fit in the memory. Getting data from disk is *a lot* slower than getting it from the memory, so you should avoid swapping at all cost.

are you saying that working on "memblock" is much less efficient than using push_back to put the file in a vector?

On the contrary. push_back() might result in relocation of all of the data in the vector.

[mickey]

hi, What does that mean?

...... but in parts that fit in the memory.

So is "memblock = new char [size];" is the best solution?
thanks

[jacek]

What does that mean?

If you have 512MB of RAM, you shouldn't try to read more than that.

So is "memblock = new char [size];" is the best solution?

std::vector and std::string will be OK too, but only if you won't change their size too often.

[mickey]

hi, I tried read with the solution above with memblock on a 200mb text file. it's goes fast.
Then I tried to read file and put it into a vector:

vector<string> text;
copy (it_file, eos, inserter (text, text.begin()) );

vector<string> text;
copy (it_file, eos, inserter (text, text.begin()) );

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this goes very slow (I didn't see the end of the copy!)
I tried this operation of "copy" because I need to sort the lines of the file too. Then I thought to put the lines in a vector and then call sort() on it. But the "reading" is too slow and I need speed.
I'm thinking of implement my own sort on memblock. But maybe it won't so fast as vector.sort().......What do you suggest, please?

[fullmetalcoder]

If you have 512MB of RAM, you shouldn't try to read more than that.

This doesn't make much sense... In most cases (users who dropped text-based interface) the OS/Desktop/backgroud tasks... occupy about half of the available memory (sometimes more). Thus it is highly recommened not to load a full file which is bigger than 40% of your available memory (yet it is possible). In such cases the best way is to read pieces of the file, perform some tasks and then discard them before loading some other new pieces... It should be a little tricky to work this way to sort lines of text but it's still doable.