I'm making a simple bug tracker and am using a text file as the database. Right now I'm reading in all the information through keys and importing them into specific arrays.
for (int i = 0; i < 5; i++)
{
getline(bugDB, title[i], '#');
getline(bugDB, importance[i], '!');
getline(bugDB, type[i], '$');
getline(bugDB, description[i], '*');
}
Here is what's in my (terribly unreadable) file
Cant jump#Moderate!Bug$Every time I enter the cave of doom, I'm unable
to jump.*Horse too expensive#Moderate!Improvement$The horses cost way
too much gold, please lower the costs.*Crash on startup#Severe!Bug$I'm
crashing on startup on my Win8.1 machine, seems to be a 8.1
bug.*Floating tree at Imperial March#Minimal!Bug$There is a tree
floating about half a foot over the ground near the crafting
area.*Allow us to instance our group#Moderate!Improvement$We would
like a feature that gives us the ability to play with our groups alone
inside dungeons.*
Output:
This works great for me, but I'd like to be able to delete specific bugs. I'd be able to do this by letting the user choose a bug by number, find the corresponding * key, and delete all information until the program reaches the next * key.
I'd appreciate any suggestions, I don't know where to start here.
There is no direct mechanism for deleting some chunk of data from the middle of the file, no delete(file, start, end) function. To perform such a deletion you have to move the data which appears after the region; To delete ten bytes from the middle of a file you'd have to move all of the subsequent bytes back ten, looping over the data, then truncate to make the file ten bytes smaller.
In your case however, you've already written code to parse the file into memory, populating your arrays. Why not just implement a function to write the contents of the arrays back to a file? Truncate the file (open in mode "w" rather than "w+"), loop over the arrays writing their contents back to the file in your preferred format, but skip the entry that you want to delete.
its only possible by manually copying the data from input file to output file and leaving out the entry you want to delete.
but: i strongly encourage the usage of some small database for keeping the informations (look at sqlite)
Also its a bad bugtracker if solving the bug means "delete it from database" (its not even is a tracker). give it a status field (open, refused, duplicate, fixed, working, ...).
Additional remarks:
use one array that keeps some structure with n informations and not n arrays.
please remind that someone may use your delimiter characters in the descriptions (use some uncommon character and replace its usage in saved text)
explanation for 1.:
instead of using
std::vector<std::string> title;
std::vector<int> importance;
std::vector<std::string> description;
define a structure or class and create a vector of this structure.
struct Bug{
std::string title;
int importance; // better define an enum for importance
std::string description;
};
std::vector<Bug> bugs;
Related
What is the best way to cut the end off of a fstream file in C++ 11
I am writing a data persistence class to store audio for my audio editor. I have chosen to use fstream (possibly a bad idea) to create a random access binary read write file.
Each time I record a little sound into my file I simply tack it onto the end of this file. Another internal data structure / file, contains pointers into the audio file and keeps track of edits.
When I undo a recording action and then do something else the last bit of the audio file becomes irrelevant. It is not referenced in the current state of the document and you cannot redo yourself back to a state where you can ever see it again. So I want to chop this part of the file off and start recording at the new end. I don’t need to cut out bitts in the middle, just off the end.
When the user quits this file will remain and be reloaded when they open the project up again.
In my application I expect the user to do this all the time and being able to do this might save me as much as 30% of the file size. This file will be long, potentially very, very long, so rewriting it to another file every time this happens is not a viable option.
Rewriting it when the user saves could be an option but it is still not that attractive.
I could stick a value at the start that says how long the file is supposed to be and then overwrite the end to recycle the space but in the mean time. If I wanted to continually update the data store file in case of crash this would mean I would be rewriting the start over and over again. I worry that this might be bad for flash drives. I could also recomputed the end of the useful part of the file on load, by analyzing the pointer file but in the mean time I would be wasting all that space potentially, and that is complicated.
Is there a simple call for this in the fstream API?
Am I using the wrong library? Note I want to stick to something generic STL I preferred, so I can keep the code as cross platform as possible.
I can’t seem to find it in the documentation and have looked for many hours. It is not the end of the earth but would make this a little simpler and potentially more efficient. Maybe I am just missing it somehow.
Thanks for your help
Andre’
Is there a simple call for this in the fstream API?
If you have C++17 compiler then use std::filesystem::resize_file. In previous standards there was no such thing in standard library.
With older compilers ... on Windows you can use SetFilePointer or SetFilePointerEx to set the current position to the size you want, then call SetEndOfFile. On Unixes you can use truncate or ftruncate. If you want portable code then you can use Boost.Filesystem. From it is simplest to migrate to std::filesystem in the future because the std::filesystem was mostly specified based on it.
If you have variable, that contains your current position in the file, you could seek back for the length of your "unnedeed chunk", and just continue to write from there.
// Somewhere in the begining of your code:
std::ofstream *file = new std::ofstream();
file->open("/home/user/my-audio/my-file.dat");
// ...... long story of writing data .......
// Lets say, we are on a one millin byte now (in the file)
int current_file_pos = 1000000;
// Your last chunk size:
int last_chunk_size = 12345;
// Your chunk, that you are saving
char *last_chunk = get_audio_chunk_to_save();
// Writing chunk
file->write(last_chunk, last_chunk_size);
// Moving pointer:
current_file_pos += last_chunk_size;
// Lets undo it now!
current_file_pos -= last_chunk_size;
file->seekp(current_file_pos);
// Now you can write new chunks from the place, where you were before writing and unding the last one!
// .....
// When you want to finally write file to disk, you just close it
file->close();
// And when, truncate it to the size of current_file_pos
truncate("/home/user/my-audio/my-file.dat", current_file_pos);
Unfortunatelly, you'll have to write a crossplatform function truncate, that would call SetEndOfFile in windows, and truncate in linux. It's easy enough with using preprocessor macros.
I am serializing multiple objects and want to save the given Strings to a file. The structure is the following:
A few string and long attributes, then a variable amount of maps<long, map<string, variant> >. My first idea was creating one valid JSONFile but this is very hard to do (all of the maps are very big and my temporary memory is not big enough). Since I cant serialize everything together I have to do it piece by piece. I am planning on doing that and I then want to save the recieved strings to a file. Here is how it will look like:
{ "Name": "StackOverflow"}
{"map1": //map here}
{"map2": //map here}
As you can see this is not one valid JSON object but 3 valid JSONObjects in one file. Now I want to deserialize and I need to give a valid JSONObject to the deserializer. I already save tellp() everytime when I write a new JSONObject to file, so in this example I would have the following adresses saved: 26, endofmap1, endofmap2.
Here is what I want to do: I want to use these addresses, to extract the strings from the file I wrote to. I need one string which is from 0 to (26-1), one string from 26 to(endofmap1-1) and one string from endofmap1 to (endofmap2-1). Since these strings would be valid JSONObjects i could deserialize them without problem.
How can I do this?
I would create a serialize and deserialize class that you can use as part of a hierarchy.
So for instance, in rough C++ psuedo-code:
class Object : public serialize, deserialize {
public:
int a;
float b;
Compound c;
bool serialize(fstream& fs) {
fs << a;
fs << b;
c->serialize(fs);
fs->flush();
}
// same for deserialize
};
class Compound : serialize, deserialize {
public:
map<> things;
bool serialize(fstream& fs) {
for(thing : things) {
fs << thing;
}
fs->flush();
}
};
With this you can use JSON as the file will be written as your walk the heirarchy.
Update:
To extract a specific string from a file you can use something like this:
// pass in an open stream (streams are good for unit testing!)
std::string extractString(fstream& fs) {
int location = /* the location of the start from file */;
int length = /* length of the string you want to extract */;
std::string str;
str.resize(length);
char* begin = *str.begin();
fs->seekp(location);
fs->read(begin, length);
return str;
}
Based on you saying "my temporary memory is not big enough", I'm going to assume two possibilities (though some kind of code example may help us help you!).
possibility one, the file is too big
The issue you would be facing here isn't a new one - a file too large for memory, assuming your algorithm isn't buffering all the data, and your stack can handle the recursion of course.
On windows you can use the MapViewOfFile function, the MSDN has plenty of detail on that. This function will effectively grab a "view" of a section of a file - allowing you to load enough of the file to modify only what you need, before closing and opening a view at a later offset.
If you are on a different platform, there will be similar functions.
possibility two, you are doing too much at once
The other option is more of a "software engineering" issue. You have so much data then when holding them in your std::maps, you run out of heap-memory.
If this is the case, you are going to need to use some clever thinking - here are some ideas!
Don't load all your data into the maps. wherever the data is coming from, take a CRC, Index, or Filename of the data-source. Store that information in the map, and leave the actual "big strings" on the hard disk. - This way you can load each item of data when you need it.
This works really well for data that needs to be sorted, or correlated.
Process or load your data when you need to write it. If you don't need to sort or correlate the data, why load it into a map beforehand at all? Just load each "big string" of data in sequence, then write them to the file with an ofstream.
I have a series of large text files (10s - 100s of thousands of lines) that I want to parse line-by-line. The idea is to check if the line has a specific word/character/phrase and to, for now, record to a secondary file if it does.
The code I've used so far is:
ifstream infile1("c:/test/test.txt");
while (getline(infile1, line)) {
if (line.empty()) continue;
if (line.find("mystring") != std::string::npos) {
outfile1 << line << '\n';
}
}
The end goal is to be writing those lines to a database. My thinking was to write them to the file first and then to import the file.
The problem I'm facing is the time taken to complete the task. I'm looking to minimize the time as far as possible, so any suggestions as to time savings on the read/write scenario above would be most welcome. Apologies if anything is obvious, I've only just started moving into C++.
Thanks
EDIT
I should say that I'm using VS2015
EDIT 2
So this was my own dumb fault, when switching to Release and changing the architecture type I had noticeable speed increases. Thanks to everyone for pointing me in that direction. I'm also looking at the mmap stuff and that's proving useful too. Thanks guys!
When you use ifstream to read and process to/from really big files, you have to increase the default buffer size that is used (normally 512 bytes).
The best buffer size depends on your needs, but as a hint you can use the partition block size of the file(s) your reading/writing. To know that information you can use a lot of tools or even code.
Example in Windows:
fsutil fsinfo ntfsinfo c:
Now, you have to create a new buffer to ifstream like this:
size_t newBufferSize = 4 * 1024; // 4K
char * newBuffer = new char[newBufferSize];
ifstream infile1;
infile1.rdbuf()->pubsetbuf(newBuffer, newBufferSize);
infile1.open("c:/test/test.txt");
while (getline(infile1, line)) {
/* ... */
}
delete newBuffer;
Do the same with the output stream and don't forget set new buffer before open file or it may not work.
You can play with values to find the very best size for you.
You'll note the difference.
C-style I/O functions are much faster than fstream.
You may use fgets/fputs to read/write each text line.
This is my first post here, so please bear with me.
I have searched high and low on the internet for an answer, but I've not been able to resolve my issue, so I have decided to write a post here.
I am trying to write(append) to a JSON array on file using C++ and JZON, at intervals of 1 write each second. The JSON file is initially written by a “Prepare” function. Another function is then called each second to a add an array to the JSON file and append an new object to the array every second.
I have tried many things, most of which resulted in all sorts of issues. My latest attempt gave me the best results and this is the code that I have included below. However, the approach I took is very inefficient as I am writing an entire array every second. This is having a massive hit on CPU utilisation as the array grows, but not so much on memory as I had first anticipated.
What I really would like to be able to do is to append to an existing array contained in a JSON file on disk, line by line, rather than having to clear the entire array from the JSON object and rewriting the entire file, each and every second.
I am hoping that some of the geniuses on this website will be able to point me in the right direction.
Thank you very much in advance.
Here is my code:
//Create some object somewhere at the top of the cpp file
Jzon::Object jsonFlight;
Jzon::Array jsonFlightPath;
Jzon::Object jsonCoordinates;
int PrepareFlight(const char* jsonfilename) {
//...SOME PREPARE FUNCTION STUFF GOES HERE...
//Add the Flight Information to the jsonFlight root JSON Object
jsonFlight.Add("Flight Number", flightnum);
jsonFlight.Add("Origin", originicao);
jsonFlight.Add("Destination", desticao);
jsonFlight.Add("Pilot in Command", pic);
//Write the jsonFlight object to a .json file on disk. Filename is passed in as a param of the function.
Jzon::FileWriter::WriteFile(jsonfilename, jsonFlight, Jzon::NoFormat);
return 0;
}
int UpdateJSON_FlightPath(ACFT_PARAM* pS, const char* jsonfilename) {
//Add the current returned coordinates to the jsonCoordinates jzon object
jsonCoordinates.Add("altitude", pS-> altitude);
jsonCoordinates.Add("latitude", pS-> latitude);
jsonCoordinates.Add("longitude", pS-> longitude);
//Add the Coordinates to the FlightPath then clear the coordinates.
jsonFlightPath.Add(jsonCoordinates);
jsonCoordinates.Clear();
//Now add the entire flightpath array to the jsonFlight object.
jsonFlight.Add("Flightpath", jsonFlightPath);
//write the jsonFlight object to a JSON file on disk.
Jzon::FileWriter::WriteFile(jsonfilename, jsonFlight, Jzon::NoFormat);
//Remove the entire jsonFlighPath array from the jsonFlight object to avoid duplicaiton next time the function executes.
jsonFlight.Remove("Flightpath");
return 0;
}
For sure you can do "flat file" storage yourself.. but this is a symptom of needing a database. Something very light like SQLite, or mid-weight & open-source like MySQL, FireBird, or PostgreSQL.
But as to your question:
1) Leave the closing ] bracket off, and just keep the file open & appending -- but if you don't close the file correctly, it will be damaged & need repair to be readable.
2) Your current option -- writing a complete file each time -- isn't safe from data loss either, as the moment you "open to overwrite" you lose all data previously stored in the file. The workaround here, is to rename the old file as a backup before you start writing.
You should also make backup copies of your file, with the first option. (Say at daily intervals). Otherwise data loss is likely to occur eventually -- on Ctrl-C, power loss, program error or system crash.
Of course if you use any of SQLlite, MySQL, Firebird or PostgreSQL all the data-integrity problems will be handled for you.
I need to read the number of lines in a file before doing some operations on that file. When I try to read the file and increment the line_count variable at each iteration until I reach EOF. It was not that fast in my case. I used both ifstream and fgets. They were both slow. Is there a hacky way to do this, which is also used by, for instance BSD, Linux kernel or berkeley db (may be by using bitwise operations).
The number of lines is in the millions in that file and it keeps getting larger, each line is about 40 or 50 characters. I'm using Linux.
Note:
I'm sure there will be people who might say use a DB idiot. But briefly in my case I can't use a db.
The only way to find the line count is to read the whole file and count the number of line-end characters. The fastest way to do this is probably to read the whole file into a large buffer with one read operation and then go through the buffer counting the '\n' characters.
As your current file size appears to be about 60Mb, this is not an attractive option. You can get some of the speed by not reading the whole file, but reading it in chunks, say of size 1Mb. You also say that a database is out of the question, but it really does look to be the best long-term solution.
Edit: I just ran a small benchmark on this and using the buffered approach (buffer size 1024K) seems to be a bit more than twice as fast as reading a line at a time with getline(). Here's the code - my tests were done with g++ using -O2 optimisation level:
#include <iostream>
#include <fstream>
#include <vector>
#include <ctime>
using namespace std;
unsigned int FileRead( istream & is, vector <char> & buff ) {
is.read( &buff[0], buff.size() );
return is.gcount();
}
unsigned int CountLines( const vector <char> & buff, int sz ) {
int newlines = 0;
const char * p = &buff[0];
for ( int i = 0; i < sz; i++ ) {
if ( p[i] == '\n' ) {
newlines++;
}
}
return newlines;
}
int main( int argc, char * argv[] ) {
time_t now = time(0);
if ( argc == 1 ) {
cout << "lines\n";
ifstream ifs( "lines.dat" );
int n = 0;
string s;
while( getline( ifs, s ) ) {
n++;
}
cout << n << endl;
}
else {
cout << "buffer\n";
const int SZ = 1024 * 1024;
std::vector <char> buff( SZ );
ifstream ifs( "lines.dat" );
int n = 0;
while( int cc = FileRead( ifs, buff ) ) {
n += CountLines( buff, cc );
}
cout << n << endl;
}
cout << time(0) - now << endl;
}
Don't use C++ stl strings and getline ( or C's fgets), just C style raw pointers and either block read in page-size chunks or mmap the file.
Then scan the block at the native word size of your system ( ie either uint32_t or uint64_t) using one of the magic algorithms 'SIMD Within A Register (SWAR) Operations' for testing the bytes within the word. An example is here; the loop with the 0x0a0a0a0a0a0a0a0aLL in it scans for line breaks. ( that code gets to around 5 cycles per input byte matching a regex on each line of a file )
If the file is only a few tens or a hundred or so megabytes, and it keeps growing (ie something keeps writing to it), then there's a good likelihood that linux has it cached in memory, so it won't be disk IO limited, but memory bandwidth limited.
If the file is only ever being appended to, you could also remember the number of lines
and previous length, and start from there.
It has been pointed out that you could use mmap with C++ stl algorithms, and create a functor to pass to std::foreach. I suggested that you shouldn't do it not because you can't do it that way, but there is no gain in writing the extra code to do so. Or you can use boost's mmapped iterator, which handles it all for you; but for the problem the code I linked to was written for this was much, much slower, and the question was about speed not style.
You wrote that it keeps getting larger.
This sounds like it is a log file or something similar where new lines are appended but existing lines are not changed. If this is the case you could try an incremental approach:
Parse to the end of file.
Remember the line count and the offset of EOF.
When the file grows fseek to the offset, parse to EOF and update the line count and the offset.
There's a difference between counting lines and counting line separators. Some common gotchas to watch out for if getting an exact line count is important:
What's the file encoding? The byte-by-byte solutions will work for ASCII and UTF-8, but watch out if you have UTF-16 or some multibyte encoding that doesn't guarantee that a byte with the value of a line feed necessarily encodes a line feed.
Many text files don't have a line separator at the end of the last line. So if your file says "Hello, World!", you could end up with a count of 0 instead of 1. Rather than just counting the line separators, you'll need a simple state machine to keep track.
Some very obscure files use Unicode U+2028 LINE SEPARATOR (or even U+2029 PARAGRAPH SEPARATOR) as line separators instead of the more common carriage return and/or line feed. You might also want to watch out for U+0085 NEXT LINE (NEL).
You'll have to consider whether you want to count some other control characters as line breakers. For example, should a U+000C FORM FEED or U+000B LINE TABULATION (a.k.a. vertical tab) be considered going to a new line?
Text files from older versions of Mac OS (before OS X) use carriage returns (U+000D) rather than line feeds (U+000A) to separate lines. If you're reading the raw bytes into a buffer (e.g., with your stream in binary mode) and scanning them, you'll come up with a count of 0 on these files. You can't count both carriage returns and line feeds, because PC files generally end a line with both. Again, you'll need a simple state machine. (Alternatively, you can read the file in text mode rather than binary mode. The text interfaces will normalize line separators to '\n' for files that conform to the convention used on your platform. If you're reading files from other platforms, you'll be back to binary mode with a state machine.)
If you ever have a super long line in the file, the getline() approach can throw an exception causing your simple line counter to fail on a small number of files. (This is particularly true if you're reading an old Mac file on a non-Mac platform, causing getline() to see the entire file as one gigantic line.) By reading chunks into a fixed-size buffer and using a state machine, you can make it bullet proof.
The code in the accepted answer suffers from most of these traps. Make it right before you make it fast.
Remember that all fstreams are buffered. So they in-effect do actually reads in chunks so you do not have to recreate this functionality. So all you need to do is scan the buffer. Don't use getline() though as this will force you to size a string. So I would just use the STL std::count and stream iterators.
#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
struct TestEOL
{
bool operator()(char c)
{
last = c;
return last == '\n';
}
char last;
};
int main()
{
std::fstream file("Plop.txt");
TestEOL test;
std::size_t count = std::count_if(std::istreambuf_iterator<char>(file),
std::istreambuf_iterator<char>(),
test);
if (test.last != '\n') // If the last character checked is not '\n'
{ // then the last line in the file has not been
++count; // counted. So increement the count so we count
} // the last line even if it is not '\n' terminated.
}
It isn't slow because of your algorithm , It is slow because IO operations are slow. I suppose you are using a simple O(n) algorithm that is simply going over the file sequentially. In that case , there is no faster algorithm that can optimize your program.
However , I said there is no faster algorithm , but there is a faster mechanism which called "Memory Mapped file " , There are some drawback for mapped files and it might not be appropiate for you case , So you'll have to read about it and figure out by yourself.
Memory mapped files won't let you implement an algorithm better then O(n) but it may will reduce IO access time.
You can only get a definitive answer by scanning the entire file looking for newline characters. There's no way around that.
However, there are a couple of possibilities which you may want to consider.
1/ If you're using a simplistic loop, reading one character at a time checking for newlines, don't. Even though the I/O may be buffered, function calls themselves are expensive, time-wise.
A better option is to read large chunks of the file (say 5M) into memory with a single I/O operation, then process that. You probably don't need to worry too much about special assembly instruction since the C runtime library will be optimized anyway - a simple strchr() should do it.
2/ If you're saying that the general line length is about 40-50 characters and you don't need an exact line count, just grab the file size and divide by 45 (or whatever average you deem to use).
3/ If this is something like a log file and you don't have to keep it in one file (may require rework on other parts of the system), consider splitting the file periodically.
For example, when it gets to 5M, move it (e.g., x.log) to a dated file name (e.g., x_20090101_1022.log) and work out how many lines there are at that point (storing it in x_20090101_1022.count, then start a new x.log log file. Characteristics of log files mean that this dated section that was created will never change so you will never have to recalculate the number of lines.
To process the log "file", you'd just cat x_*.log through some process pipe rather than cat x.log. To get the line count of the "file", do a wc -l on the current x.log (relatively fast) and add it to the sum of all the values in the x_*.count files.
The thing that takes time is loading 40+ MB into memory. The fastest way to do that is to either memorymap it, or load it in one go into a big buffer. Once you have it in memory, one way or another, a loop traversing the data looking for \n characters is almost instantaneous, no matter how it is implemented.
So really, the most important trick is to load the file into memory as fast as possible. And the fastest way to do that is to do it as a single operation.
Otherwise, plenty of tricks may exist to speed up the algorithm. If lines are only added, never modified or removed, and if you're reading the file repeatedly, you can cache the lines read previously, and the next time you have to read the file, only read the newly added lines.
Or perhaps you can maintain a separate index file showing the location of known '\n' characters, so those parts of the file can be skipped over.
Reading large amounts of data from the harddrive is slow. There's no way around that.
If your file only grows, then Ludwig Weinzierl is the best solution if you do not have control of the writers. Otherwise, you can make it even faster: increment the counter by one each time a line is written to the file. If multiple writers may try to write to the file simultaneously, then make sure to use a lock. Locking your existing file is enough. The counter can be 4 or 8 bytes written in binary in a file written under /run/<your-prog-name>/counter (which is RAM so dead fast).
Ludwig Algorithm
Initialize offset to 0
Read file from offset to EOF counting '\n' (as mentioned by others, make sure to use buffered I/O and count the '\n' inside that buffer)
Update offset with position at EOF
Save counter & offset to a file or in a variable if you only need it in your software
Repeat from "Read file ..." on a change
This is actually how various software processing log files function (i.e. fail2ban comes to mind).
The first time, it has to process a huge file. Afterward, it is very small and thus goes very fast.
Proactive Algorithm
When creating the files, reset counter to 0.
Then each time you receive a new line to add to the file:
Lock file
Write one line
Load counter
Add one to counter
Save counter
Unlock file
This is very close to what database systems do so a SELECT COUNT(*) FROM table on a table with millions of rows return instantly. Databases also do that per index. So if you add a WHERE clause which matches a specific index, you also get the total instantly. Same principle as above.
Personal note: I see a huge number of Internet software which are backward. A watchdog makes sense for various things in a software environment. However, in most cases, when something of importance happens, you should send a message at the time it happens. Not use a backward concept of checking logs to detect that something bad just happened.
For example, you detect that a user tried to access a website and entered the wrong password 5 times in a row. You want to send a instant message to the admin to make sure there wasn't a 6th time which was successful and the hacker can now see all your user's data... If you use logs, the "instant message" is going to be late by seconds if not minutes.
Don't do processing backward.