I want to access the last 6 lines in a text file using c++. Can anyone provide me with a code that reaches there in a constant time? Thanks in advance. :)
fstream myfile("test.txt");
myfile.seekg(-6,ios_base::end);
string line;
while(getline(myfile,line))
{
if(vect.size() != VSIZE)
{
vect.push_back(line);
}
else
{
vect.erase(v.begin());
vect.push_back(line);
}
}
It seems not to be working... and VSIZE is 6... please provide me with help and working code.
This line:
myfile.seekg(-6,ios_base::end);
seeks to the 6th byte before the end of the file, not 6 lines. You need to count the newline backwards or start from the beginning. So your code should work if you remove the line above.
This is quite a hard thing to do, and there are several edge cases to consider.
Broadly the strategy is:
Open the file in binary mode so you see every byte.
Seek to (end - N), where N is the size of an arbitrary buffer. About 1K should do it.
Read N bytes into a buffer. Scan backwards looking for LF characters ('\n). Skip the one at the end, if there is one.
Each line starts just after an LF, so count the lines backwards until you get to 6.
If you don't find 6 then seek backwards another N bytes, read another buffer and continue the scan.
If you reach the beginning of the file, stop.
I leave the code as an exercise.
This answer explains why what you do won't work. Below I explain what will work.
Open the file in the binary mode.
Read forward from the start storing positions of '\n' in a circular buffer of length 6. (boost::circular_buffer can help)
Dump the contents of the file starting from the smallest position in the ring buffer.
Step 2 can be improved by seeking to end-X where X is derived by some sort of bisection around the end of file.
Probably the easiest approach is to just mmap() the file. This puts its contents into your virtual address space, so you can easily scan it from the end for the first six line endings.
Since mmapping a file gives you the illusion of the entire file being in memory in a single large buffer without actually loading the parts that you don't need, it both avoids unnecessary I/O and alleviates you from managing a growing buffer as you search backwards for the line endings.
Related
Is there an efficient C or C++ way to read the last row of a CSV file? The naive approach involves reading in the entire file and then going to the end. Is there a quicker way this can be done (particularly if the CSV files are large)?
What you can do is guess the line length, then jump 2-3 lines before the end of the file and read the remaining lines. The last line you read is the last one, as long you read at least one line prior (otherwise, you still start again with a bigger offset)
I posted some sample code for doing a similar thing (reading last N lines) in this answer (in PHP, but serves as an illustration)
For implementations in a variety of languages, see
C++ : c++ fastest way to read only last line of text file?
Python : Efficiently finding the last line in a text file
Perl : How can I read lines from the end of file in Perl?
C# : Get last 10 lines of very large text file > 10GB c#
PHP : how to read only 5 last line of the txt file
Java: Read last n lines of a HUGE file
Ruby: Reading the last n lines of a file in Ruby?
Objective-C : How to read data from NSFileHandle line by line?
You can try working backwards. Read some size block of bytes from the end of the file, and look for the newline. If there is no newline in that block, then read the previous block, and so on.
Note that if the size of a row relative to the size of the file is large that this may result in worse performance, because most file caching schemes assume someone reads forward in the file.
You can use Perl module File::ReadBackwards.
Your problem falls into the same domain as searching for a string within a file. As you rightly point out, it's not always a great idea to read the entire file into memory and then search for your string. But you can always do the next best thing. Memory map your file. Then use your string searching functions to search backwards from the end of the string for your newline.
It's an extremely efficient mechanism with minimal memory footprint and optimum disk I/O.
Read with what and on what? On a Unix system, if you want the last line, it is as simple as
tail -n1 file.csv
If you want this approach from within your C++ app, you can do something like
system("tail -n1 file.csv")
if you want a quick and dirty way to accomplish this task.
I posted this over at Code Review Beta but noticed that there is much less activity there.
I have the following code and it works just fine. It's function is to grab the input from a file and display it out (to confirm that it's been grabbed). My task is to write a program that counts how many times a certain word (string) "abc" is found in the input file.
Is it better to store the input as a string or in arrays/vectors and have each line be stored separately? a[1], a[2] ect? Perhaps someone could also point me to a resource that I can use to learn how to filter through the input data.
Thanks.
input_file.open ("in.dat");
while(!input_file.eof()) // Inputs all the lines until the end of file (eof).
{
getline(input_file,STRING); // Saves the input_file in STRING.
cout<<STRING; // Prints our STRING.
}
input_file.close();
Reading as much of the file into memory is always more efficient than reading one letter or text line at a time. Disk drives take a lot of time to spin up and relocate to a sector. However, your program will run faster if you can minimize the number of reads from the file.
Memory is fast to search.
My recommendation is to read the entire file, or as much as you can into memory, then search the memory for a "word". Remember, that in English, words can have hyphens,'-', and single quotes, "don't". Word recognition may become more difficult if it is split across a line or you include abbreviations (with periods).
Good luck.
im trying to store a large list of prime numbers in a text file and if I end my program i need to be able to read the line of the file to see where I left off. I dont know how to read the last line with out reading every line of the file first.
I don't know either. Just write the last value into a separate file, and read that value to know where to resume.
You could use setg() to jump to the end of the file and do guesses how far a line is. If there's a newline between your point and the end of the file then you're in the next-to-last line and know what the last line is.
But Pete Beckers solution is a lot nicer, I'd go with that instead.
You can calculate the bytes of your numbers.
For example you have 5 number and you want to read last number.
1 integer is 4 byte. So you can move 4*4=16 Byte in file using fseek. After that you can read last line.
fseek (file , 16 , SEEK_SET);
SEEK_SET means begining of file.
Seek to the very end of the file, and just read backwards till you find the newline character which means you have found the the start of the last line
If you know the maximum length of line this will be easy.
Just go the the pointer that is the location of the end of file less this value.
Start reading lines from there and put them in a buffer. Clear buffer when the previous character was a new line
When you ran out of file the buffer will contain it.
If you do not know the maximum length you can always read the file backwards.
Is there a way that I can seek to a certain line in a file to read or write data?
Let's say I want to write some data starting on the 10th line in a text file. There might be some data already in the first few lines, or the file could even be empty. Is there a way I can seek directly to the line I want without having to worry about what's already in the file?
Only if the lines are all the same length (seek to 9 * bytes_per_line). Otherwise, you'll just have to scan your way to the appropriate spot in the file.
Also be wary of writing into the middle of a file. It may not do what you expect (insert new lines). It will simply overwrite whatever content is already there, and won't respect existing line boundaries.
You can seek to a position in a file, but that position must be a character offset from the start, end or current position - see for example fseek(). There is no way of seeking to a particular line, unless all the lines are exactly the same length.
No, you have to process the data to find the line delimiters (unless you have fixed length lines). Have a look at getline(), ftell() and fseek(). http://www.pixelbeat.org/programming/readline/cpp.cpp
The easy best way is to read the file in memory inserting for instance each line in a vector of strings, then modifying/adding whatever you want, and re-write each line in a new file.
(supposing the file fits in memory)
I have a big text file with more then 200.000 lines, and I need to read just a few lines. For instance: line 10.000 to 20.000.
Important: I don´t want to open and search the full file to extract theses lines because of performance issues.
Is this possible?
If the lines are fixed length, then it would be possible to seek to a specific byte position and load just the lines you want. If lines are variable length, the only way to find the lines you're looking for is to parse the file and count the number of end-of-line markers. If the file changes infrequently, you might be able to get sufficient performance by performing this parsing once and then keeping an index of the byte positions of each line to speed future accesses (perhaps writing that index to disk so it doesn't need to be done every time your program is run).
You will have to search through the file to count the newlines, unless you know that all lines are the same length (in which case you could seek to the offset = line_number * line_size_in_bytes, where line_number counts from zero and line_size_in_bytes includes all characters in the line).
If the lines are variable / unknown length then while reading through it once you could index the beginning offset of each line so that subsequent reads could seek to the start of a given line.
If these lines are all the same length you could compute an offset for a given line and read just those bytes.
If the lines are varying length then you really have to read the entire file to count how many lines there are. Line terminating characters are just arbitrary bytes in the file.
If the line are fixed length then you just compute the offset, no problem.
If they're not (i.e. a regular CSV file) then you'll need to go through the file, either to build an index or to just read the lines you need. To make the file reading a little faster a good idea would be to use memory mapped files (see the implementation that's part of the Boost iostreams: http://www.boost.org/doc/libs/1_39_0/libs/iostreams/doc/classes/mapped_file.html).
As others noted, if you do not have the lines of fixed width, it is impossible to do without building the index. However, if you are in control of the format of the file, you can get a ~O(log(size)) instead of O(size) performance in finding the start line, if you manage to store number of the line itself on each line, i.e. to have the file contents look something like this:
1: val1, val2, val3
2: val4
3: val5, val6
4: val7, val8, val9, val10
With this format of the file, you can quickly find the needed line by binary search: start with seeking into the middle of the file. Read till the next newline. Then read the line, and parse the number. If the number is bigger than the target, then you need to repeat the algorithm on the first half of the file, if it is smaller than the target line number, then you need to repeat it on the second half of the file.
You'd need to be careful about the corner cases (e.g.: your "beginning" of the range and "end" of the range are on the same line, etc.), but for me this approach worked excellently in the past for parsing the logfiles which had the date in it (and I needed to find the lines that are between the certain timestamps).
Of course, this still does not beat the performance of the explicitly built index or the fixed-size records.