I would like to read a text file in C++ in following manner:
Ignore the entire first line as it is simply meant as an introduction.
Only read the following lines from a specific position.
That starting position for reading is a fixed one and remains the same for every line; however, the numbers after that may be of variable length. I need to save all of these numbers from line 2 to line n into an Array.
At the moment I can read a regular 2D Array with getline.
How can I work around these things?
An example for a line I want to read could be:
Person1: 25 988.3 0.0023 7
To set the file to a position, use std::ifstream::seekg().
To set the file to the beginning of a line, you must read and count the line endings. Many text files have variable length text lines.
How can I work around these things?
You can't, unless you can ensure that all of the data lines after the first line are all the same length.
If you can't ensure that, then all you can do is read through all of the preceding lines.
An alternative I have employed in the past is to generate an 'index' of line start positions in a secondary file in binary format (so that I CAN jump directly to the right place in that file), and use that to jump to the right place in the text file. Of course that means that you need to regenerate that index file every time you replace/amend the data file.
Related
I have a very long file, in which it can be assumed (if it helps) every line of which has the same format. I want to read a specific line of the file. Is it possible in C++ to move the pointer to that line via a binary search instead of starting at the top of the file and reading line by line and counting lines? That is, is it possible to access some line_of_file pointer and move it by binary search? If not in C++, is this task possible in assembly language or some other language?
You cannot usefully use binary search to find a line by line number in a text file, because text files are not indexed by line number. In other words, there is no way to figure out the line number of a given offset in the file other than starting at the beginning, reading every character, and counting the number of new line characters.
There is only one exception, and in that case binary search won't help you either. If every line in the file is the exact same length, then you can find the offset of a specific line by multiplying that length by the line number (using 0 as the number of the first line). Don't forget to include the newline character in the line length. You can use istream::seekg or ostream::seekp to position the next input or output operation, respectively. (You need to use the two-argument version. Some other warnings apply on platforms which translate newline characters to multicharacter sequences; here's looking at you, windows.)
I have to extract information from a text file.
In the text file there is a list of strings.
This is an example of a string: AAA101;2015-01-01 00:00:00;0.784
The value after the last ; is a non integer value, which changes from line to line, so every line has different lenght of characters.
I want to map all of these lines into a structured vector as I can access to a specific line anytime I need without scan the whole file again.
I did some research and I found some threads about a command called, which permit me to reach a specific line of a text file but I read it only works if any line has the same characters lenght of the others.
I was thinking about converting all the lines in the file in a proper format in order to be able to map that file as I want but I hope there is a better and quick way
You can try TStringList*. It creates a list of AnsiStrings. Then each AnsiString can be accessed via ->operator [](numberOfTheLine).
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.