I have 10 text files (named file0.txt to file9.txt) with arbitrary lengths and number of lines. I need to randomly pick a file, randomly access 1-3 lines from that file, process them and repeat until all the lines of all the files have been processed. This only needs to be done once. For the sake of this question let's say "process" means print the lines. Does anyone have any suggestions on how I can go about doing this without loading all the text files into memory?
There's not really any way to 'randomly access' (in the sense that you can randomly access a vector) lines in a text file since the only way to find the lines is to search the file linearly for newlines. This means you'll at least need to stream through the files once to access lines even if you don't load them fully into memory.
You could achieve what you're describing by passing over all the files once to count the number of lines in them and then passing over them again to pull out randomly selected lines. I'm not sure what the benefit of that would be though. What are you really trying to achieve?
you can scan the file one to index where line starts, and keep that in memory (or even persist that if you need to do the same file more than once).
once you have that you can just seek into the line beginning and just read it till newline/eof before processing.
Suggestion:
1/ Make a copy of the files
2/ Erase a line when it is read
3/ update number of lines in file
That way you randomly pick a line that exist and that was not already read.
Lot of read/write...not efficient
Related
Let's assume that we have a header-less CSV file (I'm including a header row for clarification but the actual file doesn't contain it):
ID,BookTitle,Author,Price
110,book1,author1,price1
178,book2,author2,price2
917,book3,author3,price3
How can I acquire the ID column without having to read whole rows of data into memory? i.e. read ID: 110 and add to a vector, go to next row (line), read ID: 178 and add to vector, and so on.
You cannot. Files don't have rows and columns. The content is just characters and a \n denotes a line break. Hence, you cannot know where a line starts or ends without reading characters until you find a line break.
Situation is different when lines have a fixed width. Then you can skip ahead and start reading the next line.
For any future readers who might stumble upon this question:
#largest_prime 's answer is the reasonable one. If your row data isn't that large (which for 99.99% of times, this is the case), it makes perfect sense to read it all, acquire the ID data and finally discard the rest of the row data.
However, in that 0.01% of the times that a row might contain big data (such as a whole book's text), #z80crew 's solution might work out nicely.
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.
I have a program where I need to write text lines to a log file very frequently. I would like to limit the number of lines in the log file to 1000. When I write lines to the file, it should append them normally. Once the file reaches 1000 lines, I'd like to get rid of the first line and then append the new one. Does anyone know if there is a way to do this without rewriting the entire file each time?
Generally it's a little bit better for a case like this to remove more than one line at a time from the beginning.
That is, if your limit is 1000 lines, and you hit 1000 lines, delete the first 300 or so, and then resume writing. That way, you're not performing the delete operation with every single line written thereafter, only every 300 times. If you need to persist 1000 lines, then instead keep up to 1300 and delete 300 when 1300 is reached.
All files have to be aligned to FS cluster size. So, no, there's no way. You can append a line to a file, but you can't delete the first line without file rewriting.
You can use 2 files by turns.
Or use some buffer in memory and flush it periodically.
I think you still have to scan the file to find out how many lines are in the file at this moment. In that case, you can put it in some sort of buffer that you could easily add and delete from.
Then you do your logging and when you are done, you could "re-write" the file with the buffer (or only last 1000 lines).
Other alternatives are discussed above.
And yeah, try to avoid deleting line-by-line. Generally, it is a costly operation.
I've found some similar topics here and on CodeProject:
Small logger class;
Flexible logger class using standard streams in C++
http://www.codeproject.com/Articles/584794/Simple-logger-for-Cplusplus
Hope you find them useful :)
Any time you want to log, you can open the file, read your write index, jump to the position, and write the fixed-width log entry. When your index hits your upper threshold, simply set it back to 0.
There are a lot of warnings with this, though - first is that each proper log entry (assuming you close the file in between) will require an open, a read, a seek, a write, a seek, a write and a close - to find your index, go to it, write the new entry, then update your index. You also have the inherent issues of writing a fixed-size data element. Also, a human reader will depend on your content to know where the "beginning" of the file is. Most people expect "line 1" to be the first line.
I'm a much bigger advocate for simply having a few files and "rolling" them, so that each file on its own is coherent, but if you want just one file with a fixed number of lines, the circular buffer idea can work.
When you only want to use one file, and the length of the lines are not constant, there is no way without rewriting the whole file.
Depending on how often you are appending to the file, I don't see any problem doing so. 1000 lines of approx 100 chars are only approx 100kb, which is not to much. Additionally you may add some hysteresis.
However:
If the line length is constant (or you hard-limit the line length to some constant), you could just overwrite the oldest line. But then you have to keep track of the log file positions of old/new lines
I would use two files: The first one where you append lines. When the file gets full, rename it to a second one, and fill the first one from the beginning.
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.
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)