What will be the most eficient way to split a file in Java ?
Like to get it grid ready...
(Edit)
Modifying the question.
Basically after scouring the net I understand that there are generally two methods followed for file splitting....
Just split them by the number of bytes
I guess the advantage of this method is that it is fast, but say I have all the data in a line and suppose the file split puts half the data in one split and the other half the data in another split, then what do I do ??
Read them line by line
This will keep my data intact, fine, but I suppose this ain't as fast as the above method
Well, just read the file line by line and start saving it to a new file. Then when you decide it's time to split, start saving the lines to a new place.
Don't worry about efficiency too much unless it's a real problem later.
My first impression is that you have something like a comma separated value (csv) file. The usual way to read / parse those files is to
read them line by line
skip headers and empty lines
use String#split(String reg) to split a line into values (reg is chosen to match the delimiter)
Related
I have a java program that reads lines of a text file into a buffer and when the buffer is full it outputs the lines so that after all lines have been through the buffer the output is partially sorted.
The output will be in blocks of lines so I need a way to mark the end of each block in the output. Since the output is lines of text I'm not sure what character to use as a marker since the text can contain any characters. I'm thinking of using the ascii null or unit separator but I'm not sure if this would be reliable since it could also be in text.
You could use a Map, so you can set a key for every buffergroup something like that
Hash<int,Buffer> myMap = new HashMap<>();
if you are not sure how to discriminate lines, I suggest you take a look at a sentence tokenizer tool which is usually used in NLP. These programs contain patterns that discriminate lines from each other. That way, you can send all your date through and get the lines without worrying about wich character to use. There are plenty libraries for Java which does the job perfectly (Assuming your text is in English)
I'm currently working with pulling a CSV file from a URL and modifying it's entries. I'm currently using a StreamReader to read each line of the CSV and split it into an array, where I can modify each entry based on its position.
The CSV is generated from an e-form provider where a particular form entry is a Multi-Line field, where a user can add multiple notes. However, when a user enters a new note, they are separating each note by a line return.
CSV Example:
"FName","LName","Email","Note 1: some text
Note 2: some text"
Since my code is splitting each CSV entry by line, once it reaches these notes, it believes it to be a new CSV entry. This is causing my code that modifies the entries to not work since the element positions become incorrect. (CSV entries with empty or single line note fields work fine)
Any ideas on the best approach to take for this? I've tried adding code to replace carriage returns or to skip empty lines but it doesn't seem to help.
You can check for first column value in a row is null or not. If it is null continue to read next line.
Assuming the CSV example you have provided is supposed to be just one entry in the CSV file (with the last field spanning over several different lines due to newline breaks), you could try something like this, using 2 loops.
Keep a variable for the current CSV record (of String[] type) currentRecord and a recordList (a List or an array) to keep all the CSV records.
Read a line of the CSV file
Split it into an array of strings using the comma as the delimiter. Keep this array in a temporary variable.
If the size of this array is 1, append this string to the last element (4th) in currentRecord (if currentRecord is not null).
Keep reading lines off the CSV file, and repeating step 4 until the array size is 4.
If the size is 4, then this indicates that the record is the next record in the CSV file and you can add the currentRecord to recordList
Keep repeating steps 2 to 6 until you reach the end of the CSV file
It would be better if you can remove the line breaks in the field and clean the CSV file before parsing it though. It'll make things much simpler.
Use a proper CSV library to handle the writing and parsing. There's a few edge cases to handle here, not only the new line. Users could also insert commas or quotes in their notes and it will become very messy to handle this by yourself.
Try uniVocity-parsers as it can handle all sorts of situations when parsing and writing CSV.
Disclosure: I am the author of this library. It's open-source and free (Apache V2.0 license).
am reading xml file in List in such a way:
List<String> file_lines = Files.readAllLines(path);
My file has 317 lines, but in list appears 316. Main problem is that lat one is empty (just carriage return probably)
How can I deal with this? I need precisely every line of file, because am calculating crc32 checksum for file validation.
I need precisely every line of file, because am calculating crc32 checksum for file validation.
In that case, you are reading it the wrong way. You need to read it and checksum it as bytes rather than characters:
to avoid problems with text transcoding issues,
to avoid the problem of dealing with the last line of the file (which may or may not have an end of line ...), and
to avoid having to deal with variants in end-of-line markers ... that you typically cannot distinguish with a "read line" or "read all lines" API method.
i want to write strings to a textfile, everytime to the bottom of the file. And then if im searching for a certain string in the textfile and finds it, i want to replace that line with another.
I'm thinking this: Count rows in textfile and add +1 and then write the string i want to write to that index. But is it even possible to write to a certain linenumber in a textfile?
And how about to update a certain row to another string ?
thanks!
You do not want to do that: it is a recipe for disaster. If, during the original file modification, you fail to write to it, the original file will be corrupted.
Use a double write protocol, write the modified file to another file, and only if the write suceeds, rename that file to the original.
Provided your file is not too big, for some definition of "big", I'd recommend creating a List<String> for the destination file: read the original file line by line, add to that list; once the list processing is complete (your question is unclear what should really happen), write each String to the other file, flush and close, and if that succeeds, rename to the original.
If you want to append strings, the FileOutputStream does have an alternate constructor which you can set to true so you can open for appending.
If you'd like, say, to replace strings into a file without copying it, your best bet would be to rely in RandomAccessFile instead. However, if the line length is varying, this is unreliable. For fixed-length records, this should work as such:
Move to the offset
Write
You can also 'truncate' (via setLength), so if there's a trailing block you need to get rid, you could discard as such.
A Third Solution would be to rely in mmap. This requires on a Memory-Mapped Bytebuffer for the whole file. I'm not considering the whole feasibility of the solution (it works in plain C), but that actually 'looks' the more correct, if you consider both the Java Platform + the Operating System
I need to write a parser for textfiles (at least 20 kb), and I need to determine if words out of a set of words appear in this textfile (about 400 words and numbers). So I am looking for the most efficient possibilitie to do this (if a match is found, i need to do some further processing of this and it's previous line).
What I currently do, is to exclude lines that do not contain any information for sure (kind of metadata lines) and then compare word by word - but i don't think that only comparing word by word is the most efficient possibility.
Can anyone please provide some tips/hints/ideas/...
Thank you very much
It depends on what you mean with "efficient".
If you want a very straightforward way to code it, keep in mind that the String object in java has method String.contains(CharSequence sequence).
Then, you could put the file content into a String and then iterate on your keywords you want to check to see if any of those appear in String, using the method contains().
How about the following:
Put all your keywords in a HashSet (Set<String> keywords;)
Read the file one line at once
For each line in file:
Tokenize to words
For each word in line:
If word is contained in keywords (keywords.containes(word))
Process actual line
If previous line is available
Process previous line
Keep track of previous line (prevLine = line;)