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Read a specific line from a text file
Is there any way to read a line in a file in java.I mean if i want to read 100th line only then can i read it directly? Or I have to read the whole file until it comes to line 100.
You can use java.io.RandomAccessFile.
Moving to 100th line use the following lines:
RandomAccessFile file = new RandomAccessFile("D:\\test.txt", "rw");
int totalLines = (int)file.length();
file.seek(100);
long pointer = file.getFilePointer();
for(int pt = 100; ct < totalLines; ct++){
byte b = file.readByte(); //read byte from the file
System.out.print((char)b); //convert byte into char
}
file.close();
For more details please see the below link which will helps you:
http://tutorials.jenkov.com/java-io/randomaccessfile.html
No, no matter the abstractions, there really isn't an efficient way of "directly" reading the 100th line from a file-system. You can of course use offsets in case you have lines with fixed lengths per line (assuming CR or LF etc.) but that's it. You can't jump around in a file based on the "line" abstraction.
Under most circumstances you will need to read line-by-line starting from the start of the file.
There are exceptions to this:
If you create and maintain an index for the file that indicates the positions of each line start, you can lookup a line in the index and then seek the file to the position to read it.
If your file consists of fixed length lines, you can calculate the position of the start of a line as line_no * line_length, and then seek to that position.
Related
This question already has answers here:
Read a file line by line in reverse order
(10 answers)
Closed 9 years ago.
I'm trying to implement a log structure file system as an operating system assignment. In it most recent data is placed at end of file. That's why I want to read text file "line-by-line" in reverse order. Is it possible?
Check out ReverseLineInputStream:
https://code.google.com/p/lt2-sander-marco/source/browse/src/leertaak2/ReverseLineInputStream.java?spec=svn15&r=15
It refers to the SO question posted at How to read file from end to start (in reverse order) in Java?
in = new BufferedReader (new InputStreamReader (new ReverseLineInputStream(file)));
while(true) {
String line = in.readLine();
if (line == null) {
break;
}
System.out.println("X:" + line);
}
(Thanks, #Mark O'Donohue)
If its line by line - you could pass all the lines into an arraylist and then read it backgrounds using a reverse for loop such as for(int i = list.size()-1;i>=0;i--)
Yes, but it requires reading through the entire file first. If the file is long, what you can do is split the file into several files, read each file into memory and write the data out to a new file from last line to first line of the old file. When the operation is done, you can delete all the temporary files you created.
I want to read nth line from the end of the file. However my file size is very huge like 15MB, so I cannot go through each line to find out the last line. Is there an efficient way to get this nth line ?
I went through RandomAccessFile API however my line sizes are not constant so i was not able to move my file pointer to that nth line location from the end. Can some one help me.
You basically have to read the file backwards. The simplest approach, without using "block" reads, is to the get the length of the file, and then use RandomAccessFile to read bytes at (length--) until you have counted the required number of line feeds / carriage returns. You can then read the bytes forward for one line.
Something like this....
RandomAccessFile randomAccessFile = new RandomAccessFile("the.log", "r");
long offset = randomAccessFile.length() - 1;
int found = 0;
while (offset > 0 && found < 10) {
randomAccessFile.seek(offset--);
if (randomAccessFile.read() == 10) {
found++;
}
}
System.out.println(randomAccessFile.readLine());
Single byte reads many not be super efficient. If performance becomes a problem, you take the same approach, but read larger blocks of the file (say 8K) at a time, rather than 1 byte at a time.
Have a look at this answer, which shows that you do need to read through the file (15MB is not big). As long as you are are only storing the latest 9 rows, then you will be able to fly through the file.
This question already has answers here:
Replace lines in File with another string
(4 answers)
Closed 9 years ago.
Which class would be best to use in this case and what method should I call to replace a value at specified line ?
The answer of Kayaman is wrong.
In the programming there are in most cases many solutions. That's why "The only way is ..." is wrong.
The solution of Kayaman will only work for very smal files whose size is smaller than the available Java heap size. But then it should be specified in the problem description, like "max file size is 1MB, JVM heap size is 128MB". If you run Java with 256MB heap and your file is 20GB it will just not work.
Even reading and writing line by line may not work, because lines can be longer than the heap size.
The only way is to read all the lines, then write them back out, with the specified line modified. So BufferedReader and BufferedWriter would be your main weapons.
until eof
read line
increment line number
if the line number is the specified line
replace content
end if
write the modified line
end until
If you have the line number then use the BufferedWriter class to get to the line number and then use the BufferedWriter object to write the new line on the same line number which contains the existing line.
If you don not know the number before hand then you would need to read the File first using BufferedReader, arrive at the line number and then replace the line number following the steps above.
I am trying to read a specific line from a text file, however I don't want to load the file into memory (it can get really big).
I have been looking but every example i have found requires either to read every line (this would slow my code down as there are over 100,000 lines) or load the whole thing into an array and get the correct element (file will have alot of lines to input).
An example of what I want to do:
String line = File.getLine(5);
"code is not actual code, it is made up to show the principle of what i want"
Is there a way to do this?
-----Edit-----
I have just realized this file will be written too in between reading lines (adding to the end of the file).
Is there a way to do this?
Not unless the lines are of a fixed number of bytes each, no.
You don't have to actually keep each line in memory - but you've got to read through the whole file to get to the line you want, as otherwise you won't know where to start reading.
You have to read the file line by line. Otherwise how do you know when you have gotten to line 5 (as in your example)?
Edit:
You might also want to check out Random Access Files which could be helpful if you know how many bytes per line, as Jon Skeet has said.
The easiest way to do this would be to use a BufferedReader (http://docs.oracle.com/javase/1.5.0/docs/api/java/io/BufferedReader.html), because you can specify your buffer size. You could do something like:
BufferedReader in = new BufferedReader(new FileReader("foo.in"), 1024);
in.readLine();
in.readLine();
in.readLine();
in.readLine();
String line = in.readLine();
1) read a line which the user selects,
If you only need to read a user-selected line once or infrequently (or if the file is small enough), then you just have to read the file line-by-line from the start, until you get to the selected line.
If, on the other hand you need to read a user-selected line frequently, you should build an index of line numbers and offsets. So, for example, line 42 corresponds to an offset of 2347 bytes into the file. Ideally, then, you would only read the entire file once and store the index--for example, in a map, using the line numbers as keys and offsets as values.
2) read new lines added since
the last read. i plan to read the file every 10 seconds. i have got
the line count and can find out the new line numbers but i need to
read that line
For the second point, you can simply save the current offset into the file instead of saving the current line number--but it certainly wouldn't hurt to continue building the index if it will continue to provide a significant performance benefit.
Use RandomAccessFile.seek(long offset) to set the file pointer to the most recently saved offset (confirm the file is longer than the most recently saved offset first--if not, nothing new has been appended).
Use RandomAccessFile.readLine() to read a line of the file
Call RandomAccessFile.getFilePointer() to get the current offset after reading the line and optionally put(currLineNo+1, offset) into the index.
Repeat steps 2-3 until reaching the end of the file.
But don't get too carried away with performance optimizations unless the performance is already a problem or is highly likely to be a problem.
For small files:
String line = Files.readAllLines(Paths.get("file.txt")).get(n);
For large files:
String line;
try (Stream<String> lines = Files.lines(Paths.get("file.txt"))) {
line = lines.skip(n).findFirst().get();
}
Java 7:
String line;
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
for (int i = 0; i < n; i++)
br.readLine();
line = br.readLine();
}
Source: Reading nth line from file
The only way to do this is to build an index of where each line is (you only need to record the end of each line) Without a way to randomly access a line based on an index from the start, you have to read every byte before that line.
BTW: Reading 100,000 lines might take only one second on a fast machine.
If performance is a big concern here and you are frequently reading random lines from a static file then you can optimize this a bit by reading through the file and building an index (basically just a long[]) of the starting offset of each line of the file.
Once you have this you know exactly where to jump to in the file and then you can read up to the next newline character to retrieve the full line.
Here is a snippet of some code I had which will read a file and write every 10th line including the first line to a new file (writer.) You can always replace the try section with whatever you want to do. To change the number of lines to read just change the 0 in the if statement "lc.endsWith("0")" to whatever line you want to read. But if the file is being written to as you read it, this code will only work with the data that is contained inside the file when you run this code.
LineNumberReader lnr = new LineNumberReader(new FileReader(new File(file)));
lnr.skip(Long.MAX_VALUE);
int linecount=lnr.getLineNumber();
lnr.close();
for (int i=0; i<=linecount; i++){
//read lines
String line = bufferedReader.readLine();
String lc = String.valueOf(i);
if (lc.endsWith("0")){
try{
writer.append(line+"\n");
writer.flush();
}catch(Exception ee){
}
}
}
This is my first question here, I hope you kind sirs can help me and I thank you in advance.
I am trying to write a Java project using threads and the replicated workers paradigm. What I want to do is create a workpool of tasks. The tasks that the workers have to do is simply count the number of words in a specified file between two indices. I want to create a task like this: (file,startIndex,finishIndex). I have problems finding out what file handling class I should use to open a file and read the words from startIndex to finishIndex. I should also mention that I am given a chunk size and I am supposed to split the tasks using that. ChunkSize is an int representing the number of bytes
Bottom line: I want to read from a file from startIndex to startIndex + chunkSize.
I think you are looking for the RandomAccessFile class. It has a "seek" method that allows you to skip to a certain position in the file. Example:
int chunkSize = 64;
long startingIndex = 55;
byte[] bytesRead = new byte[chunkSize];
RandomAccessFile file = new RandomAccessFile("file.txt", "r");
file.seek(startingIndex);
file.read(bytesRead);
file.close();
Note that this will seek a number of bytes, not words. It's impossible to know where are the words in a file before reading it. Counting the spaces and adding one is a naive method that would work well in this case.