Develop Reference

How to read every second line from a file in java

Can someone tell me how to read every second line from a file in java?
BufferedReader br = new BufferedReader(new FileReader(file));
String line = br.readLine();
while(line != null){
//Do something ..
line = br.readLine()
}
br.close

One simple way would be to just maintain a counter of number of lines read:
int count = 0;
String line;
while ((line = br.readLine()) != null) {
if (count % 2 == 0) {
// do something with this line
}
++count;
}
But this still technically reads every line in the file, only choosing to process every other line. If you really only want to read every second line, then something like RandomAccessFile might be necessary.

You can do it in Java 8 fashion with very few lines :
static final int FIRST_LINE = 1;
Stream<String> lines = Files.lines(path);
String secondLine = lines.limit(2).skip(FIST_LINE).collect(Collectors.joining("\n"));
First you stream your file lines
You keep only the two first lines
Skip the first line
Note : In java 8, when using Files.lines(), you are supposed to close the stream afterwards or use it in a try-with-resource block.

This is similar to #Tim Biegeleisen's approach, but I thought I would show an alternative to get every other line using a boolean instead of a counter:
boolean skipOddLine = true;
String line;
while ((line = br.readLine()) != null) {
if (skipOddLine = !skipOddLine) {
//Use the String line here
}
}
This will toggle the boolean value every loop iteration, skipping every odd line. If you want to skip every even line instead you just need to change the initial condition to boolean skipOddLine = false;.
Note: This approach only works if you do not need to extend functionality to skip every 3rd line for example, where an approach like Tim's would be easier to modify. It also has the downside of being harder to read than the modulo approach.

This will help you to do it very well
You can use try with resource
You can use stream api java 8
You can use stream api supplier to use stream object again and again
I already hane added comment area to understand you
try (BufferedReader reader =
new BufferedReader(
new InputStreamReader(
new ByteArrayInputStream(x.getBytes()),
"UTF-8"))) { //this will help to you for various languages reading files
Supplier<Stream<String>> fileContentStream = reader::lines; // this will help you to use stream object again and again
if (FilenameUtils.getExtension(x.getOriginalFilename()).equals("txt")) { this will help you to various files extension filter
String secondLine = lines.limit(2).skip(FIST_LINE).collect(Collectors.joining("\n"));
String secondLine =
fileContentStream
.get()
.limit(2)
.skip(1)// you can skip any line with this action
.collect(Collectors.joining("\n"));
}
else if (FilenameUtils.getExtension(x.getOriginalFilename()).equals("pdf")) {
} catch (Exception ex) {
}

My Java program doesn't iterate through my entire CSV file

Code:
import java.io.BufferedReader;
import java.io.FileReader;
import java.util.Scanner;
public class dataReader {
#SuppressWarnings("rawtypes")
public static void main(String[] args) throws Exception {
String splitBy =",";
BufferedReader br = new BufferedReader(new FileReader("C:/practice/testData.csv"));
String line = br.readLine();
int counter = 0;
while ((line = br.readLine()) != null){
counter++;
String[] b = line.split(splitBy);
for (int x = 0; x < b.length; x++){
System.out.println(b[x]);
}
}
System.out.println(counter);
br.close();
}
}
When I run it, it goes through the CSV and displays some output but it starts with product ID 4000+. It basically only outputs results for the last thousand rows in the CSV. I'm wrangling with a really ugly CSV file to try to get some useful data out of it so that I can write all of it into a new database later on. I'd appreciate any tips.

As in the comments, the issue is that System.out.println prints to your console. The console will have a limit to the number of lines it can display, which is why you see the last ~1000 lines.

At a frist glance at your code, you skip the first line by assigning a readline() to your string line
String line = br.readLine();
You should change that as you read the first line and than enter the loop which will read the 2nd line before any operations on the first line took place.
Try something like
String line = "";

Your code won't handle more complicated CSVs, eg a row:
"Robert, John", 25, "Chicago, IL"
is not going to get parsed correctly. Just splitting on ',' is going to separate "Robert, John" into multiple cells though it should be just 1 cell.
The point is, you shouldn't be writing a CSV parser. I highly recommend go downloading the Apache Commons CSV library and using that! Reading CSVs is a solved problem; why reinvent the wheel?

Without seeing the structure of the file I can only speculate, but this line: String line = br.readLine(); needs to be changed to String line = "" or String line = null. As it currently stands, you are discarding your first line.

Reading a file and performing functions based on the contents of the line

I'm wondering if it's possible to read a file by the line number, each with different values and make a condition where if that line contains a certain string or number specified. If it did it would, for example, take the content specified in that line into a variable?
So in a file line one has Age: 50, line 2 has Age: 23, line 3 has Age: 34. What I'm hoping for is that I look specifically at line 3 and take the number 34 and place it in a variable for use in my program.
If it is possible, how would you go about doing this?

I would say, it is not possible to directly address a specific line unless - perhaps you know the line sizes of your file, etc... to seek through the file. But you can use this to go through your file, line by line:
String line;
BufferedReader br = new BufferedReader(new FileReader(file));
while ((line = br.readLine()) != null) {
// do some cool stuff with this line.
}
br.close();

Possible duplicate: Reading a file and performing functions based on the contents of the line
You can always iterate through each line, keeping track of the line via an int or a short.
some code:
String line;
BufferedReader br = new BufferedReader(new FileReader(FILE_HERE));
int line = 0;
while ((line = br.readLine()) != null) {
line++;
if(line == 3){
//do whatever you want
}
}
br.close();
I will add that getting something from one single line while others also have identical info is bad

you can use the scanner object to read through a file. you would use the delimiter to find the info you want and a counter to keep track of the line, then put it in an arraylist or something. depending on what you want to do.
Scanner in = new Scanner(filename);
int line = 0;
in.useDelimiter("[regex of the info you are looking for]");
while in.hasNext()) {
line++
//do something
}

How to parse a text file and create a database record from it

I'm trying to basically make a simple Test Generator. I want a button to parse a text file and add the records to my database. The questions and answers are in a text file. I have been searching the net for examples but I can't find one that matches my situation.
The text file has header information that I want to ignore up until the line that starts with "~ End of Syllabus". I want "~ End of Syllabus" to indicate the beginning of the questions. A couple of lines after that look for a line with a "(" in the seventh character position. I want that to indicate the Question Number line. The Question Number line is unique in that the "(" is in the seventh character position. I want to use that as an indicator to mark the start of a new question. In the Question Number line, the first three characters together "T1A" are the Question Group. The last part of the T1A*01* is the question number within that group.
So, as you can see I will also need to get the actual question text line and the answer lines as well. Also typically after the four Answer lines is the Question Terminator indicated by "~~". I don't know how I would be able to do this for all the questions in the text file. Do I keep adding them to an array String? How would I access this information from the file and add it to a database. This is very confusing for me and the way I feel I could learn how this works is by seeing an example that covers my situation. Here is a link to the text file I'm talking about:http://pastebin.com/3U3uwLHN
Code:
public static void main(String args[]) {
String endOfSyllabus = "~ End of Syllabus";
Path objPath = Paths.get("2014HamTechnician.txt");
String[] restOfTextFile = null;
if (Files.exists(objPath)){
File objFile = objPath.toFile();
try(BufferedReader in = new BufferedReader(
new FileReader(objFile))){
String line = in.readLine();
List<String> linesFile = new LinkedList<>();
while(line != null){
linesFile.add(line);
line = in.readLine();
}
System.out.println(linesFile);
}
catch(IOException e){
System.out.println(e);
}
}
else{
System.out.println(
objPath.toAbsolutePath() + " doesn't exist");
}
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new A19015_Form().setVisible(true);
}
});
}

Reading a text file in Java is straight forward (and there are sure to be other, more creative/efficient ways to do this):
try (BufferedReader reader = new BufferedReader(new FileReader(path))) { //try with resources needs JDK 7
int lineNum = 0;
String readLine;
while ((readLine = reader.readLine()) != null) { //read until end of stream
Skipping an arbitrary amount of lines can be accomplished like this:
if (lineNum == 0) {
lineNum++;
continue;
}
Your real problem is the text to split on. Had you been using CSV you could use String[] nextLine = readLine.split("\t"); to split each line into its respective cells based on tab separation. But your not, so you'll be stuck with reading each line, and than find something to split on.
It seems like you're in control of the text file format. If you are, go to an easier to consume format such as CSV, otherwise you're going to be designing a custom parser for your format.
A bonus to using CSV is it can mirror a database very effectivly. I.e. your CSV header column = database column.
As far as databases go, using JDBC is easy enough, just make sure you use prepared statements to insert your data to prevent against SQL injection:
public Connection connectToDatabase(){
String url = "jdbc:postgresql://url";
return DriverManager.getConnection(url);
}
Connection conn = connectToDatabase();
PreparedStatement pstInsert = conn.prepareStatement(cInsert);
pstInsert.setTimestamp(1, fromTS1);
pstInsert.setString(2, nextLine[1]);
pstInsert.execute();
pstInsert.close();
conn.close();
--Edit--
I didn't see your pastebin earlier on. It doesn't appear that you're in charge of the file format, so you're going to need to split on spaces ( each word ) and rely on regular expressions to determine if this is a question or not. Fortunately it seems the file is fairly consistent so you should be able to do this without too much problem.
--Edit 2--
As a possible solution you can try this untested code:
try{
BufferedReader reader = new BufferedReader(new FileReader("file.txt")); //try with resources needs JDK 7
boolean doRegex = false;
String readLine;
while ((readLine = reader.readLine()) != null) { //read until end of stream
if(readLine.startsWith("~~ End of Syllabus")){
doRegex = true;
continue; //immediately goto the next iteration
}
if(doRegex){
String[] line = readLine.split(" "); //split on spaces
if(line[0].matches("your regex here")){
//answer should be line[1]
//do logic with your answer here
}
}
}
} catch (IOException e) {
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
}

Detected a new line within a series of numbers in Java

Let's say I have a series of numbers like...
1 2 3 4
5 6 7 8
9 0
How could I step through each int, but stop when I reach a new line? I'm currently using nextInt() and I know that nextLine() will detect the new line, but I'm not sure how to piece that together. Is it best to take the entire line, and parse the string into separate ints? Or is there a more fluid method of doing this?
For my example, I would want the program to store 1 2 3 4, 5 6 7 8, 9 0 all in their own separate array.
For more clarification, I'm using the java.util.Scanner and I'm reading a text file.

If you want to use Scanner, read the entire line into a String, and then construct a Scanner on the String.

You can open the text file in read mode and read the entire line with readLine() method.
Then you can split the line read with the space ( ' ' ) character which will automatically give you an array.
You can do this till the end of file.
import java.io.*;
class FileRead
{
public static void main(String args[])
{
try{
// Open the file
FileInputStream fstream = new FileInputStream("textfile.txt");
// Get the object of DataInputStream
DataInputStream in = new DataInputStream(fstream);
BufferedReader br = new BufferedReader(new InputStreamReader(in));
String strLine;
delimiter = " ";
int myArr[];
//Read File Line By Line
while ((strLine = br.readLine()) != null) {
myArr = strLine.split(delimiter);
// store this array into some global array or process it in the way you want.
}
//Close the input stream
in.close();
}catch (Exception e){//Catch exception if any
System.err.println("Error: " + e.getMessage());
}
}
}
Hope this helps.