如何在Java中跳过第一个int逐行读取文件到数组？

Question

I am trying to read a file in Java and store it into an array. Now the problem is that the file has the following structure. I will give 2 example files below:

input file structure

<number of lines>
<line number> <value>
.
.
.
<line number> <value>

input1.txt

5 
1 34
2 19
3 43
4 62
5 36

input2.txt

4
1 10.3430423
2 -34.234923
3 -100.39292
4 22

As you can see the file starts with the number of lines(eg 4 or 5). In the normal input text I have there are more than 100000 lines.

So my code basically grabs the user input, opens the file, create an array size and an array of that size. Now I am stuck on reading the next line and adding the values to my elements array. The line number should not be added into the array. Now as you can see, my elements array is declared as String. Is it possible to actually read the file, get the type of the value and create an array of that type? I think it could save from converting from string to int or double or floating?

Below is my code:

   public static void main(String args[]) throws NumberFormatException, IOException{
        String inFile; //Input file name.
        int filterSize; //Filter size (odd integer >= 3).
        String outFile; //Output file name.
        int arraySize;
        String[] elements;
        int index = 0;

        //Scanner to take input file name, filter size and output file name.
        Scanner keyboardInput = new Scanner(System.in);
        System.out.println("Enter your keyboard input as follows: <data file name> <filter size(odd int >= 3> <output file name>");

        //Assigning values to variables.
        inFile = keyboardInput.next();
        filterSize = keyboardInput.nextInt();
        outFile = keyboardInput.next();


        //Reading file into array using BufferReader
        BufferedReader fileInput = new BufferedReader(new FileReader(inFile));
        arraySize = Integer.parseInt(fileInput.readLine()); //Get Array Size

        elements = new String[arraySize];

        while(fileInput.readLine() != null){
            elements[index] = fileInput.readLine();
            index += 1;
        }               
    }

Any help is appreciated. Thanks.

Answer 1

Try doing it like this:

    Scanner sc = new Scanner(new File(inFile));
    arraySize = sc.nextInt();
    elements = new String[arraySize];

    while(sc.hasNext())
    {
        sc.nextInt();
        elements[index] = sc.next();
        index += 1;
    }               

You create new Scanner and you can than read integers, booleans and so on, without any converting. Because you don't need line of current number, you just read that number and that is it. You don't need to save it anywhere. Then the next number/string you must save at elements[index] . That's it

Answer 2

Go stream-based:

Files.lines(pathToFile).skip(1) // skip the line counter. Don't trust input
   .map(line -> line.replaceFirst("\\d+", "")) // remove digits at the start
   .collect(Collectors.toList()); // collect it into a list

You can Store it into an array with .toArray() anyways

But actually you should do that with a try-with-resources :

try (Stream<String> lines = Files.lines(pathToFile).skip(1)) {
    elements = lines.map(line -> line.replaceFirst("\\d", "")).collect(Collectors.toList());
} catch (IOException e) {
    // sensible handling...
}

Answer 3

Read the number, never use it.

File file = new File("input1.txt");
Scanner in = new Scanner(file);
int lines = in.nextInt();
int []a = new int[lines];
for(int i = 0; i < lines; i++)
{
 int fakeNumber = in.nextInt();//read it but never used it
 a[i] = in.nextInt(); 
}

You could also use in#hasNextLine() also.

Answer 4

The real question here is how to get rid line number and get the value besides it.

double[] array = .... // initialize your array
int v = 0;
while(file.readLine() != null){
      // your array
      array[v++] = Double.parseDouble(file.readLine().split(" ")[1]);
}