Java: Read array of integers from file

Question

Say i have a file called "input.txt" that has a bunch of positive integers in it:

6
5
6
8
6
2
4

and so on....(one integer per line)

I want to read this file and make it into an array. The first integer (in this case 6) tells the number of indexes or elements in the array, so 6 spots. The other numbers fill in the array starting at 0. So at index 0, the number is 5, at index 1 the number is 6, and so on.

Can someone please show me how to read this file and make it into an array called A and return the integers in each index as n?

this is what i have so far:

import java.io.*;
public class inputFile {
    public static jobScheduleRecursive(int[] A, int i)
    {
        try
    {
        FileReader filereader = new FileReader("input.txt");
        BufferedReader bufferedreader = new BufferedReader(filereader);
        String line = bufferedreader.readLine();
        //While we have read in a valid line
        while (line != null) {
            //Try to parse integer from the String line
            try {
                System.out.println(Integer.parseInt(line));
            } catch (NumberFormatException nfe) {
                System.err.println("Failed to parse integer from line:" + line);
                System.err.println(nfe.getMessage());
                System.exit(1);
            }
            line = bufferedreader.readLine();
        }
    }
    catch(FileNotFoundException filenotfoundexception)
    {
        System.out.println("File not found.");
    }
    catch(IOException ioexception)
    {
        System.out.println("File input error occured!");
        ioexception.printStackTrace();
    }
    return A;
}

I think i'm doing something completely wrong. please help.

Answer 1

Using a Scanner and the Scanner.nextInt() method, you can solve this in just a few lines:

Scanner s = new Scanner(new File("input.txt"));
int[] array = new int[s.nextInt()];
for (int i = 0; i < array.length; i++)
    array[i] = s.nextInt();

Answer 2

I think you need this for ACM-like competitions:) I use following template:

import java.io.*;
import java.util.*;      

public class Task {

    private BufferedReader input;
    private PrintWriter output;
    private StringTokenizer stoken;

    String fin = "input";
    String fout = "output";


    private void solve() { // some solving code...
        int n = nextInt();
        int[] mas = new int[n];
        for (int i = 0; i<n; i++){
            mas[i] = nextInt();
        }
    }



    Task() throws IOException {
        input = new BufferedReader(new FileReader(fin + ".txt"));
        output = new PrintWriter(new FileWriter(fout + ".txt"));

        solve();

        input.close();
        output.flush();
        output.close();
    }


    int nextInt() {
        return Integer.parseInt(nextToken());
    }


    long nextLong() {
        return Long.parseLong(nextToken());
    }


    double nextFloat() {
        return Float.parseFloat(nextToken());
    }


    double nextDouble() {
        return Double.parseDouble(nextToken());
    }


    String nextToken() {
        while ((stoken == null) || (!stoken.hasMoreTokens())) {
            try {
                String line = input.readLine();
                stoken = new StringTokenizer(line);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return stoken.nextToken();
    }


    public static void main(String[] args) throws IOException {
        new Task();
    }

}

In solve() method you can see how to read one number N (length of the following number sequence) and after that in loop (0..N) I read integers from input (in this case input is a file).

Answer 3

Java 8+

int[] ints = Files.lines(Paths.get("input.txt"))
                  .mapToInt(Integer::parseInt).toArray();

Answer 4

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class filee{
    public static void main(String[] args) throws FileNotFoundException {
        File f = new File("l.txt");
        Scanner b = new Scanner(f);
        int[] arr = new int[b.nextInt()];
            for(int i = 0; i < arr.length; i++){
                arr[i] = b.nextInt();
            }
        for (int o : arr){
            System.out.println(o);
        }
    }
}

Answer 5

If file is a classpath resource:

int[] ints = Files
            .lines(Paths.get(ClassLoader.getSystemResource("input.txt")
                    .toURI())).mapToInt(Integer::parseInt).toArray();

Printing the content from file:

 Arrays.stream(ints).forEach(System.out::println);