Usage of java's scanner to read input

Question

Suppose I have the following input:

3
24 1
4358 754
305 794

When I try to read it as follows it doesn't work:

Scanner sc = new Scanner(System.in);
while(sc.hasNextLine())
{
sc.nextLine();
int a = sc.nextInt();
int b = sc.nextInt(); 
//DoSomethingWithAandB
}

I do it that way because first I want to skip the first line. Then I read the two integers. After that if there is another line I go to it with sc.nextLine() and I read the integers again. What is wrong?

Answer 1

In the input above, the first line is supposed to be the number of lines, so once you read it, you can do a loop then read the line and split it so you get your numbers.

Scanner in = new Scanner(System.in);
int numberOfLines = in.nextInt();

for (int i = 0; i < numberOfLines; ++i) {
    String line = in.nextLine();
    String[] lineSplit = line.split(" ");
    int a = Integer.parseInt(lineSplit[0]);
    int b = Integer.parseInt(lineSplit[1]);
    //DoSomethingWithAandB
}

If you don't need to read the first line, you can use while with in.hasNextLine()

Scanner in = new Scanner(System.in);
in.nextInt();

while (in.hasNextLine()) {
    String line = in.nextLine();
    String[] lineSplit = line.split(" ");
    int a = Integer.parseInt(lineSplit[0]);
    int b = Integer.parseInt(lineSplit[1]);
    //DoSomethingWithAandB
}

Answer 2

The 3 on the first line indicates how many lines you need to read, so you should use that information rather than only relying on hasNextLine() , which is not using all the information provided to you:

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

class FileInputExample {
  public static void main(String[] args) {
    File file = new File("input.txt");
    try {
      Scanner scanner = new Scanner(file);
      int n = scanner.nextInt(); // This reads the 3 in your example
      for (int line = 1; line <= n; line++) {
        int a = scanner.nextInt();
        int b = scanner.nextInt();
        // Do something with a and b like store them in ArrayLists or something
        System.out.println(String.format("A: %d, B: %d", a, b));
      }
      scanner.close();
    } 
    catch (FileNotFoundException e) {
      e.printStackTrace();
    }
  }
}

Output:

A: 24, B: 1
A: 4358, B: 754
A: 305, B: 794

input.txt:

3
24 1
4358 754
305 794

---

If you do want to use hasNextLine() in a while loop, you need to skip the first line with the 3, outside the while loop:

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

class FileInputExample {
  public static void main(String[] args) {
    File file = new File("input.txt");
    try {
      Scanner scanner = new Scanner(file);
      scanner.nextLine(); // skip the 3 on the first line
      while (scanner.hasNextLine()) {
        int a = scanner.nextInt();
        int b = scanner.nextInt();
        System.out.println(String.format("A: %d, B: %d", a, b));
      }
      scanner.close();
    } 
    catch (FileNotFoundException e) {
      e.printStackTrace();
    }
  }
}