Reading from Input Files in Java

Question

I've been trying to figure out how to do this code, but can't figure out how to change my current code to work.

The problem reads:

Write a program to read an integer N (2 <= N <= 30) from an input filename “input5_01.txt” and then write a function to create an N x N matrix with random integers from the range [10, 99]. Output that matrix to the output filename “output5_01.txt”

Currently, here's my code:

public static final int UPPER = 99, LOWER = 10;

public static void main(String[] args) throws FileNotFoundException {
    int n = 5;
    int[][] arr = new int[n][n];
    Scanner inputFile = new Scanner(new File("input5_01.txt"));

      //read in an int (n) from input file
    while (inputFile.hasNext()) {
        if (inputFile.hasNextInt() >= 2 && inputFile.hasNextInt() <= 30) {
            n = inputFile.nextInt();
        } else {
            inputFile.next();
        }
    }

     //assigning random numbers to array
    Random rand = new Random();
    for (int r = 0; r < arr.length; r++) {
        for (int c = 0; c < arr[0].length; c++) {
            arr[r][c] = rand.nextInt(UPPER - LOWER + 1) + LOWER;
        }
        System.out.println();
    }
}

Really, the only problem I have is:

if(inputFile.hasNextInt() >= 2 && inputFile.hasNextInt() <= 30)

What can I do to make this work?

Are you getting an error? What is happening with your current code? — ToddB, May 11 '15 at 14:23
As a best practice, specify a character set: `new Scanner(new File("input5_01.txt"), "UTF-8");` — Necreaux, May 11 '15 at 14:35
[how-to-use-java-util-scanner-to-correctly-read-user-input-from-system-in](http://stackoverflow.com/questions/26446599/how-to-use-java-util-scanner-to-correctly-read-user-input-from-system-in) — , May 11 '15 at 15:46

score 0 · Answer 1 · answered May 11 '15 at 14:27

As per the JavaDoc, hasNextInt() returns a boolean, denoting if there is another integer value or not within the stream, it does not return the value of the next integer. Thus, this section:

while(inputFile.hasNext()){
  if(inputFile.hasNextInt() >= 2 && inputFile.hasNextInt() <= 30)
    n = inputFile.nextInt();   
  else
    inputFile.next();
}

Would need to read something like so:

int nextInt = -1;
boolean intFound = false;

while(inputFile.hasNext()){
  if(inputFile.hasNextInt()) {
      nextInt = inputFile.nextInt();
      if((nextInt >= 2) && (nextInt <= 30) {
          intFound = true;
          break;
      }
  }   
}

if(intFound) {
     ... //Populate your matrix according to the value of nextInt
}

score 0 · Answer 2 · answered May 11 '15 at 14:34

If your input file will have only ints then I believe this should also work:

while(inputFile.hasNextInt()){
  int nextIntValue = inputFile.nextInt();
  if(nextIntValue >= 2 && nextIntValue <= 30) {
    n = nextIntValue ;   break;
  }
  else
    inputFile.next();
}

It will also avoid unnecessary calls.

Reading from Input Files in Java

2 Answers2