扫描文件中的双精度数组 | Java

Question

I was trying to scan a file that has an array of doubles. I'm using the code below but it's only outputting 0.0 for every entry. Why is this and how can I fix it?

Scanner scanner = new Scanner("file.txt");
double[] array = new double[256 * 256];
for (int i = 0; i < array.length; i++) {
    if (scanner.hasNextDouble()) {
        array[i] = scanner.nextDouble();
    }
}
System.out.println(array[0]);

An example of the file I'm scanning is

0.22131145 0.22131145 0.22131145 0.22841525 0.22841525 ...

Answer 1

The main issue is with the instantiation of the Scanner object. In this case you need to pass a File object into it, not just a string, and make sure you specify the correct file path. Refer to the official documentation for advice.

Secondly, you need to use a while-loop. An if-statement will execute only once, but you would want the Scanner to continue looking whilst there is info inside the file.

Thirdly, don't use an array for storing the values. It's too risky because you need to know the size of the array beforehand, meaning that you would need to loop twice, which would be inefficient, or you are hard coding, as you are doing here. If someone were to add or remove values from the file, you will get unexpected results. Rather use a dynamic data structure such as a List .

public static void main(String[] args) throws FileNotFoundException {
    String filepath = "file.txt";
    Scanner scanner = new Scanner(new File(filepath));
    List<Double> list = new ArrayList<>();

    while (scanner.hasNextDouble()) {
        list.add(Double.valueOf(scanner.next()));
    }

    scanner.close();
    System.out.println(list.get(0));
}

Answer 2

There are four problems with your code:

1. Blocker : Scanner expects a File object but you haven't used it in this way. You need to use the following syntax:

    Scanner scanner = new Scanner(new File("file.txt"));

2. Performance : You can improve the performance of your program by including scanner.hasNextDouble() in the condition which checks the value of i , as shown below:

    for (int i = 0; i < array.length && scanner.hasNextDouble(); i++) { array[i] = scanner.nextDouble(); }

   This will terminate the loop as soon as scanner.hasNextDouble() returns false ; otherwise, the loop in your code will continue to run until i < array.length evaluates to false irrespective of the value returned by scanner.hasNextDouble() .

3. Resource leak : You have not closed the Scanner object. Put the following line after the loop finishes:

    scanner.close();

4. Missed functionality : You haven't printed the complete array. Your statement, System.out.println(array[0]) will print only the first element in the array. Change it as follows to print the complete array:

    System.out.println(Arrays.toString(array));

   Given below the code incorporating all the above-mentioned comments:

    import java.io.File; import java.io.FileNotFoundException; import java.util.Arrays; import java.util.Scanner; public class Main { public static void main(String[] args) throws FileNotFoundException { Scanner scanner = new Scanner(new File("file.txt")); double[] array = new double[256 * 256]; for (int i = 0; i < array.length && scanner.hasNextDouble(); i++) { array[i] = scanner.nextDouble(); } scanner.close(); System.out.println(Arrays.toString(array)); } }

5. Memory utilization : You have used a fixed-sized array which is fine if the number of elements to be stored is equal to the size of the array. However, if it is not the case (ie if the number of elements to be stored can be less than or more than the specified size), you should use a Collection eg an ArrayList which is a kind of dynamic array. This will help you in many ways: (a) You will save memory if the number of elements to be stored is less than the specified size (b) You do not need to change your code in order to increase the size of the array when you need to store more elements than you have already specified (c) The Collection provides with a rich API to deal with elements. By leveraging this API, your code can be crisp, more performant, maitainable etc.

   Given below the code incorporating the 5th point:

    import java.io.File; import java.io.FileNotFoundException; import java.util.ArrayList; import java.util.List; import java.util.Scanner; public class Main { public static void main(String[] args) throws FileNotFoundException { Scanner scanner = new Scanner(new File("file.txt")); List<Double> list = new ArrayList<>(); while (scanner.hasNextDouble()) { list.add(scanner.nextDouble()); } scanner.close(); System.out.println(list); } }