扫描仪Java的偶数和奇数

Question

I try to add only even numbers to ArrayList . I work with file using scanner as the most proper tool in my opinion. Path to the file should be written in console. Also I use 2 the most popular ways to define even numbers. The problem is - not only even numbers are adding to my ArrayList . There is my code:

BufferedReader bfReader = new BufferedReader(new InputStreamReader(System.in));
InputStream inputStream = null;
List<Integer> myInts = new ArrayList<Integer>();

String filePath = null;
try {
  filePath = bfReader.readLine();
  inputStream = new FileInputStream(filePath);
} catch (IOException e) { }

Scanner scanner = new Scanner(inputStream);
while (scanner.hasNext()) {
  if ((scanner.nextInt() % 2) == 0 && scanner.nextInt() != 1) 
    myInts.add(scanner.nextInt());
  // if ((scanner.nextInt() & 1) == 0)
  //   myInts.add(scanner.nextInt());
}

for (Integer x : myInts) {
  System.out.println(x);
}

I suppose I misunderstand something about Scanner .
Would be glad to receive any answers!

Answer 1

The reason is that every new call of nextInt() read new integer from input.

Here is a modified code snippet that illustrates what you might want to try:

Scanner scanner = new Scanner(inputStream);
int myInt;

while (scanner.hasNext()) {
  myInt = scanner.nextInt();

  if ((myInt % 2) == 0 && myInt != 1) 
    myInts.add(myInt);
}

For more information look at the docs .

Answer 2

Every time you call nextInt , it takes an item out of the scanner. That means that one pass through your loop removes as many as three items, and the items being added are not the same as the ones you're doing your checks on.

Imagine your input is 4 3 1

Your code will do this:

if ((scanner.nextInt() /* 4 */ % 2) == 0 && scanner.nextInt() /* 3 */ != 1) 
    myInts.add(scanner.nextInt() /* 1 */);

And add 1 to your list.

You should change your code to this:

while (scanner.hasNext())
{
    int value = scanner.nextInt();
    if ((value % 2) == 0) 
        myInts.add(value);
}

This will only read a single value, and use it in all comparisons.

Answer 3

The problem here lies in

if ((scanner.nextInt() % 2) == 0 && scanner.nextInt() != 1) 

Every time you call scanner.nextInt() , you consume the next input. Because of this, you end up discarding most of the input. To fix this, you would need to have something like

    while (scanner.hasNext())
    {
        int i = scanner.nextInt;
        if ((i % 2) == 0 && i != 1) 
            myInts.add(i);
    }

This will properly consume the input and should work properly. The scanner javadoc, which contains this information, is found here: https://docs.oracle.com/javase/7/docs/api/java/util/Scanner.html

Answer 4

Yes I think you have understood it wrong. Whenever you use nextInt() method of the scanner class pointer which scanning file will move to the nextInt(). So it is better to save that integer values in temporary variable. Below is the modification of your code,

BufferedReader bfReader = new BufferedReader(new InputStreamReader(System.in));
    InputStream inputStream = null;
    List<Integer> myInts = new ArrayList<Integer>();

    String filePath = null;
    try
    {
        filePath = bfReader.readLine();
        inputStream = new FileInputStream(filePath);
    }
    catch (IOException e)
    {
    }

    Scanner scanner = new Scanner(inputStream);
    while (scanner.hasNext())
    {
        int firstNumber = scanner.nextInt();

        if ((firstNumber % 2) == 0 && firstNumber != 1) 
            myInts.add(firstNumber);
      //if ((scanner.nextInt() & 1) == 0)
      //    myInts.add(scanner.nextInt());
    }
    for (Integer x : myInts)
    {
        System.out.println(x);
    }

Answer 5

Every time you call scanner.nextInt() you get another number. If you want to refer to the same number multiple times, assign to a variable.

Also, having checked that a number is even, you don't also have to check that it isn't the number 1.

while (scanner.hasNext()) {
    int n = scanner.nextInt();
    if (n%2 == 0) {
        myInts.add(n);
    }
}

Answer 6

In the line

if ((scanner.nextInt() % 2) == 0 && scanner.nextInt() != 1)

You are reading two integers from the input, instead of checking the same one twice:

int nextInt = scanner.nextInt();
if ((nextInt % 2) == 0 && nextInt != 1)