如何确定哈希表中的一对值是否满足条件？

Question

I am trying to enter pairs of values (i,j) in an array to determine how many of those pairs respect the following condition: 1 <= i < j <= length of the array. The output is rarely the right answer, I do not know why. Can anyone help me? Here is my code:

public static void main(String[] args) {
        int oldVal[][] = new int [20][2];
        boolean loopAgain = true;
        Scanner scan = new Scanner(System.in);
        int num = 0;
        int i = 0;
        do {
            System.out.print("Enter input 1:");
            int input1 = scan.nextInt();

            System.out.print("Enter input2:");
            int input2 = scan.nextInt();
                
                oldVal[i][0] = input1;
                oldVal[i][1] = input2;
                i++;

            System.out.print("Enter another input (y/n)?");
            String answer = scan.next();

            if (answer.equals("y") || answer.equals("Y")) {
                continue;
            } else {
                for (int x = 0; x < oldVal.length; x++) {
                    if (oldVal[x][0] <= 1 && oldVal[x][0] < oldVal[x][1] && oldVal[x][1] <= 20) {
                        num++;
                    }
                }
                System.out.print("Output number is %d.\n" + num);
                break;
            }
            
        } while (loopAgain);
        scan.close();
    }
}

Thank you!

Answer 1

Your if condition is bad. Your pseudo code states that 1 <= i, however, your if condition states i <= 1, so changing the if would solve the issue:

if (1 <= oldVal[x][0]  && oldVal[x][0] < oldVal[x][1] && oldVal[x][1] <= 20) {
   num++;
}

On the other hand, your if condition to check for a "y" input has a bad design, you should simply negate the condition and get rid of the continue statement. I also would use a regex expression so the user may type y, Y, yes, Yes, yeah, Yeah, YEAH, YES, and all of them would work. Also, the for loop may be improved with a for each (or enhanced for). Also, for the sake of code readability you should declare the num variable inside the if block and before the for loop. ALSO, if you use the break statement to leave the while loop after printing your output whats the use of loopAgain? Just write while(true) , or better add a loopAgain=false instead of break. Adding all those changes would result in:

if (!answer.matches("[yY].*")) {
   int num = 0;
   for (int[] ints : oldVal) {
      if (1 <= ints[0] && ints[0] < ints[1] && ints[1] <= 20) {
         num++;
      }
   }
   System.out.printf("Output number is %d.%n", num);
   loopAgain=false;
}

Also, note that if you want to use the %d stuff, you should invoke System.out.printf (not println), and instead of \\n you should use %n.

Now that it works fine, I suggest you to update to java 16 and research about new ways of doing it (use a Stream<int[]> instead of a bi-dimensional array, or use a HashMap<Integer, Integer>, use lambdas, Stream.filter, Stream.count, and use var instead of explicit types in local variables). It'll help you.