我的 java 循环在我调用 function 时提前完成，但在未调用 function 时正常工作

Question

I am confused here. I am writing a program that simulates rolling a pair of dice. I have a nested for loop that includes a function call to calculate a value. When I comment out the function call my loop iterates the correct number of times. However, when I include the function call, the loop always terminates early. Can someone please help me explain why this happens?

public class RollDice {
    /** Simulates the rolling of pair of dice
     * 
     * @param num is an int between 2 and 12 inclusive
     * @return is the number of rolls it took to roll the value of num
     */
    static int roll(int num) {
        
        if (num <1 || num > 12) {  // illegal value passed
            throw new IllegalArgumentException("parameter must be between 1 and 12 inclusive");
        }
        
        int x, y;           // integers between 1 and 6 inclusive
        int count = 0;      // number of times dice were rolled
        
        /* roll dice once */
        x = ( int)(6 * Math.random());
        y = ( int)(6 * Math.random());
        
        while (x + y != num) {  // keep rolling dice until they equal num
            
            count++;
            /* roll dice again */
            x = ( int)(6 * Math.random());
            y = ( int)(6 * Math.random());
            
        }
        
        return count;
    }

    public static void main(String[] args) {
        
        /* output a header */
        
        System.out.printf("%10s  %40s", "Total on Dice", "Average Number of Rolls\n");
        System.out.println("---------------------------------------------------------");
        
        for (int num = 2; num <= 12; num++) {
            int total = 0;
            
            for (int i = 0; i <= 10000; i++) {
                total += roll(num); // num goes up to 10 when function is called instead of 12
            } // end inner loop
            
            System.out.println("num = " + num);
            
        } // end outer loop
        
    } // end main()
    
} // end class RollDice

Answer 1

This

( int)(6 * Math.random())

is a random number from 0 to 5. The reason your program stops showing any results when num gets to 11 is that x + y will never be greater than 10.

If you had

(int) (1 + 6 * Math.random())

it would give you a random number from 1 to 6, which I imagine is what you want.

Answer 2

The code is not terminating the loop early, it's actually never returning from the function when num is 11. So your application is actually in an infinite loop and never completes.

That's because you are generating random numbers from 0 to 5 (inclusive). So they never add up to 11 and the function never exits. Add 1 to the rolls and you'll find it now exits.