Java While循环与面向绝对初学者的Java编程中提出的挑战有关

Question

I am attempting to learn Java Programming on my own (without classes/teachers/tutors/etc.) so this is not a homework assignment.

My question has been alluded to in Java Challenge for beginners: Generate random numbers in a while loop , however, I have not advanced far enough into this book to have covered ArrayLists, as stated by the prior programmer who asked this question.

Answering this question definitely would help most beginner programmers better understand how to write statements for while loops, an often intricate process, especially for beginners.

The question is: Write a while loop that generates random numbers between 1 and 100 and stops looping after it generates the same number twice.

As you will recognize, running this program only creates an output of two random numbers between 1 and 100, each time it is run. I believe this error is encountered due to line 17: equal = numberCheck == values[i]; being incorrect since this is the statement defining when the while loops condition becomes true and therefore stops evaluating.

Any help is greatly appreciated. Thank you.

My code is:

import java.util.Random;

public class SameNumber {

    public static void main(String args[]) {

        Random rand = new Random();
        int[] values = new int[100];
        int counter = 0;
        boolean equal = false;

        for (int i = 0; i < 2; i++) {
            values[i] = rand.nextInt(100) + 1;
            System.out.println(values[i]);
            int numberCheck;
        }

        while (!equal) {
            for(int i = counter - 1; i >= 0; i--) {
                int numberCheck = values[i];
                equal = numberCheck == values[i];
             //}
                if (!equal) {
                    System.out.println(rand.nextInt(100) + 1);
                }

                for (int j = counter - 1; j >= 0; j--) {
                    if (numberCheck  == values[j]) {
                        break;
                    }
                }
            }
        }
    }
}

PS: I understand that this may sound ridiculous, however I have been working on this problem for about a week and trying to gather additional research, and I still could not arrive at the proper solution. I have been hesitant to ask for help since I am concerned that other programmers would answer with methods that I have not yet learned.

Answer 1

Your approach is pretty indirect...let's just try it by directly following the instructions:

1. Generate a new random number
2. Determine if you've seen this random number before
3. No? Record that you've seen this random number and go to 1.
4. Yes? Stop

Then the real question is, "how do I determine if I've seen the random number before?".

Since it's a finite set, you can have a boolean array ( boolean seen[100] ) that you initialize to false. Whenever you see a number, mark seen[<number>] as true . If seen[<number from 2>] is ever true, you've generated it before.

Using an ArrayList to track these things, by the way, wouldn't be the best choice for data structures. When you're ready to look at Java collections, check out HashSet , which has constant time (does not depend on the size of the list) execution for contains .

Answer 2

I would set values to be an array of boolean values, and set it to true if your number is there. Then you would just use your random number as the index to that array:

boolean[] values = new boolean[100];

int index = Random.nextInt(100);
while(!values[index]){
    System.out.println(index+1);
    values[index] = true;
    index = Random.nextInt(100);
}

Answer 3

try this: 1. Create a random number between 0 to 100 2. Add it to the list if it is not already inside it. 3. If the newly generated number is already inside arraylist stop

import java.util.ArrayList;
import java.util.Random;

public class SameNumber {
    public static void main(String args[]) {
        Random rand = new Random();
        ArrayList<Integer> list=new ArrayList<Integer>();       
        int newRand=rand.nextInt(100);
        while(!list.contains(newRand)){
            list.add(newRand);
            newRand=rand.nextInt(100);
        }
        System.out.print(list);
    }
}

Answer 4

Even though this is not homework (I hope this doesn't start a trend of homework questions claiming to be non-homework), I still think you are more helped by a pointer, rather than a solution.

I'll rewrite what your code actually does in pseudo-code English so you can see where you are going wrong.

Create a random number generator
Create a 100 element array
Initialize a counter
Initialize a boolean
Loop twice and assign two random numbers to the first
     two elements in the 100 element array (this is a problem)
In the loop: Print out the value of the number assign
In the loop: Meaningless line that will be ignored by the compiler
Start a loop until the boolean initialized above becomes true
Start a loop, initializing the counting variable to -1,
    that will stop when it is greater than or equal to zero, decreasing it
    every time (that means this will never run).
In the loop: Reference an illegal position in the array (-1 and below)
    and assign it to a variable (this is a problem)
In the loop: Check if the variable you just assigned equals what
    you just assigned it (this is a problem)
In the loop: If it doesn't (that can't happen) print out some random number
    unrelated to anything that has happened until now (this is a problem)
In the loop: Start a nested loop with the same issues as the other loop
In the nested loop: check through all the values
    (although they are illegally referenced) in the array equals the number
    pulled from the outer loop. [This is kind of a point, but there are just
    too many problems up to this point to even think about correcting this line]