What's wrong with my code? Generate Unique Random Number

Question

I'm trying to generate 10 unique random numbers between 1 and 10. I keep getting duplicate numbers. Can someone tell me what the problem is? What am I missing or need to fix in my code? Thank you!

students[0].id = rand() % 10 + 1;
for (int i = 1; i < 10; i++)
{
    students[i].id = rand() % 10 + 1;
    for (int j = 0; j < i; j++)
    {
        if (students[i].id == students[j].id)
        {
            students[i].id = rand() % 10 + 1 ;
        }
    }
}

for (int i = 0; i < 10; i++)
{
    printf("%d\n", students[i].id);
}

A random number is never guaranteed to be unique. You will have to think of a better idea :) — leppie
– leppie, Commented Apr 11, 2013 at 5:45
You should make a set of 10 numbers, and remove the next one picked (using rand) from the set, or keep calling rand until you get one that still remains in the set (then remove it). — GalacticJello
– GalacticJello, Commented Apr 11, 2013 at 5:55
Use this formula to generate random number M + rand() / (RAND_MAX / (N - M + 1) + 1) — Grijesh Chauhan
– Grijesh Chauhan, Commented Apr 11, 2013 at 5:59

Mani · Accepted Answer · 2013-04-11 05:51:53Z

2

if (students[i].id == students[j].id)
        {
            students[i].id = rand() % 10 + 1 ;
        }

In this line , you may get duplicate.

if students[i].id & students[j].id = 5 means it will get true. But in this line students[i].id = rand() % 10 + 1 ; ,you may get again 5

Instead of above line, you may use this code.

students[0].id = rand() % 10 + 1;
for (int i = 1; i < 10; i++)
{
    students[i].id = rand() % 10 + 1;
    for (int j = 0; j < i; j++)
    {
        if (students[i].id == students[j].id)
        {
           i--;
           break;
        }
    }
}

answered Apr 11, 2013 at 5:51

Mani

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Comments

cnicutar · Accepted Answer · 2013-04-11 05:45:46Z

2

The function rand() isn't guaranteed to generate unique random numbers. Moreover, your way of limiting the range (mod 10) is especially bad and is likely to generate many duplicates.

The simplest way to generate your sequence is probably to shuffle an array of 10 values from 1 to 10.

answered Apr 11, 2013 at 5:45

cnicutar

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1 Comment

J.Miller Over a year ago

Agreed, though it should be made clear that the numbers in the array will not be RANDOM, they will be in a RANDOM order

Community · Accepted Answer · 2017-05-23 11:57:03Z

1

rand() is random in nature, so there is no guarantee to give you unique results. You need to do something like this - keep track of all the numbers encountered so far and call the rand() till you find unique numbers

edited May 23, 2017 at 11:57

CommunityBot

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answered Apr 11, 2013 at 5:45

karthikr

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Comments

J.Miller · Accepted Answer · 2013-04-11 05:45:59Z

0

Random does not mean "No Duplicates".

answered Apr 11, 2013 at 5:45

J.Miller

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Comments

Chiara Hsieh · Accepted Answer · 2013-04-11 05:47:31Z

0

After you entered this if clause if (students[i].id == students[j].id) and modified students[i].id, you need to check for duplicates again.

answered Apr 11, 2013 at 5:47

Chiara Hsieh

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Comments

Gaurav · Accepted Answer · 2013-04-11 05:59:21Z

0

Try This One: replace if with while. I hope you get your answer.

students[0].id = rand() % 10 + 1;

for (int i = 1; i < 10; i++)

{

students[i].id = rand() % 10 + 1;
for (int j = 0; j < i; j++)
{
    while (students[i].id == students[j].id)
    {
        students[i].id = rand() % 10 + 1 ;
    }
}

}

for (int i = 0; i < 10; i++)

{

printf("%d\n", students[i].id);

}

answered Apr 11, 2013 at 5:59

Gaurav

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Comments

Nishanth · Accepted Answer · 2013-04-11 06:09:16Z

0

I think you want to sample without replacement. You can store the indices in an array, randomly pick up one & simultaneously remove it from the array.

So that when you draw next time, it doesn't repeat.

answered Apr 11, 2013 at 6:09

Nishanth

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Comments

score 0 · Accepted Answer · 2013-04-11 06:19:01Z

0

You need recursion.

public static void main(String[] args)
{
    System.out.println(generateRandomNumbers(10, new ArrayList<Integer>()));
}

private static List<Integer> generateRandomNumbers(Integer maxLimit, List<Integer>    randomNumberList)
{
    for (int i = 0; i < maxLimit; i++)
    {
        Integer tempRandom = new Random().nextInt(10);
        if (randomNumberList.contains(tempRandom))
            generateRandomNumbers(1, randomNumberList);
        else
            randomNumberList.add(tempRandom);
    }
    return randomNumberList;
}

edited Apr 11, 2013 at 6:19

answered Apr 11, 2013 at 6:11

user1969106

4 Comments

autistic Over a year ago

You're answering a C question with Java code, which uses recursion? What justification do you have that this unbound recursive function won't invoke undefined behaviour by proxy of a "stack overflow"?

user1969106 Over a year ago

Does it really matter? I've just wanted to show the algorithm. With C, there will be a function, and it'll call itself when generated random number exists in generated number list already. BTW, i don't guarentee the possibility of exception or error.

autistic Over a year ago

There are no "exceptions" in C, and C has "undefined behaviour". Code that invokes undefined behaviour may appear to function as desired on your system, while failing on other systems in subtle or devastating ways. Perhaps a segfault will be thrown if someone causes input that overflows the stack, or perhaps it'll be an opportunity for them to maliciously compromise the system...

user1969106 Over a year ago

I said i don't guarentee the possibility of exception or error. It was an example for the question. I didn't mean that i solved the question. You know C, you are the champion, solve it if you can.

autistic · Accepted Answer · 2013-04-11 06:46:06Z

What happens if RAND_MAX (commonly 32767) isn't evenly divisible by 10? You're likely to get values between 1 and 7 more often than 8 and 0. That's a bias.

I would suggest discarding any values greater than or equal to 32760 (or rather, RAND_MAX - RAND_MAX % 10), and using the division operator to construct your random number:

int x;
do {
    x = rand();
} while (x >= RAND_MAX - RAND_MAX % 10);
x /= RAND_MAX / 10;

You'll see a fairly drastic improvements from this; In fact, that seems like your most significant bias. However, the distribution of the values you get still isn't required to be uniform. Use a lookup table to discard any values you've previously selected:

int selected[10] = { 0 };
for (int i = 0; i < 10; i++) {
    int x;
    do {
        x = rand();
    } while (selected[x / RAND_MAX / 10] || x >= RAND_MAX - RAND_MAX % 10);
    x /= RAND_MAX / 10;
    selected[x] = 1;

    student[i].id = x;
}

for (int i = 0; i < 10; i++) {
    printf("student[%d].id: %d\n", i, students[i].id);
}

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