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

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);
}

Answer 1

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;
}
}
}

Answer 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.

Answer 3

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

Answer 4

Random does not mean "No Duplicates".

Answer 5

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.

Answer 6

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);
}

Answer 7

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.

Answer 8

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;
}

Answer 9

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);
}