how to create an array of random number, all different from each other?

Question

The program should just print out the elements of the array, which stores random integers between 10 and 30. I wanted the numbers to be different from each other, but my program isn't working, what is wrong with it? thanks CODE:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main()
{
    const int N=12;
    int arr[N],i,j;
    srand(time(0));
    for(i=0; i<N; i++)
        arr[i]=10+rand()%20;

    for(i=0; i<N; i++)
    {
        for(j=N-1; j == 0; j--)
        {
            do
            {
                arr[i]=10+rand()%20;
                if(arr[i]!=arr[j])
                    break;
            }
            while(arr[i]==arr[j]);
        }
        printf(">>%d\n",arr[i]);
    }

    return 0;
}

How do you that with a roulette (giving random numbers), a pencil, and a paper? How do you detect (on paper) that 12 numbers are all different? — Basile Starynkevitch
– Basile Starynkevitch, Commented Jan 31, 2021 at 10:16
@basile-starynkevitch I Don't know I tried by comparing the number with all the others,each time, but apparently isn't working — Nome A caso
– Nome A caso, Commented Jan 31, 2021 at 10:24
You first need to be able to do that yourself with a pencil & paper. If you cannot, you won't be able to program the computer doing it! — Basile Starynkevitch
– Basile Starynkevitch, Commented Jan 31, 2021 at 11:30

ojblass · Accepted Answer · 2021-01-31 09:40:20Z

1

The fact that the numbers need to be different from one another means that they are not truly random. You can create another set of numbers with elements 10 through 30 in them. Randomize that list and pull them into your array.

answered Jan 31, 2021 at 9:40

ojblass

21.8k23 gold badges89 silver badges137 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Nome A caso Over a year ago

all right but still my program can't detect equals numbers, the main problem is that one

rossum Over a year ago

If you put unique numbers into the initial list: [10, 11, 12, ... 29, 30] and then shuffle the list, the first ten numbers must be unique.

warchantua · Accepted Answer · 2021-01-31 11:46:02Z

1

C++ version:

const int begin = 10;
const int end = 30;
// creates a vector of 30-10 zeroes
std::vector<int> v(begin-end);
// fill vector with 10, 11, ..., 30.
std::iota (std::begin(v), std::end(v), begin); 
// a source for random seed
std::random_device rd;
// seed this generator with 32-bit number
std::mt19937 g(rd());
// randomly shuffle a vector
std::shuffle(std::begin(v), std::end(v), g);

const int N = 12;
std::vector<int> result(v.begin(), v.begin() + N);

C version:

#include <stdio.h>
#include <stdlib.h>

// https://stackoverflow.com/a/6127606/1953079
void shuffle(int *array, size_t n)
{
    if (n <= 1) { return; }
    
    size_t i;
    for (i = 0; i < n - 1; i++) 
    {
        size_t j = i + rand() / (RAND_MAX / (n - i) + 1);
        int t = array[j];
        array[j] = array[i];
        array[i] = t;
    }
}

int main(){
    const int begin = 10;
    const int end = 30;
    const int N = 12;
    srand(time(0));

    // array that contains elements 10, 11...30
    int nums[end-begin];
    for(int i=0;i<end-begin; i++){
        nums[i] = begin+i;
    }

    // randomly shuffle array
    shuffle(nums, end-begin);

    // take first N elements
    int result[N];
    for(int i=0;i<N;i++){
        result[i] = nums[i];
        printf("%d ", result[i]);
    }
}

edited Jan 31, 2021 at 11:46

answered Jan 31, 2021 at 10:53

warchantua

1,2611 gold badge11 silver badges28 bronze badges

3 Comments

Nome A caso Over a year ago

sorry but I was thinking of some simpler code, this looks kinda advanced and I don't understand it

warchantua Over a year ago

In short, you need to generate a sorted array of unique elements 10, 11...30, then randomly shuffle array elements (std::shuffle, or you can find its implementation and resue it), then take first N elements. This is exactly what this code does.

Luis Colorado Over a year ago

shouldn't you write rand() % (RAND_MAX / (n - i) + 1) instead?

Nome A caso · Accepted Answer · 2021-01-31 11:59:28Z

0

Thanks for the help but after some more looking I found what I was doing wrong and now works. code :

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(){
const int N=12;
int arr[N],i,j;
srand(time(0));
for(i=0;i<N;i++)
{
    arr[i]=10+rand()%30;
}
for(i=0;i<N;i++)
{
    for(j=i+1;j<N;j++)
    {
        
        if(arr[i]==arr[j])
        {
            do
            {
                arr[i]=10+rand()%30;
            }
            while(arr[i]==arr[j]);
        }
    }
    printf(">>%d\t",arr[i]);
}
 return 0;
}

answered Jan 31, 2021 at 11:59

Nome A caso

511 silver badge11 bronze badges

Collectives™ on Stack Overflow

how to create an array of random number, all different from each other?

3 Answers 3

2 Comments

3 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related