Thread: Generate random Number without repetition.... Need advice and some help..

Hi, I am writing a program to generate 6 random numbers from 1 to 40 inclusive.

So far, I have been using the code below..

However, the only problem is that from time to time, I'm getting repeating numbers.

Is there any way to solve this issue???

Here's the code:

Code:

# define M 6

int main()
{
    int j;
    int m_num[j];
    time_t seconds;
    
    int lower=1;
    int upper=40;
    
    time(&seconds);
    
    srand((unsigned int)seconds);
    
    for(j=0; j<M; j++)
    { 
        m_num[j]=rand()%(upper-lower+1)+1;   
    }
    
    printf("Generated numbers are:");
    
    for(j=0; j<M; j++)
    {
       printf(" \n %d \n", m_num[j]);
    }
    
    system("PAUSE");
    return 0;
}

note: i'm using Dev c++, and also Visual Studio sometimes..

Code:

int j;
int m_num[j];

This is problematic, because j has no known value at the time you use it to initialize m_num. Perhaps you meant M?

The easiest way to achieve what you want is probably to create an array with the values 1–40, shuffle that array, then pick off the first six values. Search for Fisher-Yates to see how to effectively shuffle an array.

Ok, I somehow managed to get another method to work..

But when generating the random numbers... 0 also is being output... and I don't want this! I just want 1-40 inclusive..

Below is the code: Any help is greatly appreciated

Code:

# define N 40


int main() {
    int ran_num[N];   
    
    srand(time(0));  
     
    for (int i=0; i<40; i++) 
    {
        ran_num[i] = i;  
    }
    
    
        for (int i=0; i<(40-1); i++) 
        
        {
            int r = i + (rand() % (40-i)); 
            int temp = ran_num[i]; 
            ran_num[i] = ran_num[r]; 
            ran_num[r] = temp;
        }
        
        for (int c=0; c<6; c++) 
        {
            printf(" %d ", ran_num[c]);  
        }
        
        printf("\n");
   
   system("PAUSE");
   return 0;
}

Many thanks.. It solved the problem..

Why would you shuffle the array, when you could simply scan it?

Code:

int array[40];
int i = 0;
int j;

while( i < 40 )
  { array[i] = (rand() % 40) + 1;
     for (j = 0; j < i; j++)
       if (array[j] == array[i])
         break;
     if ( ! (j < i) )
       i++; }

[...] rejecting repetition is a good solution.

The reason I suggested shuffling an array is that rejecting repetition has the possibility of infinitely looping. If the PRNG gets in a loop of generating numbers, none of which satisfies the necessary condition, then you've got a problem.

Of course, this is rather unlikely, but PRNGs have historically been one of the weak areas of implementation so I'd err on the side of caution. This is also one of those problems that might always work on one system no matter how much you stress test it, but could fail elsewhere.