Thread: help with generating numbers

Hi guys,
Im trying to implement a function/program that generates numbers from 1 to the input number given by the user. I've come close to generating all the numbers however there are some duplicates of the numbers still remain.
I just want to generate the number with no duplicates but my algorithm must be wrong, can anyone see anything wrong with what ive done?

Cheers.

Code:

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#define MAXSIZE 1000
int main()
{
	int k;
	int i;
	int j;
	int num;
	int n=0;
	int tmp[MAXSIZE];
	printf( "Enter integer (max 1000): " );
	scanf( "%d", &k );
	srand(1234567);
	for( i=1; ; i++ ) {
		num = 1+rand()%k;
			for( j=0; j<=MAXSIZE; j++ ) {
				if ( num == tmp[j] )
					break;
				else if ( (num != tmp[j]) ) {
					tmp[n++] = num;
					break;
				}
			}
			
		if ( n==k )
			break;
	}
	
	for( i=0; i<=n-1; i++ )
		printf( "%d ", tmp[i] );
	
	printf( "\n" );
	return 0;
}

Code:

for( j=0; j<=MAXSIZE; j++ ) {
if ( num == tmp[j] )
break;
else if ( (num != tmp[j]) ) {
tmp[n++] = num;
break;
}
}

Look at that code again... it exits as soon as a match is found, and it exits as soon as a match is not found... hmm...

>I just want to generate the number with no duplicates

Here's a way I'd done something similar, if you can stomach a goto.

hmm ive tried placing the tmp array outside of that for loop so that i allow it to scan the whole array for a match.

however if it breaks out of that loop then it will add the value to the tmp array thereafter.

the goto is hard to digest.
but the if statements and what to do next are just as annoying.
im stuck with the conditions.

>the goto is hard to digest.
Then function-ize the inner loop to return true/false.

Code:

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

int foo(const int *a, size_t i)
{
   size_t j;
   for ( j = 0; j < i; ++j )
   {
      if ( a[i] == a[j] )
      {
         return 1;
      }
   }
   return 0;
}

int main(void)
{
   size_t i;
   int n[25];
   srand(time(NULL));
   for ( i = 0; i < sizeof(n)/sizeof(*n); ++i )
   {
      do
      {
         n[i] = rand() % sizeof(n)/sizeof(*n) + 1;
      } while ( foo(n, i) );
      printf("%2d ", n[i]);
   }
   putchar('\n');
   return 0;
}

[edit]
Problems with this approach are mentioned here (if any kind of harmless registration is required, but you still fear entering, this is a teaser...)

Imagine trying to fill each cell of a dart board with its own dart. If you're a bad player like me, everything will cruise along just fine *until* the last few squares, at which point I'll need tens if not hundreds (if not thousands!) of goes to try and fill the remaining squares.

The above method suffers the same problem. You'll find that on slower machines of old, and when used against large arrays, this method can take a long time to complete! With a particularly bad rand() function, it may even lock up in an infinite loop.

I recommend joining if only to read Peter's (or Paul's) posts.
[/edit]

cheers dave

if you perform the shuffle below, when u exchange the random numbers how will it know that it is not already in the array or not already generated?
i.e. a duplicate occuring

how can the shuffle not include a duplicate?

ive seen in the card programs that deal say 52 cards with this shuffle algorithm, but im not sure how it deals/shuffles without no duplicates..

sorry

when you say unique, the same generated random number cannot appear again?