Rearranging contents of an array

I have a program to randomly pick 90 questions from a large database(295), so what I did is made an index array of ints of ints labelled 1-90, but I want the array to not have duplicates.

Code:

	srand(time(0));
	for(int i=0;i<MAXQ;i++)
		randoms[i] = rand()%295;

theres a little snippit.
I then sort the random array, but I get duplicates of an index. How can I arrange 295 questions into 90 but without duplicates?

Thanks

You could try something like this:

Code:

int j = 0;
for(int i = 0; i < MAXQ; i++) {
   randoms[i] = rand() % 295;
		
   while(j < i) {
      if (randoms[i] == randoms[j])
         randoms[i] = rand() % 295;
      else 
         j++;
   }
}

I'm guessing this int array stores Identifiers for reading the database. You could check to see if one has already come up easily.

Code:

//!!!! FILL randoms WITH -1!!!!

srand(time(0));
int number = 0;
bool picked = false;

for(int i=0; i<MAXQ; i++)
{
    number = rand()%295;
    for( int j=0; j<i; j++ )
    {
        if( randoms[j] == number )
            picked = true;
     }

     if( picked == true )
         i--; // will go through loop again
     else
         randoms[i] = number;
     
      picked = false
}

Code:

#include <vector>
#include <algorithm>  // for random_shuffle
using namespace std;

int main()
{
	vector<int> randoms(295);

	/* Generate a vector containing the numbers 0..294 */
	for (vector<int>::size_type i = 0; i < randoms.size(); i++) randoms[i] = i;

	/* Shuffle */
	random_shuffle(randoms.begin(), randoms.end());

	/* Keep only the first 90 numbers */
	randoms.resize(90);
}

sweet, thanks guys, all good now.

random_shuffle(). Nice.

If you didn't want to destroy your vector with the 295 numbers, you could modify the program slightly:

Code:

#include <iostream>
#include <vector>
#include <algorithm>  // for random_shuffle
using namespace std;

int main()
{
	vector<int> randoms(295);

	/* Generate a vector containing the numbers 0..294 */
	for (vector<int>::size_type i = 0; i < randoms.size(); i++) 
		randoms[i] = i;

	/* Shuffle */
	random_shuffle(randoms.begin(), randoms.end());

	/* Keep only the first 90 numbers */
//	randoms.resize(90);

	vector<int> ninety(90);
	vector<int>::iterator begin = randoms.begin();
	copy(begin, begin + 90, ninety.begin());  //(beginning of range, end of range, destination)
	
	//output the numbers to the screen:
	copy(ninety.begin(), ninety.end(), ostream_iterator<int>(cout, "\n"));
	
	return 0;
}

As a reminder, you should still call srand at the beginning of your program when using the random_shuffle function. The random_shuffle function makes use of rand internally and if you don't call srand you will wind up with the same sequence of "random" values each time the program is run.

[edit]Resizing the vector or copying the first 90 elements into another container after the shuffle doesn't hurt but it is unnecessary. You could just simply iterate/loop over the first 90 values and then just stop at that point.[/edit]

Also, while vectors are definitely preferred, random_shuffle works on C style arrays as well:

Code:

random_shuffle(randoms, randoms + 295); // assuming randoms has a size of 295

what i came up with the help of u guys...

Code:

srand(time(0)); //generate random int array
	for(int i=0;i<MAXQ;i++)
	{
		number = rand()%295;
		for(int j=0;j<i;j++)
		{
			if(randoms[j] == number)
				picked = true;
		}
		if(picked)
			 i--; // will go through loop again
		else
			randoms[i] = number;
		picked = false;
	}
sorted = new int[ASSIGNED];
	for(int i=0;i<ASSIGNED;i++) //get 90 questions from random array
		sorted[i] = randoms[i];
	Bubblesort(sorted,ASSIGNED);// sort it

That method should work, but it is very inefficient. If you don't want to use the built-in solution of random_shuffle, then you can use the same technique it uses.

Create an index variable and start it at the end (which is MAXQ-1). Get a random number from 0 to that index. Then swap the value at that index with the value at the index of that random number. Decrement the index and repeat the process until the index is 0.

This will effectively shuffle the array without the inefficiency of looking up previously picked numbers. You can then use the first 90 numbers of the array (you can also modify the algorithm to only shuffle 90 numbers, but that is harder so you should start with the basic algorithm).