randomly picking numbers

[Henry Addo]

hi
i'm stuck in a situation, i want to pick 10 random elements at a time from a vector.what i have done so far is, i generate a random number uses the generated random number as the index of the element be picked the element from the vector. Now my problem i don't want to pick the element which has already been picked and i know it does that because the indexes being generated repeats due to the random number i'm generating. To get view what i'm really talking about i'm posting a code. Please how do i avoid element already selected. Thanks

public String pickNumber( String [] numbers ) {
        
        /** vector to contain the elements */
        Vector <String>vector = new Vector<String>();
        
        //adds elements of the string array to the vector
        for ( int i = 0; i < numbers.length; i++ ) {
            
            /** add array elements to the vector */
            vector.add( numbers[ i ] );
        }
 
        /* generate random number */
        int rndNumber = ( int )( Math.random() * vector.size()-1 )  + 1;
        
        String selectedNumber = "";
        
        try {
            selectedNumber = vector.elementAt( rndNumber );
            
        }catch( ArrayIndexOutOfBoundsException e ){
           JOptionPane.showMessageDialog( this, "No number");
        }
        
        vector.removeElementAt( rndNumber );
        
        return selectedNumber;
    }
   // this method is called when a button is called.
   

[ July 28, 2006: Message edited by: Henry Addo ]

[Joanne Neal]

Is this an exercise in using random numbers or can you use other methods.
Collections.shuffle() may be worth looking at.

[pascal betz]

i would shuffle the List/Vector then pick the 10 (or any other number) elements starting at index 0 (of course you need to have at least 10 elements in the List then).
Look at java.util.Collections for shuffling and at the List.subList() method for creating a sub list (picking the first 10 elements)

This might not be the most performant solution but you can write it in two or three lines.

if you want to do this without shuffling then you need to create "unique" random numbers:

// pseudocode for selecting non repeating random numbers
numbers = some sort of a Set
for (i < 10; i++) {
  randomNumber = ... 
  while (randomNumber is in numbers) {
    randomNumber = ...
  }
  add randomNumber to numbers
}

this might be inefficient to (when the number of items you want to pick is close to the number of total items in the list the "algorithm" is not so smart :-) )

pascal

[Henry Addo]

no is not an assignment. Random number generation was what came to mind the first time i thought about how to solve it. I will be trying all what has been said and will post code when i'm stack. Thanks

[Arno Reper]

Hi,
why don't you use the java.util.Random? its easy to use...
watch here the javadoc :
http://www.docjar.com/docs/api/java/util/Random.html
arno

[pascal betz]

Arno, i think that's what he did. He created random numbers (and each number would be the index to an element in a List) but he does not want the same index more than once therefore you need to remember what numbers were generated before (or he will pick the same element from the List more than once).

[Christian Nash]

Hi Hope this helps!!!

   
Random randomValue=new Random();
 
ArrayList randomNumbersList= new ArrayList();
randomNumbersList.clear();
int countOfNumbers=10;
do{
if(randomNumbersList.isEmpty()){
randomNumbersList.add(new Integer(randomValue.nextInt(10)));
}
else{
int randCheck=randomValue.nextInt(10);
if(!(randomNumbersList.contains(new Integer(randCheck)))){
        randomNumbersList.add(new Integer(randCheck));
}
}


}while(randomNumbersList.size()<10);
 

[Henry Addo]

i realised where the problem was lying. i found out that when the button is clicked to call the "pickNumber" function, a new instance of the array to be passed to the "pickNumber" function is created so the element that i remove from the vector does goes away but a new but the same element are added to vector. To solve this, i created a new function that accepts a vector, then a randomly element is remove from that vector. in the actionPerform method i set a counter, when the counter is zero it should call the pickNumber other wise it should call the second function i created which accepts the vector created in the pickNumber funtion. To really explain the english, here is the code

    /**
     * pickNumVector - accepts the vector of the initialized in the 
     *  pickNumber function  
     *
     * @param String Vector vector
     * @return String a number
     */
    public String pickNumVector( Vector<String> vector ) {
        
        /* generate random number */
        int rndNumber = ( int )( Math.random() * vector.size()-1 )  + 1;
        
        String selectedNumber = "";
  
        try {
            selectedNumber = vector.elementAt( rndNumber );
            vector.removeElementAt( rndNumber );
        }catch( ArrayIndexOutOfBoundsException e ){
           JOptionPane.showMessageDialog( this, "No number");
        }
        
        return selectedNumber;
    }
 
    // in the actionPerformed method this is what i did
     button.addActionListener( new ActionListener(){
            public void actionPerformed( ActionEvent e ){
              
              /* splitting comma sperated string into array */
              
              // counter to test if it is the first click
          
              if( counter++ == 0 ){
                   str  = numbersHolder.split(",");
                   selectedNumber.setText( pickNumber(str) );
              
              }else {
                 selectedNumber.setText( pickNumVector( vector ) );  
              }
            }
        });

This is not smart enough for me if you think there is a better way of doing this please advice

[ July 28, 2006: Message edited by: Henry Addo ]

[ July 28, 2006: Message edited by: Henry Addo ]
[ July 28, 2006: Message edited by: Henry Addo ]