Question

I’ve some strange situation here and i thought that you may help me. I have an int array populated with numbers from 1 to 10. I want to generate random number from this array and save it to another int array. I used class Random to pick any number and since random throws 0 also i modify it like that ( so it throws numbers from 1 to 10 )

randNum = rand.nextInt(numbers.length-min+1)+min;   

Following code makes sure that if it generates same random number, it skips it. Program is actually working and i’m getting in another array randomly positioned numbers from 1 to 10. That’s what i wanted. But sometimes i’m missing one number from 1 – 10 AND iam Getting ZERO instead. Why??

int[] numbers = {1,2,3,4,5,6,7,8,9,10};
int[] usednum = new int[10];
Random rand = new Random();
int randNum;
int min = 1;

for (int x = 0; x<numbers.length; x++) {        
  for (int i = 0; i<usednum.length; i++) { 
    randNum = rand.nextInt(numbers.length-min+1) + min;
    for (int f = 0; f<usednum.length; f++) {
      if (usednum[f] == randNum) {
        break;
      } else if (usednum[f] == 0) { 
        usednum[x] = randNum;   
      }
    }
  } 
}

for (int c = 0; c<usednum.length; c++) {
  System.out.println(usednum[c]);
}

Answer 1

You’re inner-most for loop only checks if the current random number is in the usednum[] array. And the for loop immediately outer of that only checks 10 times total. It gives up too quickly because it only tries 10 random numbers. If all 10 are already used, nothing will get stored in that slot of usednum[] (thus it will be 0), try adding a while loop around that and get rid of the extraneous outer-most for loop:

        for(int i = 0; i<usednum.length; i++) {
           while(usednum[i]==0) {
              randNum = rand.nextInt(numbers.length-min+1)+min;
              for(int f = 0; f<usednum.length; f++) {
                 if(usednum[f] == randNum) {
                    break;
                 } //if                                                                                                                                        
                 else if (usednum[f] == 0) {
                    usednum[i] = randNum;
                 }
              }
           }
        }

Also note that the assignment is for usednum[i] = randNum;.

This is essentially replacing the middle for loop (the one that goes from i=0 to 9) with the while loop.