random number generation and exclusion of the generated numbers

Question

Is it possible to do this in Java ? I want to generate a random number such that given a range say for example: between 1 and 70 - everytime the random number is generated it should be excluded from generation results.
so [1,70] rand = 56 (now 56 shouldn't be considered next time)
[1,70] = 63 (now 56,63 should be excluded from generation till my code runs)

Duplicate to question : **[Java Creating Random Numbers with No Duplicates](http://stackoverflow.com/questions/4040001/java-creating-random-numbers-with-no-duplicates)** — lschin, Apr 28 '11 at 03:42

score 3 · Answer 1 · answered Apr 28 '11 at 03:42

3

This is equivalent to shuffling an array of numbers containing [1..70] and then dealing them out one at a time. Look up "shuffle algorithm" on Google. Here's a link http://www.vogella.de/articles/JavaAlgorithmsShuffle/article.html

answered Apr 28 '11 at 03:42

Jim Garrison

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Or better use the shuffle algorithm that comes with JDK. http://download.oracle.com/javase/6/docs/api/java/util/Collections.html#shuffle(java.util.List) – Aravind Yarram Apr 28 '11 at 03:47
Yes, I forgot that was in there. – Jim Garrison Apr 28 '11 at 03:48

score 1 · Answer 2 · edited May 23 '17 at 11:55

1

I asked the same question here: How can I generate a random number within a range but exclude some?

The general strategy is something like filling an array with 70 values. Just remove the values that you randomly generate as per the link above.

edited May 23 '17 at 11:55

Community

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answered Apr 28 '11 at 03:43

Pete

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score 1 · Answer 3 · answered Apr 28 '11 at 03:43

1

you could populate the range into an array and shuffle the array. This would be inefficient though for very large ranges

answered Apr 28 '11 at 03:43

A D

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score 1 · Answer 4 · answered Apr 28 '11 at 05:30

Another trivial alternative, using HashMaps to keep track of random numbers. It is sort of quick and dirty.

HashMap<Integer,Integer> hmRandomNum = new HashMap<Integer,Integer>();

Integer a = < generate random number>

if( hmRandomNum.get(a) == null)
{
     hmRandomNum.put(a,a);
}
else
{
     // ignore this random number. this was already selected and present in the hashmap.
}

//Iterate depending on how many numbers you want.

random number generation and exclusion of the generated numbers

4 Answers4