get random BigInteger in range (x, y)

Question

I need to get a random BigInteger that is bigger than 2^511 and lower than 2^512.

Here is example code of [Prefactored Random BigInteger](http://stanford.edu/~rezab/useful/PrefactoredRandom.java.html) — Subhrajyoti Majumder, Apr 14 '14 at 07:36

score 3 · Answer 1 · answered Apr 14 '14 at 07:34

3

byte[] bytes = new byte[64];    // 512 bits
new Random().nextBytes(bytes);
bytes[0] |= 0x80;               // set the most significant bit
return new BigInteger(1, bytes);

answered Apr 14 '14 at 07:34

apangin

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score 2 · Answer 2 · edited Jun 20 '20 at 09:12

From the doc :

BigInteger(int numBits, Random rnd)

Constructs a randomly generated BigInteger, uniformly distributed over the range 0 to (2numBits - 1), inclusive.

So something like that should work :

    BigInteger number = new BigInteger(512, new Random()); //Give you a number between 0 and 2^512 - 1
    number = number.setBit(0); //Set the first bit so number is between 2^511 and 2^512 - 1

score 2 · Answer 3 · answered Apr 14 '14 at 07:41

This solution creates at first a BigInteger with value 2^511 and afterwards adds an value between 0 and 2^511 - 1:

StringBuilder builder = new StringBuilder("1");
for (int bit = 0; bit < 511; bit++) builder.append("0");
BigInteger value = new BigInteger(builder.toString(), 2).add(new BigInteger(511, new Random()));

score 0 · Answer 4 · answered Apr 14 '14 at 07:37

0

You could try that

public static void main(String[] args) {
    int min = 511;
    double rand = Math.random(); //between 0 and 1
    double exp = min + rand; //between 511 et 512

    Double result = Math.pow(2, exp);

    System.out.println("result = ["+result+"]");

}

It is maybe not optimised, but its works. With the double result, you can obtain a integer.

answered Apr 14 '14 at 07:37

Error418

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I guess he wanted a uniform distribution, not a log-uniform one – John Dvorak Apr 14 '14 at 07:38
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There are 2^511-1 integers between 2^511 and 2^512, none of which will fit into a double variable; – VH-NZZ Apr 14 '14 at 07:39
@JanDvorak it is not specified, may be you're right – Error418 Apr 14 '14 at 07:39
@okiharaherbst The maximum value of a `Double` is 2^1024. The range is fine, even if the precision is not even slightly enough. – John Dvorak Apr 14 '14 at 07:41
@JanDvorak Of course, (although, technically, my comment stating _double_ is not wrong – but i'll gladly concede i was too quick to post :) – VH-NZZ Apr 14 '14 at 07:44

get random BigInteger in range (x, y)

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