I'm trying to generate a random number in a range - MicroPython

1

I’m trying to generate a random number in a range (between 4 and 96), I’ve used import urandom at the beginning of my code and tried the code listed below - it’s not working, this seems to be the usual syntax for most Python and MicroPython versions I’ve used before.

ix = urandom.randint(4, 96)

Thanks,

Brian

2

Here is the answer. So the parameter is the number of bits. getrandbits(8) would give a random value between 0-255.

3

I needed a range, this seems to work:

# (screen <4-96> or 92 pixels, 5 bits = rnd # 0 to 31, * 3 = 93, -1 = 92 + 4 for left edge (or + 3))
            ix = (urandom.getrandbits(5) * 3) + 3
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4

Here is an implementation of randint(start, stop) I wrote based on the code in Javitto Random:

def randint(start, stop):
    bound = stop - start
    
    mask = bound
    mask |= mask >> 16
    mask |= mask >> 8
    mask |= mask >> 4
    mask |= mask >> 2
    mask |= mask >> 1
    
    ret = urandom.getrandbits(30) & mask
    while ret > bound:
        ret = urandom.getrandbits(30) & mask
    
    return start + ret
5

Thanks! this looks interesting, I’ll have to see if I can implement it into my code.

6

Too complicated, normally you if you have a random integer N you get it to range <A,B> as A + N mod (B - A + 1). I.e. a + urandom.getrandbits(32) % (b - a + 1).

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7

I don’t know about the mask business, but the while ret > bound part likely gets rid of modulo bias.

Not that it’s necessarily an issue, depending on the use case.

1 Like
8

Yep, it doesn’t have modulo bias, but I’m guessing it doesn’t matter in this case. So better use the version drummyfish posted.

Just change getrandbits(32) to getrandbits(30). Otherwise it gives following error: OverflowError: small int overflow.

9

The mask business reduces the amount of retries in the while loop without incurring modulo bias.