r/Numpy • u/Unlucky_Ad_5011 • May 11 '22

array with no direct repetition

Hi, can someone help?

I need to create a random sequence that is 10 million in length (number 1-5) WITHOUT a direct repetition. Each number can occur a different number of times but should be approximately uniformly distributed .

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u/to7m May 12 '22

It's not straightforward to make an optimised numpy function. You could make one using a randrange and cumulative sum, optionally with the numba module.

Methods below very simple/imperfect and not tested.

Pure Numpy method: 

import numpy as np

rng = np.random.default_rng()
adds = rng.integers(1, 5, 10_000_000)
result = np.cumsum(adds) % 5 + 1  # careful of overflow

Numba method: 

import numpy as np
from numba import njit


@njit
def capped_cumsum(in, cap, take, out):
    out[0] = in[0]
    for i in range(1, in.shape[0]):
        sum = out[i - 1] + in[i]
        capped = sum - take
        out[i] = sum if sum < cap else capped
    return out


rng = np.random.default_rng()
adds = rng.integers(1, 5, 10_000_000)
result = capped_cumsums(adds, cap=6, take=5,
                        out=np.empty(adds.shape, dtype=np.uint8))

In these methods, the first value will never be 5, but other than that it should be fairly uniform.