Convert NumPy array to 0 or 1 based on threshold

np.where

np.where(a > 0.5, 1, 0)
# array([0, 0, 0, 1, 1, 1])

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Boolean basking with astype

(a > .5).astype(int)
# array([0, 0, 0, 1, 1, 1])

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np.select

np.select([a <= .5, a>.5], [np.zeros_like(a), np.ones_like(a)])
# array([ 0.,  0.,  0.,  1.,  1.,  1.])

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Special case: np.round

This is the best solution if your array values are floating values between 0 and 1 and your threshold is 0.5.

a.round()
# array([0., 0., 0., 1., 1., 1.])