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diff --git a/doc/source/reference/random/generator.rst b/doc/source/reference/random/generator.rst index a2cbb493a..8706e1de2 100644 --- a/doc/source/reference/random/generator.rst +++ b/doc/source/reference/random/generator.rst @@ -36,11 +36,105 @@ Simple random data Permutations ============ +The methods for randomly permuting a sequence are + .. autosummary:: :toctree: generated/ ~numpy.random.Generator.shuffle ~numpy.random.Generator.permutation + ~numpy.random.Generator.permuted + +The following table summarizes the behaviors of the methods. + ++--------------+-------------------+------------------+ +| method | copy/in-place | axis handling | ++==============+===================+==================+ +| shuffle | in-place | as if 1d | ++--------------+-------------------+------------------+ +| permutation | copy | as if 1d | ++--------------+-------------------+------------------+ +| permuted | either (use 'out' | axis independent | +| | for in-place) | | ++--------------+-------------------+------------------+ + +The following subsections provide more details about the differences. + +In-place vs. copy +~~~~~~~~~~~~~~~~~ +The main difference between `Generator.shuffle` and `Generator.permutation` +is that `Generator.shuffle` operates in-place, while `Generator.permutation` +returns a copy. + +By default, `Generator.permuted` returns a copy. To operate in-place with +`Generator.permuted`, pass the same array as the first argument *and* as +the value of the ``out`` parameter. For example, + + >>> rg = np.random.default_rng() + >>> x = np.arange(0, 15).reshape(3, 5) + >>> x + array([[ 0, 1, 2, 3, 4], + [ 5, 6, 7, 8, 9], + [10, 11, 12, 13, 14]]) + >>> y = rg.permuted(x, axis=1, out=x) + >>> x + array([[ 1, 0, 2, 4, 3], # random + [ 6, 7, 8, 9, 5], + [10, 14, 11, 13, 12]]) + +Note that when ``out`` is given, the return value is ``out``: + + >>> y is x + True + +Handling the ``axis`` parameter +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +An important distinction for these methods is how they handle the ``axis`` +parameter. Both `Generator.shuffle` and `Generator.permutation` treat the +input as a one-dimensional sequence, and the ``axis`` parameter determines +which dimension of the input array to use as the sequence. In the case of a +two-dimensional array, ``axis=0`` will, in effect, rearrange the rows of the +array, and ``axis=1`` will rearrange the columns. For example + + >>> rg = np.random.default_rng() + >>> x = np.arange(0, 15).reshape(3, 5) + >>> x + array([[ 0, 1, 2, 3, 4], + [ 5, 6, 7, 8, 9], + [10, 11, 12, 13, 14]]) + >>> rg.permutation(x, axis=1) + array([[ 1, 3, 2, 0, 4], # random + [ 6, 8, 7, 5, 9], + [11, 13, 12, 10, 14]]) + +Note that the columns have been rearranged "in bulk": the values within +each column have not changed. + +The method `Generator.permuted` treats the ``axis`` parameter similar to +how `numpy.sort` treats it. Each slice along the given axis is shuffled +independently of the others. Compare the following example of the use of +`Generator.permuted` to the above example of `Generator.permutation`: + + >>> rg.permuted(x, axis=1) + array([[ 1, 0, 2, 4, 3], # random + [ 5, 7, 6, 9, 8], + [10, 14, 12, 13, 11]]) + +In this example, the values within each row (i.e. the values along +``axis=1``) have been shuffled independently. This is not a "bulk" +shuffle of the columns. + +Shuffling non-NumPy sequences +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +`Generator.shuffle` works on non-NumPy sequences. That is, if it is given +a sequence that is not a NumPy array, it shuffles that sequence in-place. +For example, + + >>> rg = np.random.default_rng() + >>> a = ['A', 'B', 'C', 'D', 'E'] + >>> rg.shuffle(a) # shuffle the list in-place + >>> a + ['B', 'D', 'A', 'E', 'C'] # random Distributions ============= |