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Fonction choose - module numpy.matlib

Signature de la fonction choose

def choose(a, choices, out=None, mode='raise') 

Description

choose.__doc__

    Construct an array from an index array and a set of arrays to choose from.

    First of all, if confused or uncertain, definitely look at the Examples -
    in its full generality, this function is less simple than it might
    seem from the following code description (below ndi =
    `numpy.lib.index_tricks`):

    ``np.choose(a,c) == np.array([c[a[I]][I] for I in ndi.ndindex(a.shape)])``.

    But this omits some subtleties.  Here is a fully general summary:

    Given an "index" array (`a`) of integers and a sequence of `n` arrays
    (`choices`), `a` and each choice array are first broadcast, as necessary,
    to arrays of a common shape; calling these *Ba* and *Bchoices[i], i =
    0,...,n-1* we have that, necessarily, ``Ba.shape == Bchoices[i].shape``
    for each `i`.  Then, a new array with shape ``Ba.shape`` is created as
    follows:

    * if ``mode=raise`` (the default), then, first of all, each element of
      `a` (and thus `Ba`) must be in the range `[0, n-1]`; now, suppose that
      `i` (in that range) is the value at the `(j0, j1, ..., jm)` position
      in `Ba` - then the value at the same position in the new array is the
      value in `Bchoices[i]` at that same position;

    * if ``mode=wrap``, values in `a` (and thus `Ba`) may be any (signed)
      integer; modular arithmetic is used to map integers outside the range
      `[0, n-1]` back into that range; and then the new array is constructed
      as above;

    * if ``mode=clip``, values in `a` (and thus `Ba`) may be any (signed)
      integer; negative integers are mapped to 0; values greater than `n-1`
      are mapped to `n-1`; and then the new array is constructed as above.

    Parameters
    ----------
    a : int array
        This array must contain integers in `[0, n-1]`, where `n` is the number
        of choices, unless ``mode=wrap`` or ``mode=clip``, in which cases any
        integers are permissible.
    choices : sequence of arrays
        Choice arrays. `a` and all of the choices must be broadcastable to the
        same shape.  If `choices` is itself an array (not recommended), then
        its outermost dimension (i.e., the one corresponding to
        ``choices.shape[0]``) is taken as defining the "sequence".
    out : array, optional
        If provided, the result will be inserted into this array. It should
        be of the appropriate shape and dtype. Note that `out` is always
        buffered if `mode='raise'`; use other modes for better performance.
    mode : {'raise' (default), 'wrap', 'clip'}, optional
        Specifies how indices outside `[0, n-1]` will be treated:

          * 'raise' : an exception is raised
          * 'wrap' : value becomes value mod `n`
          * 'clip' : values < 0 are mapped to 0, values > n-1 are mapped to n-1

    Returns
    -------
    merged_array : array
        The merged result.

    Raises
    ------
    ValueError: shape mismatch
        If `a` and each choice array are not all broadcastable to the same
        shape.

    See Also
    --------
    ndarray.choose : equivalent method
    numpy.take_along_axis : Preferable if `choices` is an array

    Notes
    -----
    To reduce the chance of misinterpretation, even though the following
    "abuse" is nominally supported, `choices` should neither be, nor be
    thought of as, a single array, i.e., the outermost sequence-like container
    should be either a list or a tuple.

    Examples
    --------

    >>> choices = [[0, 1, 2, 3], [10, 11, 12, 13],
    ...   [20, 21, 22, 23], [30, 31, 32, 33]]
    >>> np.choose([2, 3, 1, 0], choices
    ... # the first element of the result will be the first element of the
    ... # third (2+1) "array" in choices, namely, 20; the second element
    ... # will be the second element of the fourth (3+1) choice array, i.e.,
    ... # 31, etc.
    ... )
    array([20, 31, 12,  3])
    >>> np.choose([2, 4, 1, 0], choices, mode='clip') # 4 goes to 3 (4-1)
    array([20, 31, 12,  3])
    >>> # because there are 4 choice arrays
    >>> np.choose([2, 4, 1, 0], choices, mode='wrap') # 4 goes to (4 mod 4)
    array([20,  1, 12,  3])
    >>> # i.e., 0

    A couple examples illustrating how choose broadcasts:

    >>> a = [[1, 0, 1], [0, 1, 0], [1, 0, 1]]
    >>> choices = [-10, 10]
    >>> np.choose(a, choices)
    array([[ 10, -10,  10],
           [-10,  10, -10],
           [ 10, -10,  10]])

    >>> # With thanks to Anne Archibald
    >>> a = np.array([0, 1]).reshape((2,1,1))
    >>> c1 = np.array([1, 2, 3]).reshape((1,3,1))
    >>> c2 = np.array([-1, -2, -3, -4, -5]).reshape((1,1,5))
    >>> np.choose(a, (c1, c2)) # result is 2x3x5, res[0,:,:]=c1, res[1,:,:]=c2
    array([[[ 1,  1,  1,  1,  1],
            [ 2,  2,  2,  2,  2],
            [ 3,  3,  3,  3,  3]],
           [[-1, -2, -3, -4, -5],
            [-1, -2, -3, -4, -5],
            [-1, -2, -3, -4, -5]]])