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How Can NumPy Arrays Be Efficiently Justified (Shifted)?

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Dec 24, 2024 am 12:16 AM

How Can NumPy Arrays Be Efficiently Justified (Shifted)?

Justification of NumPy Arrays

Problem:

Optimizing code for shifting content in a matrix is sought for use in a 2048 game demo. Specifically, functions are needed to move non-zero values in the matrix towards the left, right, up, or down.

Solution Using NumPy:

The provided code offers a vectorized approach inspired by another post:

def justify(a, invalid_val=0, axis=1, side='left'):
    """
    Justifies a 2D array

    Parameters
    ----------
    A : ndarray
        Input array to be justified
    axis : int
        Axis along which justification is to be made
    side : str
        Direction of justification. It could be 'left', 'right', 'up', 'down'
        It should be 'left' or 'right' for axis=1 and 'up' or 'down' for axis=0.

    """

    if invalid_val is np.nan:
        mask = ~np.isnan(a)
    else:
        mask = a!=invalid_val
    justified_mask = np.sort(mask,axis=axis)
    if (side=='up') | (side=='left'):
        justified_mask = np.flip(justified_mask,axis=axis)
    out = np.full(a.shape, invalid_val) 
    if axis==1:
        out[justified_mask] = a[mask]
    else:
        out.T[justified_mask.T] = a.T[mask.T]
    return out

Sample Runs:

In [473]: a # input array
Out[473]: 
array([[1, 0, 2, 0],
       [3, 0, 4, 0],
       [5, 0, 6, 0],
       [6, 7, 0, 8]])

In [474]: justify(a, axis=0, side='up')
Out[474]: 
array([[1, 7, 2, 8],
       [3, 0, 4, 0],
       [5, 0, 6, 0],
       [6, 0, 0, 0]])

In [475]: justify(a, axis=0, side='down')
Out[475]: 
array([[1, 0, 0, 0],
       [3, 0, 2, 0],
       [5, 0, 4, 0],
       [6, 7, 6, 8]])

In [476]: justify(a, axis=1, side='left')
Out[476]: 
array([[1, 2, 0, 0],
       [3, 4, 0, 0],
       [5, 6, 0, 0],
       [6, 7, 8, 0]])

In [477]: justify(a, axis=1, side='right')
Out[477]: 
array([[0, 0, 1, 2],
       [0, 0, 3, 4],
       [0, 0, 5, 6],
       [0, 6, 7, 8]])

Generic Case (ndarray):

For a generic n-dimensional array, the code can be modified as follows:

def justify_nd(a, invalid_val, axis, side):    
    """
    Justify ndarray for the valid elements (that are not invalid_val).

    Parameters
    ----------
    A : ndarray
        Input array to be justified
    invalid_val : scalar
        invalid value
    axis : int
        Axis along which justification is to be made
    side : str
        Direction of justification. Must be 'front' or 'end'.
        So, with 'front', valid elements are pushed to the front and
        with 'end' valid elements are pushed to the end along specified axis.
    """
    
    pushax = lambda a: np.moveaxis(a, axis, -1)
    if invalid_val is np.nan:
        mask = ~np.isnan(a)
    else:
        mask = a!=invalid_val
    justified_mask = np.sort(mask,axis=axis)
    
    if side=='front':
        justified_mask = np.flip(justified_mask,axis=axis)
            
    out = np.full(a.shape, invalid_val)
    if (axis==-1) or (axis==a.ndim-1):
        out[justified_mask] = a[mask]
    else:
        pushax(out)[pushax(justified_mask)] = pushax(a)[pushax(mask)]
    return out

Sample Runs (ndarray):

In [87]: a
Out[87]: 
array([[[54, 57,  0, 77],
        [77,  0,  0, 31],
        [46,  0,  0, 98],
        [98, 22, 68, 75]],

       [[49,  0,  0, 98],
        [ 0, 47,  0, 87],
        [82, 19,  0, 90],
        [79, 89, 57, 74]],

       [[ 0,  0,  0,  0],
        [29,  0,  0, 49],
        [42, 75,  0, 67],
        [42, 41, 84, 33]],

       [[ 0,  0,  0, 38],
        [44, 10,  0,  0],
        [63,  0,  0,  0],
        [89, 14,  0,  0]]])

To 'front', along axis =0 :

In [88]: justify_nd(a, invalid_val=0, axis=0, side='front')
Out[88]: 
array([[[54, 57,  0, 77],
        [77, 47,  0, 31],
        [46, 19,  0, 98],
        [98, 22, 68, 75]],

       [[49,  0,  0, 98],
        [29, 10,  0, 87],
        [82, 75,  0, 90],
        [79, 89, 57, 74]],

       [[ 0,  0,  0, 38],
        [44,  0,  0, 49],
        [42,  0,  0, 67],
        [42, 41, 84, 33]],

       [[ 0,  0,  0,  0],
        [ 0,  0,  0,  0],
        [63,  0,  0,  0],
        [89, 14,  0,  0]]])

Along axis=1 :

In [89]: justify_nd(a, invalid_val=0, axis=1, side='front')
Out[89]: 
array([[[54, 57, 68, 77],
        [77, 22,  0, 31],
        [46,  0,  0, 98],
        [98,  0,  0, 75]],

       [[49, 47, 57, 98],
        [82, 19,  0, 87],
        [79, 89,  0, 90],
        [ 0,  0,  0, 74]],

       [[29, 75, 84, 49],
        [42, 41,  0, 67],
        [42,  0,  0, 33],
        [ 0,  0,  0,  0]],

       [[44, 10,  0, 38],
        [63, 14,  0,  0],
        [89,  0,  0,  0],
        [ 0,  0,  0,  0]]])

Along axis=2 :

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