Python: Justifying NumPy array
Here's a vectorized approach inspired by this other post and generalized to cover non-zeros for all four directions -
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 ndarray, we could modify it to -
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 -
Input array :
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 :
In [90]: justify_nd(a, invalid_val=0, axis=2, side='front')
Out[90]:
array([[[54, 57, 77, 0],
[77, 31, 0, 0],
[46, 98, 0, 0],
[98, 22, 68, 75]],
[[49, 98, 0, 0],
[47, 87, 0, 0],
[82, 19, 90, 0],
[79, 89, 57, 74]],
[[ 0, 0, 0, 0],
[29, 49, 0, 0],
[42, 75, 67, 0],
[42, 41, 84, 33]],
[[38, 0, 0, 0],
[44, 10, 0, 0],
[63, 0, 0, 0],
[89, 14, 0, 0]]])
To the 'end' :
In [94]: justify_nd(a, invalid_val=0, axis=2, side='end')
Out[94]:
array([[[ 0, 54, 57, 77],
[ 0, 0, 77, 31],
[ 0, 0, 46, 98],
[98, 22, 68, 75]],
[[ 0, 0, 49, 98],
[ 0, 0, 47, 87],
[ 0, 82, 19, 90],
[79, 89, 57, 74]],
[[ 0, 0, 0, 0],
[ 0, 0, 29, 49],
[ 0, 42, 75, 67],
[42, 41, 84, 33]],
[[ 0, 0, 0, 38],
[ 0, 0, 44, 10],
[ 0, 0, 0, 63],
[ 0, 0, 89, 14]]])