Masked arrays are arrays that may have missing or invalid entries.
The numpy.ma module provides a nearly work-alike replacement for numpy that supports data arrays with masks.
A masked array is the combination of a standard numpy.ndarray and a mask.
A mask is either nomask, indicating that no value of the associated array is invalid, or an array of booleans that determines for each element of the associated array is valid and is said to be unmasked.
When an element of the mask if False, the corresponding of the associated array is valid and is said to be unmasked.
When an element of the mask if True, the corresponding element of the associated array is said to be masked(invalid).
The package ensures that masked entries are not used in computations.
