Function:
maskregion mask out the data in the nc file as per the shape
file provided as the fourth argument
Syntax:
[data_masked]=maskregion(lon,lat,data,shapefile_var);

Inputs:
It takes first three arguments as the latitude, longitude and
data respectively. shapefile argument should be the shapefile,
which contains X and Y coordinates of the region(s) you wish
to crop.

Example:

W=shaperead('us_states.shp');
[data_masked]=maskregion(lon,lat,data,shapefile_var);