Python Array Slice With Comma?
It is being used to extract a specific column from a 2D array. Refer to the first examples here.
So your example would extract column 0 (the first column) from the first 2048 rows (0 to 2047). Note however that this syntax will only work for numpy arrays and not general python lists.
Empirically - create an array using numpy
m = np.fromfunction(lambda i, j: (i +1)* 10 + j + 1, (9, 4), dtype=int)
which assigns an array like below to m
array( [[11, 12, 13, 14], [21, 22, 23, 24], [31, 32, 33, 34], [41, 42, 43, 44], [51, 52, 53, 54], [61, 62, 63, 64], [71, 72, 73, 74], [81, 82, 83, 84], [91, 92, 93, 94]])
Now for the slice
m[:,0]
giving us
array([11, 21, 31, 41, 51, 61, 71, 81, 91])
I may have misinterpreted Khan Academy (so take with grain of salt):
In linear algebra terms,
m[:,n]
is taking thenth
column vector of the matrixm
See Abhranil's note how this specific interpretation only applies to numpy
It slices with a tuple. What exactly the tuple means depends on the object being sliced. In NumPy arrays, it performs a m-dimensional slice on a n-dimensional array.
>>> class C(object):... def __getitem__(self, val):... print val... >>> c = C()>>> c[1:2,3:4](slice(1, 2, None), slice(3, 4, None))>>> c[5:6,7](slice(5, 6, None), 7)