Example
Slice syntax is i:j:k where i is the starting index (inclusive), j is the stopping index (exclusive) and k is the step size. Like other python data structures, the first element has an index of 0:
x = np.arange(10)
x[0]
# Out: 0
x[0:4]
# Out: array([0, 1, 2, 3])
x[0:4:2]
# Out:array([0, 2])
Negative values count in from the end of the array. -1 therefore accesses the last element in an array:
x[-1]
# Out: 9
x[-1:0:-1]
# Out: array([9, 8, 7, 6, 5, 4, 3, 2, 1])
Multi-dimensional arrays can be accessed by specifying each dimension separated by commas. All previous rules apply.
x = np.arange(16).reshape((4,4))
x
# Out:
# array([[ 0, 1, 2, 3],
# [ 4, 5, 6, 7],
# [ 8, 9, 10, 11],
# [12, 13, 14, 15]])
x[1,1]
# Out: 5
x[0:3,0]
# Out: array([0, 4, 8])
x[0:3, 0:3]
# Out:
# array([[ 0, 1, 2],
# [ 4, 5, 6],
# [ 8, 9, 10]])
x[0:3:2, 0:3:2]
# Out:
# array([[ 0, 2],
# [ 8, 10]])
