Starting from the basic. Indexing/Slicing in NumPy has this formula [start:stop:step]
Example 1: y = [1,2,3,4,5,6]
y[2:6:3]
Output is [3,6]
This means starting from index 2 which is number 3 in the array and goes all the way till index 6, which is number six and skip by 2.
Same logic
Example 2: y[-3::-2] returns [4,2]
This means starting from the 3th element from the end, which is number 4, and counting backwards by 2 (skipping 1) until the beginning of the array is reached.
Skipping backwards
Example3: y[::-1] returns [6,5,4,3,2,1]
y[::-3] returns [6,3]
Now using it in a multi-dimensional array
Below r[::3]goes from the first row and then skip 2 rows. The number here means row number. Since there are only 6 rows, r[::7] and r[::10] only return the first row.
Reshape, however, first making the array ‘flat’ and then the code works as if on a single dimensional array.
The first ‘:’ means by row.
