Python Language Indexing and Slicing Basic Slicing
Example
For any iterable (for eg. a string, list, etc), Python allows you to slice and return a substring or sublist of its data.
Format for slicing:
iterable_name[start:stop:step]
where,
startis the first index of the slice. Defaults to 0 (the index of the first element)stopone past the last index of the slice. Defaults to len(iterable)stepis the step size (better explained by the examples below)
Examples:
a = "abcdef"
a # "abcdef"
# Same as a[:] or a[::] since it uses the defaults for all three indices
a[-1] # "f"
a[:] # "abcdef"
a[::] # "abcdef"
a[3:] # "def" (from index 3, to end(defaults to size of iterable))
a[:4] # "abcd" (from beginning(default 0) to position 4 (excluded))
a[2:4] # "cd" (from position 2, to position 4 (excluded))
In addition, any of the above can be used with the step size defined:
a[::2] # "ace" (every 2nd element)
a[1:4:2] # "bd" (from index 1, to index 4 (excluded), every 2nd element)
Indices can be negative, in which case they're computed from the end of the sequence
a[:-1] # "abcde" (from index 0 (default), to the second last element (last element - 1))
a[:-2] # "abcd" (from index 0 (default), to the third last element (last element -2))
a[-1:] # "f" (from the last element to the end (default len())
Step sizes can also be negative, in which case slice will iterate through the list in reverse order:
a[3:1:-1] # "dc" (from index 2 to None (default), in reverse order)
This construct is useful for reversing an iterable
a[::-1] # "fedcba" (from last element (default len()-1), to first, in reverse order(-1))
Notice that for negative steps the default end_index is None (see http://stackoverflow.com/a/12521981 )
a[5:None:-1] # "fedcba" (this is equivalent to a[::-1])
a[5:0:-1] # "fedcb" (from the last element (index 5) to second element (index 1)
