Sequences

Lists, tuples and strings are examples of sequences, but what are sequences and what is so special about
them? Two of the main features of a sequence is the indexing operation which allows us to fetch a particular
item in the sequence directly and the slicing operation which allows us to retrieve a slice of the
sequence i.e. a part of the sequence.
Using Sequences
Example 9.5. Using Sequences
#!/usr/bin/python
# Filename: seq.py
shoplist = ['apple', 'mango', 'carrot', 'banana']
# Indexing or 'Subscription' operation
print 'Item 0 is', shoplist[0]
print 'Item 1 is', shoplist[1]
print 'Item 2 is', shoplist[2]
print 'Item 3 is', shoplist[3]
print 'Item -1 is', shoplist[-1]
print 'Item -2 is', shoplist[-2]
# Slicing on a list
print 'Item 1 to 3 is', shoplist[1:3]
print 'Item 2 to end is', shoplist[2:]
print 'Item 1 to -1 is', shoplist[1:-1]
print 'Item start to end is', shoplist[:]
# Slicing on a string
name = 'swaroop'
print 'characters 1 to 3 is', name[1:3]
print 'characters 2 to end is', name[2:]
print 'characters 1 to -1 is', name[1:-1]
print 'characters start to end is', name[:]
Output
$ python seq.py
Item 0 is apple
Item 1 is mango
Item 2 is carrot
Item 3 is banana

Item -1 is banana
Item -2 is carrot
Item 1 to 3 is ['mango', 'carrot']
Item 2 to end is ['carrot', 'banana']
Item 1 to -1 is ['mango', 'carrot']
Item start to end is ['apple', 'mango', 'carrot', 'banana']
characters 1 to 3 is wa
characters 2 to end is aroop
characters 1 to -1 is waroo
characters start to end is swaroop
How It Works
First, we see how to use indexes to get individual items of a sequence. This is also referred to as the subscription
operation. Whenever you specify a number to a sequence within square brackets as shown
above, Python will fetch you the item corresponding to that position in the sequence. Remember that Python
starts counting numbers from 0. Hence, shoplist[0] fetches the first item and shoplist[3]
fetches the fourth item in the shoplist sequence.
The index can also be a negative number, in which case, the position is calculated from the end of the
sequence. Therefore, shoplist[-1] refers to the last item in the sequence and shoplist[-2]
fetches the second last item in the sequence.
The slicing operation is used by specifying the name of the sequence followed by an optional pair of
numbers separated by a colon within square brackets. Note that this is very very similar to the indexing
operation you have been using til lnow. Remember the numbers are optional but the colon isn't.
The first number (before the colon) in the slicing operation refers to the position from where the slice
starts and the second number (after the colon) indicates where the slice will stop at. If the first number is
not specified, Python will start at the beginning of the sequence. If the second number is left out, Python
will stop at the end of the sequence. Note that the slice returned starts at the start position and will end
just before the end position i.e. the start position is included but the end position is excluded from the sequence
slice.
Thus, shoplist[1:3] returns a slice of the sequence starting at position 1, includes position 2 but
stops at position 3 and therefore a slice of two items is returned. Similarly, shoplist[:] returns a
copy of the whole sequence.
You can also do slicing with negative positions. Negative numbers are used for positions from the end of
the sequence. For example, shoplist[:-1] will return a slice of the sequence which excludes the
last item of the sequence but contains everything else.
Try various combinations of such slice specifications using the Python interpreter interactively i.e. the
prompt so that you can see the results immediately. The great thing about sequences is that you can access
tuples, lists and strings all in the same way!