# 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[:]
$ 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
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 andshoplist[3] fetches the fourth item in theshoplist 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 andshoplist[-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!