String Indexing

In Python, we can access a certain/individual item in a sequence if the data type is indexable.

To index/access a single item from a string, we use [ ] square brackets.

# Example
'''
Looking at: 'Hello!'

 |  H  |  e  |  l  |  l  |  o  |  !  |
 0     1     2     3     4     5     6

'''

word = 'Hello!'

print('word[2]:', word[2])
print('word[1]:', word[1])
print('word[0]:', word[0])
print('word[5]:', word[5])
print('word[6]:', word[6])

word[2]: l
word[1]: e
word[0]: H
word[5]: !



---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

<ipython-input-3-f291ab6028d1> in <module>
     14 print('word[0]:', word[0])
     15 print('word[5]:', word[5])
---> 16 print('word[6]:', word[6])


IndexError: string index out of range

NOTE:

• Indexing always starts at 0 (zero)

• Indexing always accesses a single item from the sequence, for strings: a character from the targetted string

• The index value cannot go beyond the limit of the sequence. Mathematically, the index value cannot be greater than length of the sequence subtracted by one.

  • Examine the error for print('word[6]:', word[6]): gives us index out of range error

Looking at: 'Hello!'

 |  H  |  e  |  l  |  l  |  o  |  !  |
 0     1     2     3     4     5     6

• Notice that our index value in this diagram starts at the left of the item

• When we index, it will go to the given location and look at the item to the right of the index

  • Therefore, print('word[1]:', word[1]) gives us: e

Negative Indexing

Just like how we can use positive integer-indexes to access values, we can also use negative integer-indexes to access values as well.

# Example
'''
Looking at: 'Hello!'

 |  H  |  e  |  l  |  l  |  o  |  !  |  H  |  e  |  l  |  l  |  o  |  !  |
-6    -5    -4    -3    -2    -1     0     1     2     3     4     5     

'''

word = 'Hello!'

print('word[-1]:', word[-1])
print('word[-3]:', word[-3])
print('word[-5]:', word[-5])
print('word[-6]:', word[-6])
print('word[-7]:', word[-7])

word[-1]: !
word[-3]: l
word[-5]: e
word[-6]: H



---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

<ipython-input-6-5d7a5ddd18cd> in <module>
     14 print('word[-5]:', word[-5])
     15 print('word[-6]:', word[-6])
---> 16 print('word[-7]:', word[-7])


IndexError: string index out of range

NOTE:

• Index of -1 is the quickest way to grab the last value of an indexable sequence

• Examine that at -6 and 0 we got the value of H for Hello!, they are located at the same place

Looking at: 'Hello!'

 |  H  |  e  |  l  |  l  |  o  |  !   
-6    -5    -4    -3    -2    -1      

• Much like how we look at the right of positive indexes, we do the same for negative indexes

• The indexes are laid out like an integer line with the string itself duplicated; however, Python does not actually create the string as a double

  • It is smart enough to know how to handle positive and negative indexes with a single instance of the sequence