Introduction to Python

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Data Parsing & Extraction: List Operations and String Slicing

lists and list subscripting

Subscripting allows us to select individual elements of a list.

fields = ['jw234', 'Joe', 'Wilson', 'Smithtown', 'NJ', '2015585894']

var = fields[0]           # 'jw234'
var2 = fields[4]          # 'NJ'
var3 = fields[-1]         # '2015585894' (-1 means last index)

• the list is a sequence of objects of any type (here they are strings)
• subscripting means accessing an individual item within the list
• square brackets specify an element index, starting at 0
• the last index can also be specified using -1
• indices count from 0 at the start, or -1 at the

lists: slicing

Slicing allows us to select multiple items from a list.

letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
first_four = letters[0:4]
print(first_four)                     # ['a', 'b', 'c', 'd']

# no upper bound takes us to the end
print(letters[5:])                    # ['f', 'g', 'h']

Here are the rules for slicing:

   1) the first index is 0
   2) the lower bound is the 1st element to be included
   3) the upper bound is one higher the last element to be included
   4) no upper bound means "to the end"

strings: slicing

Slicing a string selects characters the way that slicing a list selects items.

mystr = '2014-03-13 15:33:00'
year =  mystr[0:4]               # '2014'
month = mystr[5:7]               # '03'
day =   mystr[8:10]              # '13'

Again, please review the rules for slicing:

   1) the first index is 0 (first character)
   2) the lower bound is the 1st character to be included
   3) the upper bound is one higher the last character to be included
   4) no upper bound means "to the end"

the IndexError exception

An IndexError exception indicates use of an index for a list element that doesn't exist.

mylist = ['a', 'b', 'c']

print(mylist[5])            # IndexError:  list index out of range

Since mylist does not contain a sixth item (i.e., at index 5), Python tells us it cannot complete this operation.