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Wednesday, August 16, 2017
 
Python Lists in 5 Minutes or Less .. Part 2

SEE ALSO: Python Lists in 5 Minutes or Less .. Part 1


Extracting Part of a List (sublist)

Lists can be sliced using [:] construction. list[start:end] (slicing) returns a new list containing elements in the original list from index position "start" to "(end-1)". Index starts at 0. "start" and "end" positions are optional. When "start" position was omitted, start of the list is assumed. When "end" position was omitted, end of the list is assumed. If both are omitted, entire list is returned as is.

eg.,
>>> record
['Gary', 'Doe', 25, 'Network Ct, Twin Peaks, WA 90201', 140.32, '925-93-2176', 'John', 'Keats', 'Sears Tower, Chicago, IL 46371']

>>> record[4:7]
[140.32, '925-93-2176', 'John']

>>> record[:4]
['Gary', 'Doe', 25, 'Network Ct, Twin Peaks, WA 90201']

>>> record[4:]
[140.32, '925-93-2176', 'John', 'Keats', 'Sears Tower, Chicago, IL 46371']

>>> record[:]
['Gary', 'Doe', 25, 'Network Ct, Twin Peaks, WA 90201', 140.32, '925-93-2176', 'John', 'Keats', 'Sears Tower, Chicago, IL 46371']

Negative index counts back from the end of the list (right-to-left). -1 refers to the right most (last) element in the list.

eg.,
Extract last 3 elements in the list using negative index.

>>> record[-3:]
['John', 'Keats', 'Sears Tower, Chicago, IL 46371']

Extract all elements except the last 3.

>>> record[:-3]
['Gary', 'Doe', 25, 'Network Ct, Twin Peaks, WA 90201', 140.32, '925-93-2176']

An optional "step" parameter can be specified to skip few elements from the list, or put another way to get every nth element from the list. Syntax: list[start:end:step]

eg.,
Fetch every alternate element from the beginning of the list. 

>>> record[::2]
['Gary', 25, 140.32, 'John', 'Sears Tower, Chicago, IL 46371']

Reverse the list.

>>> record[::-1]
['Sears Tower, Chicago, IL 46371', 'Keats', 'John', '925-93-2176', 140.32, 'Network Ct, Twin Peaks, WA 90201', 25, 'Doe', 'Gary']

Note that the new lists returned from slicing a list are in reality shallow copies so contain references to elements from the original list.


Sorting a List

sort() method of the list object returns nothing (None) but modifies the original list by sorting in place. sort method supports 3 optional parameters -- method to be used for sorting (cmp), function to be executed with key element (key) and the reverse option. Syntax: sort(cmp, key, reserve).

eg.,
>>> record
['Gary', 'Doe', 25, 'Network Ct, Twin Peaks, WA 90201', 140.32, '925-93-2176', 'John', 'Keats', 'Sears Tower, Chicago, IL 46371']

>>> record.sort()
>>> record
[25, 140.32, '925-93-2176', 'Doe', 'Gary', 'John', 'Keats', 'Network Ct, Twin Peaks, WA 90201', 'Sears Tower, Chicago, IL 46371']

>>> record=['Gary', 'Doe', 25, 'Network Ct, Twin Peaks, WA 90201', 140.32, '925-93-2176', 'John', 'Keats', 'Sears Tower, Chicago, IL 46371']

>>> record.sort(reverse=True)
>>> record
['Sears Tower, Chicago, IL 46371', 'Network Ct, Twin Peaks, WA 90201', 'Keats', 'John', 'Gary', 'Doe', '925-93-2176', 140.32, 25]

If you don't want the original list to be modified, one option is to make a copy of the list and call sort() on the copy. Another option is to use the built-in sorted() function. The sorted() function returns the sorted list. It also accepts reverse parameter similar to sort() of the list.

eg.,
>>> record=['Gary', 'Doe', 25, 'Network Ct, Twin Peaks, WA 90201', 140.32, '925-93-2176', 'John', 'Keats', 'Sears Tower, Chicago, IL 46371']

>>> import copy
>>> recorddup = copy.copy(record)
>>> recorddup.sort()

>>> recorddup
[25, 140.32, '925-93-2176', 'Doe', 'Gary', 'John', 'Keats', 'Network Ct, Twin Peaks, WA 90201', 'Sears Tower, Chicago, IL 46371']

>>> record
['Gary', 'Doe', 25, 'Network Ct, Twin Peaks, WA 90201', 140.32, '925-93-2176', 'John', 'Keats', 'Sears Tower, Chicago, IL 46371']

>>> sorted(record)
[25, 140.32, '925-93-2176', 'Doe', 'Gary', 'John', 'Keats', 'Network Ct, Twin Peaks, WA 90201', 'Sears Tower, Chicago, IL 46371']

>>> sorted(record, reverse=True)
['Sears Tower, Chicago, IL 46371', 'Network Ct, Twin Peaks, WA 90201', 'Keats', 'John', 'Gary', 'Doe', '925-93-2176', 140.32, 25]

Comparing Lists

Lists can be compared for equality using == operator. Lexicographical comparison is possible with the help of less-than and greater-than operators.

eg.,
>>> record
['Gary', 'Doe', 25, 'Network Ct, Twin Peaks, WA 90201', 140.32, '925-93-2176', 'John', 'Keats', 'Sears Tower, Chicago, IL 46371']

>>> recorddup
[25, 140.32, '925-93-2176', 'Doe', 'Gary', 'John', 'Keats', 'Network Ct, Twin Peaks, WA 90201', 'Sears Tower, Chicago, IL 46371']

>>> record == recorddup
False

>>> sorted(record) == recorddup
True

>>> record > recorddup
True

>>> record < recorddup
False

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