CSC1018F:Functional Programming

Diving into Python Ch. 16

Lecture Outline

Recap of Regular Expressions [week 3]Functional Programming

The Zen of Functional ProgrammingFinding Path NamesFiltering and Mapping RevisitedRevision Exercise: Chess Notation

Recap of Regular Expressions

A powerful mechanism for identifying patterns in textBut need to be handled with care

An entirely new syntax is required

Verbose regular expressions include whitespace and comments

The verbose form is preferred

Introduction to Functional Programming

Data-centric thought patternFunctions are simply treated as another form of dataCan be operated on and passed as arguments!Enables higher level logic and avoid “busywork coding”

A philosophy of focusing on the data and how it needs to be transformed

Finding Path Names

Functions useful in finding the name and path of the currently executing script:

sys.argv - a list with the name and command line arguments of the scriptos.path.dirname(filename) - separates out and returns the path portion of filenameos.path.abspath(pathname) - expands a pathname from partial to full. Performs normalizationos.getcwd() - returns the current working directory

Filtering Lists Differently

Pass a function and a list to a filter. List elements are passed to the function which returns True for elements in the final filtered list

Syntax: filter(fn, li)fn takes a single argument and evaluates to true or false

Example: >>> def odd(n):

... return n % 2

...

>>> li = range(10)

>>> filter(odd, li)

[1,3,5,7,9]

>>> [e for e in li if odd(e)]

Mapping Lists Differently

Pass a function and a list to a map. List elements are passed through the function:

Syntax: map(fn, li)fn takes a single argument, operates on it and returns a new version

Example: >>> def double(n):

... return n * 2

...

>>> li = range(10)

>>> map(double, li)

[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

>>> [double(e) for e in li]

Example: Dynamically Importing Modules

__import__ - a built in function that takes a module name as a string and returns the moduleAllows us to load and access modules that are only named at run-timeExample:>>> moduleNames = [‘sys’, ‘os’, ‘re’]

>>> modules = map(__import__, moduleNames)

>>> modules[<module 'sys' (built-in)>, <module 'os' from '/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/os.pyc'>, <module 're' from '/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/re.pyc'>]

>>> modules[0].version'2.3.3 (#2, Dec 23 2003, 22:56:29) \n[GCC 3.1 20020420 (prerelease)]'

Revision Exercise

You are tasked with coding an “anonymization” program. This replaces names with initials in order to hide identity:

To begin, read in a list of names from a file “idprotect.txt”, which contains names in the form: “Firstname” and also “Firstname Surname” on separate linesThe main text should be read in from “plaintext.txt” and all occurrences of idprotect names replaced with their corresponding initial. Thus “John” becomes “J.” and “John Smith” becomes “J. S.” The final anonymous text should be output to the file “anontext.txt”

Do this using regular expressions, maps and filters, where appropriate