EXAMINATIONS 2012 MID-YEAR NWEN 241 SYSTEMS PROGRAMMING. The examination contains 5 questions. You must answer ALL questions

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1 EXAMINATIONS 2012 MID-YEAR NWEN 241 SYSTEMS PROGRAMMING Time allowed: Instructions: THREE HOURS The examination contains 5 questions. You must answer ALL questions Each question is worth 36 marks. The exam consists of 180 marks in total. Non-programmable calculators are allowed. Paper foreign to English language dictionaries are allowed. Electronic dictionaries and programmable calculators are not allowed. NWEN241 Page 1 of 9 continued

2 Question 1 - Python fundamentals [36 marks] a) [5 marks] Some of following strings are legal Python identifiers and some are not. For each one state if the name is legal and if not, explain why: i) lambda ii) 4squared iii) days_of_week iv) square-root v) daysofweek vi) 1) not legal Python keyword 2) not legal variables can t start with an number 3) legal variables can contain alphanumerics and underscores ( _ ) 4) not legal hyphen ( - ) is not a legal character 5) legal uppercase letters are OK NWEN241 Page 2 of 9 continued

3 b) [16 marks] For each of the following Python keywords, briefly explain what it does and write a short piece of code to illustrate its use. i) try The try keyword is used to allow a section of code to be run and have an exception handled if it fails:!/usr/bin/env python3 f = None f = open('films', 'r') for i in f: print i, except IOError: print "Error reading file" finally: if f: f.close() ii) return The return keyword:!/usr/bin/env python3 def square(x): return x * x a = square(2) print (a) The example demonstrates a simple function. The function will calculate the square of a number. The return keyword is closely connected with a function definition. The keyword exits the function and returns a value. iii) in NWEN241 Page 3 of 9 continued

4 The in keyword.!/usr/bin/env python3 print (4 in (2, 3, 5, 6)) for i in range(25): print (i, end= ) print( ) In this example, the in keyword tests if the number four is in the tuple. The second usage is traversing a tuple in a for loop. The built-in function range() returns integers iv) import The import statement is used to include Python modules into programs and make the features and functions of the module available to the program. In this example the square root function can be used after the math module is imported.!/usr/bin/env python3 import math x = 25 y = math.sqrt(x) print('the square root of', x, 'is', y) NWEN241 Page 4 of 9 continued

5 Question 2 Writing and reading Python programs [36 marks] a) [10 marks] Write a Python program that reads two non negative integers as parameters on the command line and prints them out. For example, it might be called by: $ python3 testprog.py 2 1 Your program should check the parameters and issue appropriate error messages and return values.!/usr/bin/env python3 import Python libraries import os import sys get the name of our program with leading dirname removed progname = os.path.basename(sys.argv[0]) def : print('usage: %s <non-negative integer> <non-negative integer>' % progname) sys.exit(-1) process the inputs from the user if len(sys.argv) == 3: read inputs and convert to integers m = int(sys.argv[1]) n = int(sys.argv[2]) except ValueError: tell user inputs were not acceptable if m < 0 or n < 0: tell user inputs were not acceptable if inputs are non-negative print them print(m, n) sys.exit(0) NWEN241 Page 5 of 9 continued

6 b) [10 marks] Modify your program to prompt the user for input if no command line arguments were given. You may choose to show just the changes, if you prefer.!/usr/bin/env python3 import Python libraries import os import sys get the name of our program with leading dirname removed progname = os.path.basename(sys.argv[0]) def : print('usage: %s <non-negative integer> <non-negative integer>' % progname) sys.exit(-1) process the inputs from the user if len(sys.argv) == 1: no arguments on command line prompt the user for the details m = int(input('non-negative integer: ').strip()) n = int(input('non-negative integer: ').strip()) except ValueError: elif len(sys.argv) == 3: read inputs and convert to integers m = int(sys.argv[1]) n = int(sys.argv[2]) except ValueError: tell user inputs were not acceptable if m < 0 or n < 0: tell user inputs were not acceptable if inputs are non-negative print them print(m, n) sys.exit(0) NWEN241 Page 6 of 9 continued

7 c) [16 marks] Consider the following python program which retrieves data from a website and analyses it. You are to add 10 suitable comments to the code explaining the section following.!/usr/bin/env python3 import datetime import urllib.request urlbase = ' months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'] (1) thisyear = datetime.datetime.now().year (2) years = range(1998, thisyear + 1) def extract(text, sub1, sub2): (3) return text.split(sub1, 1)[-1].split(sub2, 1)[0] for year in years: for month in months: (4) (5) (6) listurl = urlbase + str(year) + '-' + month + '/subject.html' fp = urllib.request.urlopen(listurl) mybytes = fp.read() fp.close() except: break encoding = extract(str(mybytes).lower(), 'charset=', '"') if encoding == None: print("encoding type not found!") break lines = mybytes.decode(encoding).split('\n') NWEN241 Page 7 of 9 continued

8 totals = dict() total_subjects = 0 total_postings = 0 print (month, year, '\n***************') (7) (8) for line in lines: if '<LI>' in line: posting = line.split('>') url = posting[1].split('"')[1] subject = posting[2].lstrip() if subject not in totals: totals[subject] = 1 total_subjects += 1 totals[subject] += 1 total_postings += 1 except: break (9) (10) for subject in sorted(totals, key=totals.get, reverse=true): print('{0:60} {1:10}'.format(subject, totals[subject])) print('\npostings: {0:}, Subjects: {1:}\n'.format(total_postings,total_subjects)) Additional information Data returned from the fp.read() statement in the code is a sequence of bytes that are encoded in a particular character set. You can assume that the HTML data retrieved will contain lines like the following: <meta http-equiv="content-type" content="text/html; charset=iso "> <LI><A HREF=" html">[nznog] APNIC 34 Conference - Call for Papers </A><A NAME="19115"> </A> <LI><A HREF=" html">[nznog] Beeping rack at Sky tower </A><A NAME="19073"> </A> <LI><A HREF=" html">[nznog] Beeping rack at Sky tower </A><A NAME="19074"> </A> NWEN241 Page 8 of 9 continued

9 1. Use the datetime library to work out the current year 2. Use the range function to create a generator for the list of years 3. Get the text between two substrings 4. Construct a URL based on the year and month 5. Extract the encoding of the document from the html data 6. Decode the document based on the encoding noted in the document 7. Loop through each line of data in the document 8. Split each line into a number of fields 9. Print a formatted and sorted list of the subject lines found and their counts 10. Print out a formatted summary of the month s data NWEN241 Page 9 of 9 continued