Terms Test 3 April 2014 NWEN 241 SYSTEMS PROGRAMMING

Terms Test
3 April 2014
NWEN 241
SYSTEMS PROGRAMMING
Time allowed:
50 minutes
Instructions:
The test contains 4 questions. You should attempt ALL questions
Each question is worth 10 marks.
The exam consists of 40 marks in total.
Non-programmable calculators are allowed.
Paper foreign to English language dictionaries are allowed.
Electronic dictionaries and programmable calculators are not allowed.
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Question 1 - Python fundamentals
[10 marks]
a) [4 marks] Some of following strings are legal python3 identifiers and some are not. For each one
state if the name is legal or not and if not, explain why:
i) global
not legal – Python keyword
ii) 4squared
not legal – variables can’t start with an number
iii) square-root
not legal – hyphen (‘-‘) is not a legal character
iv) daysOfWeek
legal – uppercase letters are OK
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b) [6 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
try:
f = open('films', 'r')
for i in f:
print i,
except IOError:
print "Error reading file"
finally:
if f:
f.close()
ii)
in
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 0 .. 24.
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Question 2 – Writing a Python program
[10 marks]
a) [10 marks] Write a python3 program that reads a filename from the command line,
opens a file and prints the contents on the screen. For example, it might be called by:
$ python3 printfile.py filename
Your program 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 usage():
print('Usage: {0} <filename>'.format(progname))
sys.exit(­1)
# process the inputs from the user
if len(sys.argv) == 2:
# read inputs and convert to integers
try:
filename = sys.argv[1]
except ValueError:
usage()
else:
# tell user inputs were not acceptable
usage()
sys.exit(­1)
try:
with open(filename) as fin:
for line in fin:
print(line.rstrip())
fin.close()
except:
print("ERROR: {0} couldn't open file '{1}'".format(progname, filename))
sys.exit(­1)
sys.exit(0)
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Question 3 – Reading a Python program
[10 marks]
Consider the following python3 program.
#! /usr/bin/env python3
#
# Declare a string with all the alphanumeric characters
#
alphabet='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'
#
# (1)
#
def some_varencode(number):
#
# Check that number is an integer
#
if not isinstance(number, int):
raise TypeError('number must be an integer')
#
# Zero is a special case
#
if number == 0:
return '0'
#
# (2)
#
sign = ''
if number < 0:
sign = '-'
number = - number
#
# (3)
#
some_var = ''
while number != 0:
number, i = divmod(number, len(alphabet))
some_var = alphabet[i] + some_var
#
# (4)
#
return sign + some_var
#
# (5)
#
def some_vardecode(some_var):
return int(some_var, len(alphabet))
print(some_varencode(34))
print(some_varencode(1234))
print(some_vardecode('R'))
print(some_vardecode('QCH'))
HINT: The built-in function divmod(x, y) takes two numbers as arguments and returns a pair of
numbers consisting of their quotient and remainder when using long division
e.g. divmod(7,3) -> (2, 1).
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a) [5 marks] The code contains 5 numbered places where comments are required. Write 5 suitable
comments in the box below. Comments (1) and (5) should describe what the complete function does
and comments (2) to (4) should describe the section that follows.
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1) Encode a base 10 number in base 36
2) Record the 'sign' of the input and make sure the number to be converted is positive.
3) Convert the number to base 36 using divmod(x,y) to calculate the result
4) Return the converted value with the given 'sign'
5) Convert a base 36 number to base 10
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b) [5marks] The functions in the program above are called with the following values:
print(some_varencode(34))
print(some_varencode(1234))
34th element of the alphabet => Y
print(some_vardecode('R'))
print(some_vardecode('QCH')
>>> divmod(1234,36)
(34, 10)
34 -> 'Y'
10 -> 'A'
YA
'R' - > 27th element of the alphabet
'Q' -> 26
'C' -> 12
'H' -> 17
(((26 * 36) + 12) * 36) + 17
34145
What is the result in each case?
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Question 4 – Interfacing with other systems
[10 marks]
a) [6 marks] We can call programs written in C (and other languages) from python3 using a toolkit
like the Simplified Wrapper and Interface Generator (SWIG). Briefly outline three tasks that need to
be done by the SWIG extension code to make this possible.
We need extension code to:
1.  Convert data values from Python to C,
2.  Perform a function call to a C routine using the converted values, and
3.  Convert the data values from the call from C to Python.
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b) [5 marks] The following commands were used to delete some records on a database server.
# Prepare SQL query to DELETE required records
sql = "DELETE FROM EMPLOYEE WHERE AGE > 20”
# Execute the SQL command
cur.execute(sql)
The psycopg2 library provides two operations (conn.commit() and conn.rollback()) that we
need to use after we make such changes. Write a short code fragment to illustrate the correct use of
these operations. You should include comments that describe what the operations are doing.
# Prepare SQL query to DELETE required records
sql = "DELETE FROM EMPLOYEE WHERE AGE > 20”
try:
# Execute the SQL command
cur.execute(sql)
# Commit your changes in the database
conn.commit()
except:
# Rollback in case there is any error
conn.rollback()
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