COMP90059 Semester 1, 2022
Introduction to Programming
School of Computing and Information Systems
Faculty of Engineering and Information Technology
Lecture 6: Iteration
Dr Simon D’Alfonso
COMP90059 Introduction to Programming 2
Lecture 5 Challenges
1. Write a function ascii_match(), which accepts two
required input arguments, an integer and a string. If
the ASCII numbers of all the characters in the string
add up to the value of the input integer, return True.
Otherwise, return False.
2. Write a function avg_three() that returns the average
(mean) of three integers between 1 and 9. The
function should allow for three optional input
arguments (num1, num2, num3), but if any of these
arguments is not provided when the function is
called, then a random number is generated for that
value. See
https://www.w3schools.com/python/ref_random_ran
dint.asp for how to generate random integers.
COMP90059 Introduction to Programming 3
Notes
An update on the current plans for the Mid-
Semester Test (MST) and Assignment 1 (A1):
• A1 will be released via Grok on the Friday at
the end of Week 6 (April 8) and will be due 2
weeks after that.
• The MST will be held during the lecture period
of Week 7. It will be conducted online via LMS.
I will put up some sample questions soon.
• With MST, be careful in cut and pasting.
• Don’t worry.
• Discussion forum: tutors will also respond.
COMP90059 Introduction to Programming 4
Overview
• For Loops
• While loops
• Nested loops
• Good practice for writing code and the PEP8
standard
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Iteration
• Repeating or looping instructions to make
the computer do something repeatedly.
• Repeat something a fixed number of times:
• times a number by itself n times
• print each element in a collection
• print 7 instances of something
• Repeat something until something happens
(e.g., scroll while button held)
COMP90059 Introduction to Programming 6
Loops in Python
• The Python programming language provides
the following types of loops to handle
looping requirements:
• for loop - iterates over the members of a sequence
in order, executing the statement(s) inside each
time.
• while loop - repeats a statement or group of
statements while a given condition is true. It tests
the condition before executing the loop body.
• These loop types can also be nested: can
use one or more loops inside any another
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The for loop
• A for loop is used for iterating over Python
data collections (strings, lists, tuples,
dictionaries, sets)
• During each pass or iteration of the loop
statement(s) inside will be executed
• Basic form:
for in :
statement(s)
• Note: in here is not (quite) the same as the
comparison operator of the same name
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For loops and sequences
• So for loops are used with sequences, which
are ordered collections
• We have seen three types of sequences so
far (to varying degrees of detail):
• Strings
• Lists
• Tuples
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For Loop and String Example
for char in 'COMP90059':
print(char)
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For Loop and Tuple Example
sum = 0
for i in (1, 2, 3):
sum = sum + i
print(sum)
The code above is equivalent to the code below.
sum = 0
sum = sum + 1
sum = sum + 2
sum = sum + 3
print(sum)
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For Loop and List Example
sum = 0
for i in [1, 2, 3]:
sum = sum + i
print(sum)
The code above is equivalent to the code in the
previous slide.
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Exercise 1
Write a function is_pangram(s), which
determines if an input string s is a pangram
and returns True or False accordingly. Recall
that a pangram is a string in which each
alphabetical characters appears at least once.
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Exercise 1 Solution
def is_pangram(s):
something = ? #what goes here
for char in something:
if char not in s.lower():
return False
return True
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Using range()
• The useful Python function range() returns a sequence of
numbers.
• It is used with loops such as for to conveniently iterate a
certain number of times.
• Take the two code examples below to print out the
numbers 1, 2, 3.
#approach using a tuple with ordered numbers each specified.
for i in (1, 2, 3):
print(i)
#approach using a range
for i in range(1, 4):
print(i)
• If we needed to go through 1 – 1000, the latter approach
would be much better than the former.
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Range()
There are three ways you can call range():
• range(stop) takes one argument.
• Starts at 0 and stops at but does not include stop
• For example, range(3) would generate the numbers
0, 1, 2
• range(start, stop) takes two arguments.
• Starts at integer start and stop is same as above.
• range(start, stop, step) takes three
arguments.
• Start and stop same as above, step argument
determines the increment between each integer in
the sequence
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Range() Examples
for i in range(3):
print(i)
for i in range(5, 10):
print(i)
for i in range(0, 10, 2):
print(i)
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Points to remember about range()
• All three arguments must be integers
(positive or negative).
• The step value must not be zero. If a step is
zero python raises a ValueError exception.
• range() is a type in Python
• Users can access items in a range() by
index, just as can be done with strings, lists
and tuples.
• Technically start can be greater than stop,
though this would effectively give a null
range.
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Range() Examples
for i in range(1, 10, 0):
print(i)
for i in range(-10, -1):
print(i)
for i in range(10, 5):
print(i)
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For over lists
• Lists (arrays) are sequences that are mutable
• They can contain various types of data,
including mixtures of different data types
• Share many similarities with strings, and can
similarly be accessed with indexes
>>> highest_scorers = ['Sally', 'Jessica', 'Anne']
>>> highest_scorers[0]
'Sally'
>>> highest_scorers[2] = 'Brenda'
>>> highest_scorers[-1]
'Brenda'
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List Methods
• Python has a set of built-in methods that you can use for lists:
https://www.w3schools.com/python/python_lists_methods.asp
>>> my_numbers = [4, 5, 6.6, 3.3]
>>> my_numbers.append(1)
>>> my_numbers.sort()
>>> my_numbers
[1, 3.3, 4, 5, 6.6]
>>> my_numbers = [4, 5, 6.6, 3.3, 1]
>>> sorted(my_numbers) #sorted sorts collections
[1, 3.3, 4, 5, 6.6]
>>> my_numbers
[4, 5, 6.6, 3.3, 1]
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List Sort Trivia
What would happen with the following:
>>> my_list = [5, 6, 1, False]
>>> sorted(my_list)
?
>>> my_list = [4, 'four', True,
3.3]
>>> sorted(my_list)
?
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For, range() and lists
# using range for iteration over a list
my_list = [10, 20, 30, 40]
for i in range(len(my_list)):
print(my_list[i], end=" ")
print()
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The while loop
• The other type of loop is a while loop, which
is a conditional loop.
• The general idea is that we continue
repeating a block of code as long as a given
condition holds.
• Basic form:
while :
statement(s)
COMP90059 Introduction to Programming 24
While Loop Example
number_guess = None
while number_guess != "7":
number_guess = input('Guess the lucky
number: ')
print(number_guess + " is correct!")
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Break statement
Another way to end loops is via a break in the block of code.
This prematurely and unconditionally exits from the loop.
Below is an example that is an alternative to the code in the
previous slide.
number_guess = None
while True:
number_guess = input('Guess the lucky
number: ')
if number_guess == "7":
break
print('Sorry, wrong guess')
print(number_guess + " is correct!")
COMP90059 Introduction to Programming 26
Break example with for loop
my_list = [1, 5, 8, 9]
for i in my_list:
print(i)
if i == 8:
break
What is the output of this program?
COMP90059 Introduction to Programming 27
Beware the infinite loop
while True:
print(True):
#---
i = 1
j = 10
while i < j:
print(i)
How could we do something so that we keep
print(i) but it stops after printing out 1, 2, 3, …
COMP90059 Introduction to Programming 28
Choosing between for and while loops
• If you need to iterate over all items of an
iterable, use a for loop
• If there is a well defined end-point to the
iteration which doesn’t involve iterating over
all items, use a while loop
• With a for loop, avoid modifying the object
you are iterating over within the block of
code
• Given a choice between the two, for loops
are generally more elegant/safer/easier to
understand
COMP90059 Introduction to Programming 29
Exercise 2
• The Fibonacci Sequence is the series of
numbers: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...
• Starting with 0 and 1, the next number is
found by adding up the two numbers before
it.
• Write a function that gets an integer n via
input() and using for generates a list with the
first n Fibonacci numbers.
COMP90059 Introduction to Programming 30
Exercise 2 Solution
n = int(input("Number of elements in Fibonacci Series, n, (n>=2) :
"))
#initialize the list with starting elements: 0, 1
fibonacci_list = [0, 1]
if n > 2:
for i in range(2, n):
#next elment in series = sum of its previous two numbers
next_element = fibonacci_list[i-1] + fibonacci_list[i-2]
#append the element to the series
fibonacci_list.append(next_element)
print(fibonacci_list)
COMP90059 Introduction to Programming 31
Nested for loops
Syntax for nested for:
for var1 in sequence:
for var2 in sequence:
statements(s) # inner loop
statements(s) # outer loop
statement(s) # outside loops
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Nested for Example 1
n = int(input('enter a number:'))
for i in range(1, n+1):
for j in range(1, n+1):
print(i, j)
print("good bye")
COMP90059 Introduction to Programming 33
Nested for Example 2
#Example code that checks whether
numbers between 1 and 10 are prime.
for num in range(1, 11):
prime = True
for x in range(2, num):
if not(num % x):
prime = False
if prime:
print(num, "is a prime")
else:
print(num, "is not a prime")
COMP90059 Introduction to Programming 34
Nested for Example 3
#display the 12x12 multiplication tables
for i in range(1, 13):
for j in range(1, 13):
print(i * j, ' ' * (3 - len(str(i * j))), '|', end = "")
print('')
COMP90059 Introduction to Programming 35
Nested while
Syntax for nested while:
while :
while :
statement(s) # inner loop
statement(s) # outer loop
statement(s) # outside loops
COMP90059 Introduction to Programming 36
Using for/while and else
In Python we can use else with a for/while
loop. The else block will be executed only if the
loop terminates naturally (not due to a break).
def digits_in_word(word):
for i in range(len(word)):
if word[i].isdigit():
print("The word contains digits")
break
else:
print("The word does not contain digits")
COMP90059 Introduction to Programming 37
Good practice for writing code
• PEP8 provides a set of standards or best
practices for writing good, neat code:
https://peps.python.org/pep-0008/
• Why a standard:
• To improve readability; code is read more often than
it is written.
• Make code more consistent, which promotes code
sharing and reusability.
COMP90059 Introduction to Programming 38
Overview of PEP8
• 4 spaces per indent line level.
• Lines must not exceed 79 characters.
• Use blank lines to separate logical sections
(e.g., between functions and major sections of
code associated with a given comment).
• Always start a new line after if, elif, else, while,
for, etc.
• Function names should be lowercase, with
words separated by underscores.
• Constants should be written in all capitals , with
words separated by underscores.
• Don’t compare Boolean values to True or False
using ==
COMP90059 Introduction to Programming 39
Summary
Today we covered:
• The for loop
• The range function
• The while loop
• Nested loops
• Good coding style
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Lecture 6 Challenges
• Convert the code on the Nested for Example 1
slide to a functionally equivalent piece of code
that uses while loops instead of for loops.
• Modify the Exercise 1 Solution so that the
function instead returns a list containing all
characters that don’t appear in the string. If the
string is a pangram, then return an empty list.
• Write a program that uses a while loop to print
out each individual character in a given string
line by line.
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Lecture Identification and Acknowledgement
Coordinator / Lecturer: Simon D’Alfonso
Semester: Semester 1, 2022
© University of Melbourne
These slides include materials from 2020 - 2021
instances of COMP90059 run by Kylie McColl or
Wally Smith