Introduction to programming using Python

Session 4

Matthieu Choplin

matthieu.choplin@city.ac.uk

http://moodle.city.ac.uk/

Objectives

• To come back on the notion of object and type.
• To introduce to the type "List" and its methods.
• To use the len, min/max, sum, and random.shuffle functions for a list.
• To develop and invoke functions with list arguments and return value.
• To access list elements using indexed variables.
• To obtain a sublist using the slicing operator [start:end].
• To use +, *, and in/not in operators on lists.
• To traverse elements in a list using a for-each loop.
• To create lists using list comprehension.
• To split a string to a list using the str’s split method.
• To copy contents from one list to another.

What is the difference between an object and a type?

A type or a class is what is going to create an object

Type and object seen so far:

An object has methods

You can find the method of an object with the function dir(), which returns the attributes of an object.

>>> dir("abc")
['__add__', '__class__', '__contains__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'capitalize', 'casefold', 'center', 'count', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'format_map', 'index', 'isalnum', 'isalpha', 'isdecimal', 'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

NB: the dunder methods (with double underscore), are "special methods" in python that can be overridden. We will come back on that later.

Difference between methods of objects and builtin functions

The methods of an object can only be called on an object.

>>> "speak louder".upper()
'SPEAK LOUDER'

A builtin function does not need an object to be called.

>>> len("number of character")
19

NB: len() give the number of element in a sequence

The type List

Creating list using the list constructor

list1 = list() # Create an empty list
list2 = list([2, 3, 4]) # Create a list with elements 2, 3, 4
list3 = list(["red", "green", "blue"]) # Create a list of strings
list4 = list(range(3, 6)) # Create a list with elements 3, 4, 5
list5 = list("abcd") # Create a list with characters a, b, c

That is the equivalent of:

list1 = [] # Same as list()
list2 = [2, 3, 4] # Same as list([2, 3, 4])
list3 = ["red", "green"] # Same as list(["red", "green"])

The List methods

You can find the different methods of a list thanks to the function dir()

>>> dir([])
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'append', 'clear', 'copy', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']

We are going to look at: 'append', 'clear', 'copy', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort'

How to see what a method can do

Look at the builtin help:

>>> help([].append)
Help on built-in function append:

append(...) method of builtins.list instance
    L.append(object) -> None -- append object to end

Experiment in the interpreter:

Summary of the list methods

Exercise 1

Write a program that reads integers from the user and stores them in a list (use input() and append()). Your program should continue reading values until the user enters 'q' (the sentinel value). Then it should display all of the values entered by the user in order from smallest to largest, with one value appearing on each line. Use either the sort method or the sorted built in function to sort the list.

data = []
num = input("Enter an integer ('q' to quit): ")
while num != 'q':
    data.append(int(num))
    num = input("Enter an integer ('q' to quit): ")
data.sort()
print("The values, sorted into ascending order are:")
for element in data:
    print(element)

Builtin function for list or sequences

>>> list1 = [2, 3, 4, 1, 32]
>>> len(list1)
5
>>> max(list1)
32
>>> min(list1)
1
>>> sum(list1)
42
>>> import random
>>> random.shuffle(list1) # Shuffle the items in the list
>>> list1
[4, 1, 2, 32, 3]

Iterating on a list

The list is a sequence on which you can iterate.

With for:

With while:

Reminder about functions

We define the function like this:

def main():
    print('The function', main.__name__, 'has been called')

And we call the functions like this:

main()

NB: notice the brackets: when we define and when we call!

Try to use functions in the next exercises.

Exercise 2: Chinese Zodiac sign with list

Simplify the exercise we saw at the end of session 2 by using a list of string storing all the animals name, instead of multiple if and elif statement.

def main():
    year = int(input("Enter a year: "))
    animals = ["monkey", "rooster", "dog", "pig", "rat", "ox",
               "tiger", "rabbit", "dragon", "snake", "horse", "sheep"]
    print(year, "is", animals[year % 12])

main()

Passing Lists to Functions

Returning a List from a Function

Example: a function that returns a reversed list

The function reverse actually exists for doing the same thing

Exercise 3:

Complete this program to get the minimum number of the list and its index

import random
random_list = [random.choice(list(range(1, 100))) for _ in range(10)]
def get_min(random_list):
    # to complete
    pass
get_min(random_list)

Solution without using built in functions non list methods

import random
random_list = [random.choice(list(range(1, 100))) for _ in range(10)]
def get_min_index(any_list):
    min = 100
    index = 0
    for i in any_list:
        if i <= min:
            min = i
            min_index = index
        index += 1
    print("the min is", min)
    print("its index is", min_index)

print(random_list)
get_min_index(random_list)

Solution using built in functions and list methods

import random
random_list = [random.choice(list(range(1, 100))) for _ in range(10)]
def get_min_index(any_list):
    print("the min is", min(any_list))  # using the min builtin
    print("its index is", random_list.index(min(random_list))) # using the index method

print(random_list)
get_min_index(random_list)

Reminder

The string is a sequence

The items of a sequence can be accessed through indexes

Get the first element of the sequence:

my_string_variable = "abracadabra"
first_elem = my_string_variable[0]

Manipulate element of a List with indexes

You can also access element of a list with indexes BUT you can also modify them:

contrary to the string type.

Difference between mutable and immutable objects

• You cannot modify an immutable object such as a string.
• You can modify a mutable object such as a list.

The +, *, [ : ], and in Operators (1/2)

+ is for concatenating list

* is for repeating a list

[ : ] is the slice operator, for extracting a sublist from a list

>>> list1 = [2, 3]
>>> list2 = [1, 9]
>>> list3 = list1 + list2
>>> list3
[2, 3, 1, 9]
>>> list3 = 2 * list1
>>> list3
[2, 3, 2, 3]
>>> list4 = list3[2:4]
>>> list4
[2, 3]

The +, *, [ : ], and in Operators (2/2)

• Get the last element of a list with a negative index
• Check if an element is in a list with the in operator

>>> list1 = [2, 3, 5, 2, 33, 21]
>>> list1[-1]
21
>>> list1[-3]
2
>>> list1 = [2, 3, 5, 2, 33, 21]
>>> 2 in list1
True
>>> list1 = [2, 3, 5, 2, 33, 21]
>>> 2.5 in list1
False

List comprehensions

• List comprehensions provide a concise way to create lists
• Transforming a list with operation on each element
• Filtering a list, keeping only elements that satisfy a condition

>>> list1 = [x for x in range(0, 5)]
>>> list1
[0, 1, 2, 3, 4]
>>> list2 = [0.5 * x for x in list1]
>>> list2
[0.0, 0.5, 1.0, 1.5, 2.0]
>>> list3 = [x for x in list2 if x < 1.5]
>>> list3
[0.0, 0.5, 1.0]

Splitting a String to a List

You can convert a string to a list with the split function on string.

>>> items = "Welcome to the UK".split()
>>> print(items)
['Welcome', 'to', 'the', 'UK']
>>> items = "34#13#78#45".split("#")
>>> print(items)
['34', '13', '78', '45']

You can convert back a list to a string with the join function on string

>>> print(items)
['Welcome', 'to', 'the', 'UK']
>>> print(" ".join(items))
'Welcome to the UK'

Exercise 4 - Eliminate duplicates

Write a function that returns a new list by eliminating the duplicate values in the list. Use the following function header:

def eliminateDuplicates(lst):

Write a test program that reads in a list of integers, invokes the function, and displays the result. Here is the sample run of the program:

Enter ten numbers: 1 2 3 2 1 6 3 4 5 2
The distinct numbers are: 1 2 3 6 4 5

Solution

def main():
    # Read numbers as a string from the console
    s = input("Enter numbers: ")
    items = s.split() # Extracts items from the string
    numbers = [ int(x) for x in items ] # Convert items to numbers

    print("The distinct numbers are:", eliminateDuplicates(numbers))

def eliminateDuplicates(list):
    result = []
    for element in list:
        if not (element in result):
            result.append(element)

    return result

main()

Exercise 5 = Anagrams

Write a function that checks whether two words are anagrams. Two words are anagrams if they contain the same letters. For example, silent and listen are anagrams. The header of the function is:

def isAnagram(s1, s2):

(Hint: Obtain two lists for the two strings. Sort the lists and check if two lists are identical.)

Write a test program that prompts the user to enter two strings and, if they are anagrams, displays is an anagram; otherwise, it displays is not an anagram.

Solution

def main():
    s1 = input("Enter the first string: ").strip()
    s2 = input("Enter the second string: ").strip()

    print(s1, "and", s2, "are",
      ("anagram." if isAnagram(s1, s2) else "not anagram."))

def isAnagram(s1, s2):
    if len(s1) != len(s2):
        return False

    newS1 = sort(s1);
    newS2 = sort(s2);

    return newS1 == newS2

def sort(s):
    r = list(s)
    r.sort()

    result = ""
    for ch in r:
        result += ch

    return result

main()

Copying Lists

Often, in a program, you need to duplicate a list or a part of a list. In such cases you could attempt to use the assignment statement (=):

list1 = [1, 2, 3]
list2=list1

But you are not copying the list here! You are copying its reference.

What is happening in memory

Copying a list the correct way

>>> list2 = [x for x in list1]
>>> list2 = list1[:]
>>> list2 = list(list1)
>>> list2 = list(list1)
>>> import copy
>>> list2 = copy.copy(list1)
>>> list2 = copy.deepcopy(list1) # will copy the object as well

What is happening in memory for a real copy

Pass By Value

There are important differences between passing immutable or mutable objects as arguments to a function.

String and numeric values (integer and float) are immutable, they do not get changed

Lists are mutable, they can be changed

Example

Exercise 6: Hangman

Write a hangman game that randomly generates a word and prompts the user to guess one letter at a time, as shown in the sample run.

Each letter in the word is displayed as an asterisk. When the user makes a correct guess, the actual letter is then displayed. When the user finishes a word, display the number of misses and ask the user whether to continue playing. Create a list to store the words, as follows:

words = ["write", "that", "program", ...]

(Guess) Enter a letter in word ******* > p
(Guess) Enter a letter in word p****** > r
(Guess) Enter a letter in word pr**r** > p
    p is already in the word
(Guess) Enter a letter in word pr**r** > o
(Guess) Enter a letter in word pro*r** > g
(Guess) Enter a letter in word progr** > n
    n is not in the word
(Guess) Enter a letter in word progr** > m
(Guess) Enter a letter in word progr*m > a
The word is program. You missed 1 time
Do you want to guess another word? Enter y or n>