Lecture 2 : Correction¶

November, 2021 - updated on April, 2022 - François HU

Master of Science - EPITA

This lecture is available here: https://curiousml.github.io/

image-2.png

Table of contents¶

Correction and remarks

Correction and feedback ¶

You will find bellow the correction of the exercices of the first lecture.

Exercice 1.¶

Create the following list with loops:

[['car', 0, 1, 4, 9, 16],
 ['bus', 1, 4, 9, 16, 25],
 ['train', 4, 9, 16, 25, 36],
 ['boat', 9, 16, 25, 36, 49]]

Create a script that prints this list in the following manner:

car      0    1    4    9   16
bus      1    4    9   16   25
train    4    9   16   25   36
boat     9    16  25   36   49

Common mistake 1¶

Instead of generating the list with for loops, many of you gave me the answer:

In [1]:
# No: need at least a loop instead of manually putting the numbers
L = [['car', 0, 1, 4, 9, 16],
 ['bus', 1, 4, 9, 16, 25],
 ['train', 4, 9, 16, 25, 36],
 ['boat', 9, 16, 25, 36, 49]]

L
Out[1]:
[['car', 0, 1, 4, 9, 16],
 ['bus', 1, 4, 9, 16, 25],
 ['train', 4, 9, 16, 25, 36],
 ['boat', 9, 16, 25, 36, 49]]

Common mistake 2¶

some of you gave me four independant lists instead of a single list (of lists)

In [2]:
# No: create a list including these independant lists
l1 = ["car"] + [i**2 for i in range(5)]
l2 = ["bus"] + [i**2 for i in range(1, 6)]
l3 = ["train"] + [i**2 for i in range(2, 7)]
l4 = ["boat"] + [i**2 for i in range(3, 8)]

print(l1)
print(l2)
print(l3)
print(l4)

# one should add for example the line: L = [l1, l2, l3, l4]

['car', 0, 1, 4, 9, 16]
['bus', 1, 4, 9, 16, 25]
['train', 4, 9, 16, 25, 36]
['boat', 9, 16, 25, 36, 49]

Common mistake 3¶

In [3]:
first_index = ['car','bus','train','boat']
list=[]

#Populating list
for i in first_index:
    row_first_number = len(list)
    list.append([i])
    for j in range(row_first_number,(row_first_number+5)):
        if j==0:  
            list[row_first_number].append(0) 
        else:
            list[row_first_number].append(j**2)

#Printing list
for i in list:
    for j in i:
        print(j,'\t',end=" ")
    print("\n")

car 	 0 	 1 	 4 	 9 	 16 	 

bus 	 1 	 4 	 9 	 16 	 25 	 

train 	 4 	 9 	 16 	 25 	 36 	 

boat 	 9 	 16 	 25 	 36 	 49
  • Be carfeful of your variable names. list is already a python-object, therefore you may encounter some conflict. Suggested: write other name for instantiating your list (like lst)
In [4]:
l = list()
print(l)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/var/folders/55/c2kg8h2d2wnfzx5wnypmt8kr0000gn/T/ipykernel_2143/2873245483.py in <module>
----> 1 l = list()
      2 print(l)

TypeError: 'list' object is not callable
In [ ]:
del list
l = list()
print(l)

Solution proposed by a student 1¶

In [5]:
x = ['car','bus','train','boat']
odd = [i**2 for i in range(15)]

num = []
lign = 0 # not needed
index = 0 # not needed
temp = [] # not needed
for lign in range(4):
    temp = []
    temp.append(x[lign])
    for index in range(5):
        temp.append( odd[index+lign] )
    num.append(temp)
    
num
Out[5]:
[['car', 0, 1, 4, 9, 16],
 ['bus', 1, 4, 9, 16, 25],
 ['train', 4, 9, 16, 25, 36],
 ['boat', 9, 16, 25, 36, 49]]
  • It works !
  • pros: we compute $i^2$ one time for all lists so we gain in time and space complexity
  • cons: there are some unnecessary lines here

Solution proposed by a student 2¶

In [6]:
trans = ["car", "bus", "train", "boat"]
total = []

for a in range(4):
    temp = []
    temp.append(trans[a])
    for b in range(1,6):
        temp.append(a**2)
        a = a + 1 # it works but dangerous
    total.append(temp)

total
Out[6]:
[['car', 0, 1, 4, 9, 16],
 ['bus', 1, 4, 9, 16, 25],
 ['train', 4, 9, 16, 25, 36],
 ['boat', 9, 16, 25, 36, 49]]
  • It works !
  • pros: very intuitive
  • cons: when one uses a for loop, it is often advised to not reassign the items of the iterator

Suggested correction¶

In [7]:
# There are many ways to do it.
transports = ["car", "bus", "train", "boat"]

L = [] # my final list (of lists)
for index, transport in enumerate(transports):
    line = [transport] + [i**2 for i in range(index, 5+index)]
    L.append(line)
    
L
Out[7]:
[['car', 0, 1, 4, 9, 16],
 ['bus', 1, 4, 9, 16, 25],
 ['train', 4, 9, 16, 25, 36],
 ['boat', 9, 16, 25, 36, 49]]
In [8]:
for line in L:
    for item in line:
        print(item, end="\t")
    print()

car	0	1	4	9	16	
bus	1	4	9	16	25	
train	4	9	16	25	36	
boat	9	16	25	36	49

Another way to print

In [9]:
str_to_print = ""
for line in L:
    for item in line:
        str_to_print += "\t"
        str_to_print += str(item)
    str_to_print += "\n"
        
print(str_to_print)

	car	0	1	4	9	16
	bus	1	4	9	16	25
	train	4	9	16	25	36
	boat	9	16	25	36	49
In [10]:
for line in L:
    print( * line, sep = '\t')

car	0	1	4	9	16
bus	1	4	9	16	25
train	4	9	16	25	36
boat	9	16	25	36	49

Exercice 2.¶

For the following list of dictionaries, write a script that add a field registrations which is twice the number of accepted papers.

In [11]:
confs = [{"Name": "NeurIPS", "Date": 2016, "Location": "Barcelona", "acc_papers": 300},
             {"Name": "ICML", "Date": 2016, "Location": "New York City", "acc_papers": 450},
             {"Name": "ICML", "Date": 2015, "Location": "Lille", "acc_papers": 250},
             {"Name": "AISTATS", "Date": 2016, "Location": "Cadiz", "acc_papers": 100}]

Common mistake 1¶

confs needs some clarification . This is a list of dictionaries. More precisely it is one list containing four dictionaries and each dictionary contains four fields (or keys) : "Name", "Date", "Location" and "acc_papers" -> Here in each dictionary of the list we want to add fifth field named registration which the value is twice the number of accepted paper.

Suggested correction¶

In [12]:
for conf in confs:
    conf["registrations"] = conf["acc_papers"] * 2
    
confs
Out[12]:
[{'Name': 'NeurIPS',
  'Date': 2016,
  'Location': 'Barcelona',
  'acc_papers': 300,
  'registrations': 600},
 {'Name': 'ICML',
  'Date': 2016,
  'Location': 'New York City',
  'acc_papers': 450,
  'registrations': 900},
 {'Name': 'ICML',
  'Date': 2015,
  'Location': 'Lille',
  'acc_papers': 250,
  'registrations': 500},
 {'Name': 'AISTATS',
  'Date': 2016,
  'Location': 'Cadiz',
  'acc_papers': 100,
  'registrations': 200}]

Exercice 3.¶

Write a function include, that produces the following results:

>>> l = [0]
>>> include(l, 2)
>>> print(l)
[0, 2]
>>> include(l, "-1")
>>> print(l)
[0, 2, '-1']
>>> include(l)
>>> print(l)
[]

Let us clarify this exercice: Our objective is to create a function that:

  • appends item to the list (in an inline manner)
  • clears the list if we don't give a second argument (in an inline manner)

Common mistake 1¶

In [13]:
def include(l, item):
    l.append(item)
    return l
In [14]:
l = [0]
include(l, 2)
print(l)
include(l, "-1")
print(l)
include(l)
print(l)

[0, 2]
[0, 2, '-1']

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/var/folders/55/c2kg8h2d2wnfzx5wnypmt8kr0000gn/T/ipykernel_2143/2280774624.py in <module>
      4 include(l, "-1")
      5 print(l)
----> 6 include(l)
      7 print(l)

TypeError: include() missing 1 required positional argument: 'item'

Common mistake 2¶

In [15]:
def include(l, item = None):
    if item == None: # recommended to use `is None`
        l = []
        return l
    else:
        l.append(item) # or `l += [item]`
        return l
In [16]:
l = [0]
include(l, 2)
print(l)
include(l, "-1")
print(l)
include(l)
print(l)

[0, 2]
[0, 2, '-1']
[0, 2, '-1']
  • No need to return a value here
  • Error with l = []

Common mistake 3¶

In [17]:
def include(l, item=None):
    if item:
        l.append(item)
        return l
    else:
        l.clear()
        return []
In [18]:
l = [0]
include(l, 2)
print(l)
include(l, "-1")
print(l)
include(l)
print(l)

[0, 2]
[0, 2, '-1']
[]
  • include(l, 0) will return [].
In [19]:
include(l, 0)
print(l)

[]

Suggested correction¶

In [20]:
def include(l, item = None):
    if item is None:
        del l[:]
    else:
        l.append(item)
In [21]:
l = [0]
include(l, 2)
print(l)
include(l, "-1")
print(l)
include(l)
print(l)

[0, 2]
[0, 2, '-1']
[]

Or alternatively

In [22]:
def include(l, item = None):
    if item is None:
        l.clear()
    else:
        l.append(item)
In [23]:
l = [0]
include(l, 2)
print(l)
include(l, "-1")
print(l)
include(l)
print(l)

[0, 2]
[0, 2, '-1']
[]

Exercice 4.¶

Create a module, that contains a function fibonacci(n) computing the $n^{th}$ Fibonacci number. Then create a script using this function.

Common mistake 1¶

Functions of a module should be in a python file (extension .py) not in a ipython notebook file (extension .ipynb)

image-3.png

Common mistake 2¶

In [24]:
def fibo(n):
    """
    mistake 1 : print instead of return
    mistake 2 : do not use a and b in the statements
    """
    a,b=0,1
    if n<a:
        print('negative')
    elif n==a:
        print(a) # here: problem
    elif n==b:
        print(b) # here: problem
    else:
        while a<n:
            print(a, end=' ')
            a, b=b, a+b # here: problem
In [25]:
fibo(10) # should be 55

0 1 1 2 3 5 8

Common mistake 3¶

In [27]:
def fibonacci(n):
    b = 0
    if n == 1:
        b = 1
    elif n > 1:
        a = 0
        sum = 1
        for i in range(n-1):
            b = sum + a
            a = sum
            sum = b
    return b

fibonacci(10)
Out[27]:
55
  • don't take account negative numbers
  • sum is a built-in function name
  • we can swap values with a, b = b, a
In [28]:
a = 1
b = 2
print(a,b)

a, b = b, a
print(a,b)

1 2
2 1

Suggested correction¶

  • fibonacci_rec: recursive approach
  • fibonacci_ite: iterative approach
In [29]:
def fibonacci_rec(n): # recursive approach
    if n < 0:
        raise ValueError("Negative arguments not implemented")
    elif n == 0:
        return 0
    elif n == 1:
        return 1
    else:
        return fibonacci_rec(n-1) + fibonacci_rec(n-2)
In [30]:
fibonacci_rec(10)
Out[30]:
55
In [31]:
fibonacci_rec(-1)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/var/folders/55/c2kg8h2d2wnfzx5wnypmt8kr0000gn/T/ipykernel_2143/4136678502.py in <module>
----> 1 fibonacci_rec(-1)

/var/folders/55/c2kg8h2d2wnfzx5wnypmt8kr0000gn/T/ipykernel_2143/3168884607.py in fibonacci_rec(n)
      1 def fibonacci_rec(n): # recursive approach
      2     if n < 0:
----> 3         raise ValueError("Negative arguments not implemented")
      4     elif n == 0:
      5         return 0

ValueError: Negative arguments not implemented
In [32]:
def fibonacci_ite(n): # iterative approach
    if n < 0:
        raise ValueError("Negative arguments not implemented")
    F = [0, 1]
    for i in range(1, n):
        F.append(F[i] + F[i-1])
    return F[n]
In [33]:
fibonacci_ite(10)
Out[33]:
55
In [34]:
fibonacci_ite(-1)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/var/folders/55/c2kg8h2d2wnfzx5wnypmt8kr0000gn/T/ipykernel_2143/3775388262.py in <module>
----> 1 fibonacci_ite(-1)

/var/folders/55/c2kg8h2d2wnfzx5wnypmt8kr0000gn/T/ipykernel_2143/3172447240.py in fibonacci_ite(n)
      1 def fibonacci_ite(n): # iterative approach
      2     if n < 0:
----> 3         raise ValueError("Negative arguments not implemented")
      4     F = [0, 1]
      5     for i in range(1, n):

ValueError: Negative arguments not implemented

Exercice 5.¶

Change the class Point (the toy example bellow) in order to take account a 3rd coordinate z.

class Point:
    def __init__(self, x, y):          
        self.x = x                     
        self.y = y                     
        self.norm = self.compute_norm()

    def compute_norm(self):
        norm = (self.x**2 + self.y**2)**(0.5) 
        return norm

    def __add__(self, another_point) :
         return Point(self.x + another_point.x, self.y + another_point.y)

Then execute the following script.

p1 = Point(1, -1, 1)
p2 = Point(1, 1, 3)
p3 = p1 + p2

print(f"p1 = ({p1.x}, {p1.y}, {p1.z})")
print(f"p2 = ({p2.x}, {p2.y}, {p2.z})")
print(f"p3 = ({p3.x}, {p3.y}, {p3.z})")

Suggested correction¶

In [35]:
class Point:
    def __init__(self, x, y, z):           # add a new argument: z
        self.x = x
        self.y = y
        self.z = z                         # initialize the attribute z
        self.norm = self.compute_norm()

    def compute_norm(self):
        norm = (self.x**2 + self.y**2 + self.z**2)**(0.5) # for the norm computation, take z into account
        return norm

    def __add__(self, another_point):      # dont forget to add the 3rd coordinate
         return Point(self.x + another_point.x, self.y + another_point.y, self.z + another_point.z)
In [36]:
p1 = Point(1, -1, 1)
p2 = Point(1, 1, 3)
p3 = p1 + p2

print(f"p1 = ({p1.x}, {p1.y}, {p1.z})")
print(f"p2 = ({p2.x}, {p2.y}, {p2.z})")
print(f"p3 = ({p3.x}, {p3.y}, {p3.z})")

p1 = (1, -1, 1)
p2 = (1, 1, 3)
p3 = (2, 0, 4)