Lecture 7 - Recap

• Working Lists/Dictionary
• Loops
• List/dictionary comprehensions

Working with lists

2 +3 

5

3.6 + 4

7.6

2**100

1267650600228229401496703205376

items = [1, 2, 3, 4, 5]

items

[1, 2, 3, 4, 5]

items[0] # index starts with zero

1

items[4] # index ends with one less than length of list

5

items[-1] # give me last item

5

words = ["one", "two", "three", "four", "five"]

words[1:3] # subset of list which starts at index 1 but ends at 3(excluded)

['two', 'three']

words[2:] # drop 2 items 

['three', 'four', 'five']

words[:2] # take first 2 items

['one', 'two']

words[:] # take everything

['one', 'two', 'three', 'four', 'five']

words[::2] # give everything from start to end ...but alternate item

['one', 'three', 'five']

list[start:end:step]

words[::-1] # reverse the list

['five', 'four', 'three', 'two', 'one']

scores = {
    "Prathamesh" : 20,
    "Priya": 25,
    "Prafulla": 24,
    "Priyanka":22
}

scores

{'Prathamesh': 20, 'Priya': 25, 'Prafulla': 24, 'Priyanka': 22}

scores['Priya']

25

nums = {1:"one",
        2:"two",
        3:"three"
       }

nums

{1: 'one', 2: 'two', 3: 'three'}

nums[1]

'one'

d = {
    [1, 1, 1]: "value1",
    [1]:["value2"]
}

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-25-83cc877e91b1> in <module>
----> 1 d = {
      2     [1, 1, 1]: "value1",
      3     [1]:["value2"]
      4 }

TypeError: unhashable type: 'list'

What can be stored in dictionary

• You can have anything as value
• but only hashable (immutable) objects as keys
• ints, strings, floats, tuple these are some immutables objects

Loops

classroom = ["Prathamesh", "Hema", "Pranav", "Sanika", "Preetee", "Sidhhu"]

i = 0
while i < len(classroom):
    i = i + 1 
    print(classroom[i])

Hema
Pranav
Sanika
Preetee
Sidhhu

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-28-63aba0778bd3> in <module>
      2 while i < len(classroom):
      3     i = i + 1
----> 4     print(classroom[i])

IndexError: list index out of range

i = 0
while i < len(classroom):
    print(classroom[i])## <--- this is where errors are if any!
    i = i + 1 

Prathamesh
Hema
Pranav
Sanika
Preetee
Sidhhu

for student in classroom:
    print(student) # this loop will never fail!

Prathamesh
Hema
Pranav
Sanika
Preetee
Sidhhu

for member in scores: # this will go over the keys
    print(member)

Prathamesh
Priya
Prafulla
Priyanka

for member in scores: # this will go over the keys
    print(member, scores[member])

Prathamesh 20
Priya 25
Prafulla 24
Priyanka 22

for member, score in scores.items():
    print(member, score)

Prathamesh 20
Priya 25
Prafulla 24
Priyanka 22

x, y = 3, 4

print(x, y)

3 4

list(scores.items())

[('Prathamesh', 20), ('Priya', 25), ('Prafulla', 24), ('Priyanka', 22)]

Patterns of looping

for student in classroom:
    print(student)

Prathamesh
Hema
Pranav
Sanika
Preetee
Sidhhu

for student in reversed(classroom):
    print(student)

Sidhhu
Preetee
Sanika
Pranav
Hema
Prathamesh

for rollno, student in enumerate(classroom):
    print(rollno, student)

0 Prathamesh
1 Hema
2 Pranav
3 Sanika
4 Preetee
5 Sidhhu

for rollno, student in enumerate(classroom, start=1):
    print(rollno, student)

1 Prathamesh
2 Hema
3 Pranav
4 Sanika
5 Preetee
6 Sidhhu

testmarks = [20, 19, 18, 18, 20, 19]

classroom

['Prathamesh', 'Hema', 'Pranav', 'Sanika', 'Preetee', 'Sidhhu']

testmarks

[20, 19, 18, 18, 20, 19]

for student, marks in zip(classroom, testmarks):
    print(student, marks)

Prathamesh 20
Hema 19
Pranav 18
Sanika 18
Preetee 20
Sidhhu 19

import os

all_files = os.listdir()

all_files

['index.html',
 'Lecture6-Comprehensions-1.ipynb',
 'presenty5.txt',
 'sample.html',
 'presenty3.txt',
 'Welcome.html',
 'push',
 'Lecture4-Working_With_Data.html',
 'joy.py',
 'presenty6.txt',
 'Lecture2-Quick_Tour.ipynb',
 'Lecture7-Recap.html',
 'sample.ipynb',
 '#presenty4.txt#',
 '__pycache__',
 'Untitled.html',
 'Lecture4-Working_With_Data.ipynb',
 'Lecture5-Comprehensions.html',
 'Lecture1-Fundamental_ideas_of_programming.ipynb.ipynb',
 'index.ipynb',
 'Lecture7-Recap.ipynb',
 'presenty5.txt~',
 'Lecture6-Comprehensions-1.html',
 'test.html',
 'Lecture5-Comprehensions.ipynb',
 'Lecture6-Recap.html',
 'Lecture3-Quick_Tour.ipynb',
 'Lecture1-Fundamental_ideas_of_programming.ipynb.html',
 'Makefile',
 'Lecture2-Quick_Tour.html',
 '.ipynb_checkpoints',
 'Lecture3-Quick_Tour.html']

def read_files(filenames):
    filedata = {}
    for file in filenames:
        if os.path.isfile(file):
            with open(file) as f:
                print("Reading file:", file)
                filedata[file] = f.read()
    return filedata

data = read_files(all_files)

Reading file: index.html
Reading file: Lecture6-Comprehensions-1.ipynb
Reading file: presenty5.txt
Reading file: sample.html
Reading file: presenty3.txt
Reading file: Welcome.html
Reading file: push
Reading file: Lecture4-Working_With_Data.html
Reading file: joy.py
Reading file: presenty6.txt
Reading file: Lecture2-Quick_Tour.ipynb
Reading file: Lecture7-Recap.html
Reading file: sample.ipynb
Reading file: #presenty4.txt#
Reading file: Untitled.html
Reading file: Lecture4-Working_With_Data.ipynb
Reading file: Lecture5-Comprehensions.html
Reading file: Lecture1-Fundamental_ideas_of_programming.ipynb.ipynb
Reading file: index.ipynb
Reading file: Lecture7-Recap.ipynb
Reading file: presenty5.txt~
Reading file: Lecture6-Comprehensions-1.html
Reading file: test.html
Reading file: Lecture5-Comprehensions.ipynb
Reading file: Lecture6-Recap.html
Reading file: Lecture3-Quick_Tour.ipynb
Reading file: Lecture1-Fundamental_ideas_of_programming.ipynb.html
Reading file: Makefile
Reading file: Lecture2-Quick_Tour.html
Reading file: Lecture3-Quick_Tour.html

data.keys()

dict_keys(['index.html', 'Lecture6-Comprehensions-1.ipynb', 'presenty5.txt', 'sample.html', 'presenty3.txt', 'Welcome.html', 'push', 'Lecture4-Working_With_Data.html', 'joy.py', 'presenty6.txt', 'Lecture2-Quick_Tour.ipynb', 'Lecture7-Recap.html', 'sample.ipynb', '#presenty4.txt#', 'Untitled.html', 'Lecture4-Working_With_Data.ipynb', 'Lecture5-Comprehensions.html', 'Lecture1-Fundamental_ideas_of_programming.ipynb.ipynb', 'index.ipynb', 'Lecture7-Recap.ipynb', 'presenty5.txt~', 'Lecture6-Comprehensions-1.html', 'test.html', 'Lecture5-Comprehensions.ipynb', 'Lecture6-Recap.html', 'Lecture3-Quick_Tour.ipynb', 'Lecture1-Fundamental_ideas_of_programming.ipynb.html', 'Makefile', 'Lecture2-Quick_Tour.html', 'Lecture3-Quick_Tour.html'])

List compresions

testmarks

[20, 19, 18, 18, 20, 19]

def square_list(numbers):
    sqrs = []
    for n in numbers:
        sqrs.append(n*n)
    return sqrs

def filtertext_files(files):
    textfiles = []
    for file in files:
        if file.endswith(".txt"):
            textfiles.append(file)
    return textfiles

square_list(testmarks)

[400, 361, 324, 324, 400, 361]

filtertext_files(all_files)

['presenty5.txt', 'presenty3.txt', 'presenty6.txt']

textfiles = filtertext_files(all_files)

textfiles

['presenty5.txt', 'presenty3.txt', 'presenty6.txt']

read_files(textfiles)

Reading file: presenty5.txt
Reading file: presenty3.txt
Reading file: presenty6.txt

{'presenty5.txt': 'Varsha Bendre18:37\nok\nYou18:40\nhttps://notes.pipal.in/2021/pccoe-python-ml/\nlink for live notes\nTEETC322 Nisha Hiremath18:43\nno sir \nVarsha Bendre18:43\nno sir\nTEETC322 Nisha Hiremath18:43\nwill try it today\nTEETA145 INDRARAJ19:28\nTEETA145\nTEETC322 Nisha Hiremath19:29\nTEETC322 Nisha Hiremath\n',
 'presenty3.txt': 'TEETC340 Sameer Pathak20:08\nSameer Pathak\nTEETC340\nBEETC316 Vedant Kharche20:08\nBEETC316 Vedant kharche \nTEETC323 Renuka Jadhav20:08\nRenuka Jadhav TEETC323\nTEETA108_ Kartikkumar Athani20:08\nKartikkumar Athani  TEETA108                          \nNisha Hiremath20:08\nTEETC322 Nisha Hiremath\n',
 'presenty6.txt': 'TEETC322 Nisha Hiremath18:57\nOk sir\nTEETC323 Renuka Jadhav19:01\nsir can you give some hint\nTEETC340 Sameer Pathak19:03\nZip\nTEETC340 Sameer Pathak19:05\nYes sir\nTEETC322 Nisha Hiremath19:58\nTEETC322 Nisha Hiremath\nTEETC340 Sameer Pathak19:58\nTEETC340 Sameer Pathak\nTEETC323 Renuka Jadhav19:58\nTEETC323 Renuka Jadhav\n'}

def square_list(numbers):
    sqrs = []
    for n in numbers:
        sqrs.append(n*n)
    return sqrs

numbers = [1, 2, 3, 4, 5,6 ,7 ]

[n*n for n in numbers] # list compehsnions

[1, 4, 9, 16, 25, 36, 49]

square_list(numbers)

[1, 4, 9, 16, 25, 36, 49]

Geeral gudelines while coding loops

• do not put indices for collection unless very necessary
• try to put for loop directly over the collection
• next is it possible to convert your loop to list comprehension

[student.upper() for student in classroom]

['PRATHAMESH', 'HEMA', 'PRANAV', 'SANIKA', 'PREETEE', 'SIDHHU']

scores

{'Prathamesh': 20, 'Priya': 25, 'Prafulla': 24, 'Priyanka': 22}

scaled_score = {}
for student, score in scores.items():
    scaled_score[student] = score/25
print(scaled_score)

{'Prathamesh': 0.8, 'Priya': 1.0, 'Prafulla': 0.96, 'Priyanka': 0.88}

{student:score/25 for student, score in scores.items()}

{'Prathamesh': 0.8, 'Priya': 1.0, 'Prafulla': 0.96, 'Priyanka': 0.88}

Examples

• Filter out text files from all files in current directory

[file for file in os.listdir() if file.endswith(".txt")]

['presenty5.txt', 'presenty3.txt', 'presenty6.txt']

[f for f in os.listdir() if f.endswith(".html")]

['index.html',
 'sample.html',
 'Welcome.html',
 'Lecture4-Working_With_Data.html',
 'Lecture7-Recap.html',
 'Untitled.html',
 'Lecture5-Comprehensions.html',
 'Lecture6-Comprehensions-1.html',
 'test.html',
 'Lecture6-Recap.html',
 'Lecture1-Fundamental_ideas_of_programming.ipynb.html',
 'Lecture2-Quick_Tour.html',
 'Lecture3-Quick_Tour.html']

[f for f in os.listdir() if f.endswith(".ipynb") and f.startswith("Lecture")]

['Lecture6-Comprehensions-1.ipynb',
 'Lecture2-Quick_Tour.ipynb',
 'Lecture4-Working_With_Data.ipynb',
 'Lecture1-Fundamental_ideas_of_programming.ipynb.ipynb',
 'Lecture7-Recap.ipynb',
 'Lecture5-Comprehensions.ipynb',
 'Lecture3-Quick_Tour.ipynb']

[n*n for n in numbers if n%2==0]

[4, 16, 36]

Problems for home

• Write a list comprehension to generate multiplication tables of 1-5
• Write a function to find all factors of given number. (make use of list comprehesions
• Find sum of all integers divisible by 13 or 17 and less than 1000

sum(numbers)

28