Programming in Python for High-Tech

Fall 2021Recitation 2: Lists and Loops

Programming in Python for High-Tech

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Plan

� Strings� Lists� For loop � While loop� Break/Continue

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Reminder: String SlicingThink of string indexes as pointers to characters.>>> a = 'Hello'>>> a[1:3]'el'>>> a[1:]'ello'>>> a[1:5:2]'el'>>> a[-4:-2]'el'>>> a[-2:-4:-1]'ll'>>> a[:-3]'He'>>> a[-3:]'llo'>>> 'hello'[1:5:2]'el'

olleH

0 1 2 3 4

-5 -4 -3 -2 -1

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Strings

>>> s = "While My Guitar Gently Weeps"

# Lowercase representation of a string>>> s.lower()

# find the index of the first occurrence of ‘e’>>> s.find("e")

# What does first contain?>>> n_pos = s.find("e") + 1>>> first = s[:n_pos]>>> first

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'while my guitar gently weeps'

'While'

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ListsA list is an ordered collection of elements (of any type). To create a list with specific elements, enclose them in square brackets:

>>> my_list = [2,3,5,7,11]

>>> print(my_list)

[2, 3, 5, 7, 11]

>>> empty_list = []

>>> mixed_types = [2, "hi",3.4]

To initialize a list of n elements set to zero do:>>> a = [0] * n # what about +?

(you may replace 0, with any other number)

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int str float

Indexing

� Indexing and Slicing works the same as for Strings.

>>> my_list = ["George", "Paul", "John", "Ringo"]

>>> my_list[0]

>>> my_list[2]

>>> my_list[4]

6

0 1 2 3

"George"

"John"

Traceback (most recent call last):File "<pyshell#7>", line 1, in <module>my list[4]

IndexError: list index out of range

Slicing>>> my_list = ['a','b','c','d','e','f','g','h','i','j']

>>> my_list[1:5] # Slicing

['b', 'c', 'd', 'e']

# Slicing does NOT change original list

>>> my_list

['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j']

# Slicing an arithmetic progression

>>> my_list[0:9:2]

['a', 'c', 'e', 'g', 'i']

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0 1 2 3 4 5 6 7 8 9

start stop step

Adding Elements>>> my_list = [6, 2]

# Insert at position

>>> my_list.insert(0, 'begin')

>>> my_list.insert(1, 'here')

>>> my_list

# Append (insertion at end)

>>> my_list.append('at last')

>>> my_list

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['begin', 'here', 6, 2]

['begin', 'here', 6, 2 , 'at last']

Adding Elements – cont’my_list = ['begin', 'here', 6, 2 , 'at last' ]# Extend (concatenate another list to the end)

>>> my_list.extend([15, 16])

>>> my_list

� What would happen if we used append instead of extend?

>>> my_list.append([15, 16])

>>> my_list

['begin', 'here', 6, 2, 'at last', 15, 16, [15, 16]]

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['begin', 'here', 6, 2, 'at last', 15, 16]

Changing Elements>>> my_list = ["aa", "bb", 1, 2]

# Change an element inside a list

>>> my_list[0] = my_list[1] + "b"

>>> my_list[2] = my_list[2] + 5

>>> my_list

# Replace elements

>>> my_list[0:2] = [1, 12]

>>> my_list

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[1, 12, 6, 2]

['bbb', 'bb', 6, 2]

Removing Elements>>> my_list = [2, 3, "in", 4]

# Remove by value (first occurrence only)>>> my_list.remove("in")>>> my_list

# Remove by position (slice)>>> my_list[0:2] = []>>> my_list[4]

� What if we try my_list.remove(1) ?

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[2, 3, 4]

In

Use in to check for the existence of an element inside a collection:>>> my_list = [2, 3, "in", 4]

# Element exists

>>> 2 in my_list

>>> "in" in my_list

# Element does not exist

>>> 5 in my_list

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True

False

True

LengthUse len to get the number of elements in a collection:

>>> len([1,2,3,4,5])

>>> len([])

>>> len("abc")

13

5

0

3

Rangerange(a, b) creates an iterator with ordered integers. All k values satisfying a ≤ k < b

# Examples:>>> list(range(6,10))[6, 7, 8, 9]>>> list(range(5)) # same as range(0,5)[0, 1, 2, 3, 4]>>> list(range(4,2))[]>>> type(range(3))<class 'range’>

range(a, b, d) - all integers from a (including) to b (not including) taking a “step” of d in between >>> list(range(10, 0, -2))

[10, 8, 6, 4, 2]14

Range – Cont.How can we create an Arithmetic progression of n elements:

𝒂𝒊 = 𝒂𝟎 + 𝒅 ∗ 𝒊# Examples:

>>> a_0 = 4

>>> d = 2

>>> n = 6

>>> list(range(a_0, a_0 + d * n, d))

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[4, 6, 8, 10, 12, 14]

What if we try n=0? n=1? n=10?

Other useful functions

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� Reversing list:>>> l = list(range(10))

>>> l.reverse() #What’s the difference from l[::-1]?

>>> print(l)

� Sorting list:>>> l = list(range(10, 0,-1))

>>> l.sort() #What’s the difference from sorted(l)?

>>> print(l)

For other useful functions: list - useful functions

[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Nested ListsList may contain other lists:

>>> nested_list = [['a', 12], ['b', 16]]

>>> nested_list[0]

>>> nested_list[0][0] #slicing one by one'a'

� What is the length of “nested_list"?>>> len(nested_list)

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2

['a', 12]

The str function

• Use str to convert any value of built-in types into a string

>>> str(1)'1'

>>> str("hello")'hello'

>>> str(range(3))'range(0, 3)'

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For LoopSaywewanttoprinteachelementofourlist:

lst =[3,1.24,"gorilla"]print (lst[0])print(lst[1])print(lst[2])print(lst[3])

Thisisveryrepetitiveandreliesonusknowingthenumberofelementsinthelist.

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Error

For Loopfor element in iterable:

statement1statement2

lst=[3, 1.24, "gorilla"]

for elem in lst:

print(elem)

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Indentation determines the scope of the iteration.

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For Loops

lst=[3,1.24,"gorilla"]

for elem in lst:elem =

print(elem)

3

3

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For Loops



print(elem)

31.24

1.24

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For Loops



print(elem)

31.24gorilla

"gorilla"

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For LoopsWecanalsoloopoverastring:

for letter in 'ACGT':print(letter)

print('Done!')

ACGTDone!

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For LoopsLet'sgooverastringandcountthenumberofoccurrencesoftheletterP:

seq = 'MALWMRLLPLLALLALWGPDPA'

count = 0for letter in seq:

if letter == 'P':count += 1 # count = count + 1

same as: seq.count('P')

For Loops with range� Example: print the even numbers between 0-20

(each in a new line)l = range(0,21,2)

for num in _____________:print(_________)

024…20

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numlrange(0,21,2)

For Loops with range� Example: display the multiplication chart (לוח הכפל)

for i in ____________:for j in ____________:

print(___________)print("")

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range(1, 11)

range(1, 11)i*j,end=""

'end=“”' is used to accumulate output in the

same line

forces the output to be in separate lines

For Loops with rangeFor a given string, lets print each two subsequent letters:ACGT −> AC CG GT

seq ='ATGATTCCAACGCGA'

for letterin seq:print(letter….) and the next letter(?!)

seq='ATGATTCCAACGCGA'

n=len(seq)for i in range(n):print(seq[i])

But we want to printsubsequent letters


0123451014


n=len(seq)for i in range(n):print(seq[i]+seq[i+1])

What will happen when i==14?

IndexError!


0123451014


n=len(seq)

for i in range(n-1):print(seq[i]+seq[i+1])


0123451014

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lst =[3,1.24,"gorilla"]

for elem in lst:

print(elem)

Sameresultinthiscase!Butsometimeswemustusetheindices(previousslideforexample)

Usewisely,andaccordingtoneed

For Loops with / without range

for i in range(len(lst)):

print(lst[i])

Matrix representation using lists

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� A matrix (plural matrices) is a rectangular array of numbers, symbols, or expressions, arranged in rows and columns (From wiki). An 𝑀×𝑁 matrix, denotes an array of 𝑀 rows and 𝑁columns.

� An example of a 4×2 matrix is:

2 5823

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� The (i,j) element refers to the j’th element of the i’th row� What does (3,2) equals to (in 1-based indexing)? 9

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� An𝑀×𝑁 matrix,canberepresentedbynestedlists.The(i,j)elementofthematrixisstoredasthe(j-1)’th elementofthe(i-1)’th list.

� Thematrixinourexampleisrepresentedby:A=[[2,5],[8,1],[2,9],[3,0]]� Toobtainthe(i,j)element:A[i-1][j-1]� Forexample,toobtain(3,1),wewrite:A[2][0]


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� Wearegivenamatrixofintegers,wewouldliketocollectallevennumbersintoanewlist.

� Anyideas?� Initializeavariable,toholdthenewlist(initializewithwhatvalue?)

�Goovereachelementinthematrix:� Ifthecurrentelementiseven,updatethenewlist

� printthelist


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

new_list = ______________

num_rows = len(______________)

num_columns = len(______________)

for i in range(num_rows):

for j in range(num_columns):

if ____________________:

__________________________

print("The even numbers are:", new_list)

[]

AA[0]

A[i][j] % 2 == 0

new_list.append(A[i][j])

Assumptions:1. A contains at least a single row (otherwise?)2. All rows A[0], A[1],…, are lists of the same length


While Loopwhile expression:

statement1statement2…

rest of code

Example:count = 0 while count < 3:

print("The count is:", count)count += 1 # count = count + 1

print("Goodbye!")

expr

statement(s)

true

false

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The count is: 0 The count is: 1 The count is: 2 Goodbye!

Break

break - terminates the nearest enclosing loop# Write a code which prints True if all digits in a given number are smaller or equal to 5

n = 129231330

print("Number:", n)

while n > 0:

cur_dig = n % 10

if _______________:

________________

________________

____________

if _______________:

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cur_dig > 5

print(False)

break

n = n // 10

n == 0

print(True)

Continue

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continue - starts the next cycle of the nearest enclosing loop

# Example: counting spacesin_string = "Ground control to Major Tom"spaces = 0for c in in_string:

if c != " ":continue

spaces += 1 print("Number of spaces:", spaces)

While Loop – Example 1

secret = int(input("Choose a number: "))

while True:guess = int(input("Enter guess: "))if _______________:

breakelif ______________:

print("Try a higher number")else:

print("Try a lower number")

print("Right!")

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guess == secret

guess < secret

Hint (sample output)

Choose a number: 34Enter guess: 100Try a lower numberEnter guess: 50Try a lower numberEnter guess: 25Try a higher numberEnter guess: 35Try a lower numberEnter guess: 34Right!

Player 1

Player 2

Phone Book

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� We want to develop an application for managing contacts

� It will contain a database� A list of contacts� Each contact is a list in the format [name, phone]

� Functionality� Find contact(s) by full name or prefix� Add contact� Remove contact

found = Falsephone = ""for contact in phone_book:

if contact[0] == name:found = Truephone = contact[1]break

if found:print "Name:", name , "Phone:", phone

else:print "Cannot find", name , "in contact list."

phone_book = [['nir','7886'],

['cohen','0123456758']]

# Retrieve the number for a given personname = "Moshe"

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Hint (sample output)

For name == "nir" the result will be:Name: nir Phone: 7886

For name == "assaf" the result will beCannot find assaf in contact list.

Phone book

Phone book

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found = Falsephone = ""for contact in phone_book:

if contact[0] == name:found = Truephone = contact[1]break

if found:print("Name:", name , "Phone:", phone)

else:print("Cannot find", name , "in contact list.")

phone_book = [['nir','7886'],

['cohen','0123456758']]

# Retrieve the number for a given personname = "Moshe"

# Add a contact

name = "Dan"

num = 1454

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phone_book.append([name, num])

Phone book

# Remove a contact

name = "Moshe"

found = False

for contact in ___________:

if __________________:

__________________________

____________

break

if not found:

print("Cannot find contact")

phone_book.remove(contact)

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phone_book

contact[0] == name

found = True

Phone book

# Search for all contacts with given prefix.

# Ignore character case (aka, case insensitive).

prefix = "ni"

prefix = prefix.lower()

for contact in phone_book:

if contact[0].lower().startswith(prefix):

print("Name:", contact[0], "Phone:", contact[1])

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Note that we don’t break (otherwise we will only find the first contact)

Phone book

One more break# Example: find the hidden number in a string# (a hidden number is a number which begins at the first character of the string and continues until some letter appears, e.g., 23fkd => 23)

in_string = "2346897ewrwqer"out_string = ""for c in in_string:if not c.isdigit():

breakout_string += c

print("The number is:", out_string)

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