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Sequence Types

6.0001 LECTURE 3

Ordered 1D collections of objects
Common examples are strings, lists, tuples
Use the “…”, [1, 2, 3], or (1, 2, 3) syntax to make them.
We use the same set of operations to act on them.

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STRINGS

6.0001 LECTURE 3

think of as a sequence of case sensitive characters
can compare strings with ==, >, < etc.
len() is a function used to retrieve the length of the string in the parentheses

s = "abc"

len(s) → evaluates to 3

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In addition to accessing list elements one at a time, concise syntax to access sublists; this is known as slicing

6.0001 LECTURE 3

Slicing

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Slicing Example

6.0001 LECTURE 3

can slice strings using [start:stop:step]
if give two numbers, [start:stop], step=1 by default
you can also omit numbers and leave just colons

s = "abcdefgh"

s[3:6]

→ evaluates to "def", same as

s[3:6:1]

s[3:6:2] → evaluates to "df"

s[::]

s[::-1]

→ evaluates to "abcdefgh", same as s[0:len(s):1]

→ evaluates to "hgfedbca", same as s[-1:-(len(s)+1):-1]

s[4:1:-2] → evaluates to "ec"

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STRINGS

6.0001 LECTURE 3

square brackets used to perform indexing into a string

to get the value at a certain index/position

s = "abc"

s[0]	→	evaluates to "a"
s[1]	→	evaluates to "b"
s[2]	→	evaluates to "c"
s[3]	→	trying to index out of bounds, error
s[-1]	→	evaluates to "c"
s[-2] s[-3]	→ →	evaluates to "b" evaluates to "a"

index: index:

0 1 2 → indexing always starts at 0

-3 -2 -1 → last element always at index -1

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Slicing and Combining Strings

6.0001 LECTURE 3

Given: A string s of length at most 200 letters and four integers a, b, c and d.

Return: The slice of this string from indices a through b and c through d

(with space in between), inclusively. In other words, we should include elements s[b] and s[d] in our slice.

Sample Dataset

HumptyDumptysatonawallHumptyDumptyhadagreatfallAlltheKingshorsesandalltheKingsmenCouldntputHumptyDumptyinhisplaceagain. 22 27 97 102

Sample Output

Humpty Dumpty

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EXERCISE: Reversing a sequence

6.0001 LECTURE 3

Write a program that starts with a word, reverse it.

Pictorial Presentation:

word = “something”�rword = “”��… do stuff��print(“reversed word: “, rword)��

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nums = list(range(5)) # range is a built-in function

# that creates a list of integers

print(nums) # Prints "[0, 1, 2, 3, 4]"

print(nums[2:4]) # Get a slice from index 2 to 4

# (exclusive); prints "[2, 3]"

print(nums[2:]) # Get a slice from index 2 to the

# end; prints "[2, 3, 4]"

print(nums[:2]) # Get a slice from the start to

# index 2 (exclusive); prints "[0, 1]"

print(nums[:]) # Get a slice of the whole list;

# prints "[0, 1, 2, 3, 4]"

print(nums[:-1]) # Slice indices can be negative; prints

# "[0, 1, 2, 3]"

nums[2:4] = [8, 9] # Assign a new sublist to a slice

print(nums) # Prints "[0, 1, 8, 9, 4]"

SLICING

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Strings vs Lists

6.0001 LECTURE 3

strings are “immutable” – cannot be modified

s = "hello"

s[0] = 'y'

s = 'y'+s[1:len(s)]

→ gives an error

→ is allowed

"hello"

"yello"

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for LOOPS

6.0001 LECTURE 3

for loops have a loop variable that iterates over a set of values

for var in range(4):

→ var iterates over values 0,1,2,3

→ expressions inside loop executed with each value for var

for var in range(4,6): → var iterates over values 4,5

range is a way to iterate over numbers, but a for loop variable can iterate over any set of values, not just numbers!

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STRINGS AND LOOPS

6.0001 LECTURE 3

these two code snippets do the same thing

s = "abcdefgh"

for index in range(len(s)):

if s[index] == 'i' or s[index] == 'u': print("There is an i or u")

for	char in s:
	if char == 'i' or char print("There is an	== 'u': i or u")

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CODE EXAMPLE:

an_letters = "aefhilmnorsxAEFHILMNORSX"

word = ”w!o%r?d”

i = 0

while i < len(word): char = word[i]

if char in an_letters:

print(char + “is a letter")

else:

print(char + “is not a letter”)

i += 1

for char in word:

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EXERCISE: printing the common letters in a string

6.0001 LECTURE 3

s1 = "this is a string"

s2 = "xyz are letters"

for char1 in s1:

for char2 in s2:

if char1 == char2:

print("common letter: “ + char1)

else:

print(uncommon letters: “ char1 + char2)

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animals = ['cat', 'dog', 'monkey’]

for animal in animals:

print(animal)

# Prints "cat", "dog", "monkey", each on its own line.

LOOPS

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EXERCISE

Take two lists, say for example these two:

a = [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

b = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]

and write a program that returns a list that contains only the elements that are common between the lists (without duplicates).

Make sure your program works on two lists of different sizes.

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TUPLES

6.0001 LECTURE 5

an ordered sequence of elements, can mix element types
cannot change element values, immutable
represented with parentheses

te = ()

t = (2,"foo",3) t[0]

(2,"foo",3) + (5,6)

 evaluates to 2

 evaluates to (2,"foo",3,5,6)

→ slice tuple, evaluates to ("foo",)

t[1:2]

t[1:3]

len(t)

→ slice tuple, evaluates to ("foo",3)

→ evaluates to 3

t[1] = 4 → gives error, can’t modify object

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MANIPULATING TUPLES

6.0001 LECTURE 5

aTuple:(( ),( ),( ))

can iterate over tuples

nums = ()

words = ()

for t in aTuple: nums = nums +

(t[0],)

if t[1] not in words:

words = words + (t[1],)

min_n = min(nums) max_n = max(nums)

unique_words = len(words)

newTuple = (min_n, max_n, unique_words)

nums( )

words(

if not already in words

i.e. unique strings from aTuple

^??

_?)

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LISTS

6.0001 LECTURE 5

ordered sequence of information, accessible by index
a list is denoted by square brackets, []
a list contains elements

can contain mixed types (not common)

list elements can be changed so a list is mutable

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INDICES AND ORDERING

6.0001 LECTURE 5

a_list = []

L = [2, 'a', 4, [1,2]]

→ evaluates to 4

→ evaluates to 2

→ evaluates to 5

→ evaluates to [1,2], another list!

→ gives an error

len(L)

L[0]

L[2]+1

L[3]

L[4]

i = 2 L[i-1]

→ evaluates to ‘a’ since L[1]='a' above

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CHANGING ELEMENTS

6.0001 LECTURE 5

lists are mutable!
assigning to an element at an index changes the value

L = [2, 1, 3]

L[1] = 5

L is now [2, 5, 3], note this is the same object L

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ITERATING OVER A LIST

6.0001 LECTURE 5

compute the sum of elements of a list
common pattern, iterate over list elements

to len(L)-1

range(n) goes from 0

to n-1

total = 0

for i in range(len(L)): total += L[i]

print total

notice

list elements are indexed 0

total = 0

for i in L:

total += i print total

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OPERATIONS ON LISTS - ADD

6.0001 LECTURE 5

add elements to end of list with L.append(element)
mutates the list!

L = [2,1,3]

L.append(5)

→ L is now [2,1,3,5]

what is the dot?

lists are Python objects, everything in Python is an object
objects have data
objects have methods and functions
access this information by object_name.do_something()
will learn more about these later

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OPERATIONS ON LISTS - ADD

6.0001 LECTURE 5

to combine lists together use concatenation, + operator, to generate a new list
mutate list with L.extend(some_list)

L1	=	[2,1,3]
L2	=	[4,5,6]
L3	=	L1 + L2	→	L3 is [2,1,3,4,5,6] L1, L2 unchanged

L1.extend([0,6])

→ mutated L1 to [2,1,3,0,6]

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OPERATIONS ON LISTS -

REMOVE

6.0001 LECTURE 5

delete element at a specific index with del(L[index])
remove element at end of list with L.pop(), returns the removed element
remove a specific element with L.remove(element)

looks for the element and removes it
if element occurs multiple times, removes first occurrence
if element not in list, gives an error

L = [2,1,3,6,3,7,0] # do below in order L.remove(2) → mutates L = [1,3,6,3,7,0] L.remove(3) → mutates L = [1,6,3,7,0]

del(L[1])

L.pop()

→ mutates L = [1,3,7,0]

→ returns 0 and mutates L = [1,3,7]

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CONVERT LISTS TO STRINGS

AND BACK

6.0001 LECTURE 5

convert string to list with list(s), returns a list with every

character from s an element in L

can use s.split(), to split a string on a character parameter, splits on spaces if called without a parameter
use ''.join(L) to turn a list of characters into a string, can give a character in quotes to add char between every element

s = "I<3 cs" list(s) s.split('<')

L = ['a','b','c']

''.join(L)

'_'.join(L)

→ s is a string

→ returns ['I','<','3',' ','c','s']

→ returns ['I', '3 cs']

→ L is a list

→ returns "abc"

→ returns "a_b_c"

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OTHER LIST OPERATIONS

reverse()
and many more! https://docs.python.org/3/tutorial/datastructures.html

6.0001 LECTURE 5

sort()

L=[9,6,0,3]

L.sort() L.reverse()

→ mutates L=[0,3,6,9]

→ mutates L=[9,6,3,0]

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LISTS IN MEMORY

6.0001 LECTURE 5

lists are mutable
behave differently than immutable types
is an object in memory
variable name points to object
any variable pointing to that object is affected
key phrase to keep in mind when working with lists is

side effects

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ALIASES

6.0001 LECTURE 5

hot is an alias for warm – changing one changes the other!
append() has a side effect

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CLONING A LIST

6.0001 LECTURE 5

create a new list and copy every element using

chill = cool[:]

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SORTING LISTS

6.0001 LECTURE 5

calling sort()

mutates the list, returns nothing

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LISTS OF LISTS OF LISTS….

6.0001 LECTURE 5

can have nested lists
side effects still possible after mutation

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MUTATION AND ITERATION

6.0001 LECTURE 5

avoid mutating a list as you are iterating over it

for e in L1:

if e in L2:

L1.remove(e)

L1 = [1, 2, 3, 4]

L2 = [1, 2, 5, 6]

remove_dups(L1, L2)

L1 is [2,3,4] not [3,4]

Why?

Python uses an internal counter to keep track of index it is in the loop
mutating changes the list length but Python doesn’t update the counter
loop never sees element 2

L1_copy = L1[:]

for e in L1_copy: if e in L2:

L1.remove(e)

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EXERCISE

Write a program that takes a list, and returns two new lists with the input list values in sorted and reverse sorted order.

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Module Questionnaire

Funding Sources: NIH U24 EB028887, NSF 2120200, 2000281, 1720625

Previous Funding : NIH R01 GM122424

Please take a minute or two to let us know about your experience with this module by filling out the brief zoom survey

Feel free to provide additional comments and suggestions in the slack or by email to us as well (hfennel@iu.edu)