Sets

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A set is a mutable unordered collection of immutable distinct elements (that is, without duplicates). The Python datatype to represent sets is called set.

What to do

  1. Unzip exercises zip in a folder, you should obtain something like this:

sets
    sets.ipynb
    sets-sol.ipynb
    jupman.py

WARNING: to correctly visualize the notebook, it MUST be in an unzipped folder !

  1. open Jupyter Notebook from that folder. Two things should open, first a console and then a browser. The browser should show a file list: navigate the list and open the notebook sets.ipynb

  2. Go on reading the exercises file, sometimes you will find paragraphs marked Exercises graded from ✪ to ✪✪✪✪ which will ask to write Python commands in the following cells.

Shortcut keys:

  • to execute Python code inside a Jupyter cell, press Control + Enter

  • to execute Python code inside a Jupyter cell AND select next cell, press Shift + Enter

  • to execute Python code inside a Jupyter cell AND a create a new cell aftwerwards, press Alt + Enter

  • If the notebooks look stuck, try to select Kernel -> Restart

Creating a set

We can create a set using curly brackets, and separating the elements with commas ,

Let’s try a set of characters:

[2]:
s = {'b','a','d','c'}
[3]:
type(s)
[3]:
set

WARNING: SETS ARE *NOT* ORDERED !!!

DO NOT BELIEVE IN WHAT YOU SEE !!

Let’s try printing the set:

[4]:
print(s)
{'a', 'b', 'c', 'd'}

The output shows the order in which the print was made is different from the order in which we built the set. Also, according to the Python version you’re using, on your computer it might be even different!

This is because the order in sets is NOT guaranteed: the only thing that matters is whether or not an element belongs to a set.

As a further demonstration, we may ask Jupyter to show the content of the set, by writing only the variable s WITHOUT print:

[5]:
s
[5]:
{'a', 'b', 'c', 'd'}

Now it appears in alphabetical order! It happens like so because Jupyter show variables by implicitly using the pprint (pretty print), which ONLY for sets gives us the courtesy to order the result before printing it. We can thank Jupyter, but let’s not allow it to confuse us!

Elements index: since sets have no order, asking Python to extract an element at a given position would make no sense. Thus, differently from strings, lists and tuples, with sets it’s NOT possible to extract an element from an index:

s[0]

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-352-c9c96910e542> in <module>
----> 1 s[0]

TypeError: 'set' object is not subscriptable

We said that a set has only distinct elements, that is without duplicates - what happens if we try to place some duplicate anyway?

[6]:
s = {6,7,5,9,5,5,7}
[7]:
s
[7]:
{5, 6, 7, 9}

We note that Python silently removed the duplicates.

Converting sequences to sets

As for lists and strings, we can create a set from another sequence:

[8]:
set('acacia') # from string
[8]:
{'a', 'c', 'i'}
[9]:
set( [1,2,3,1,2,1,2,1,3,1] ) # from list
[9]:
{1, 2, 3}
[10]:
set( (4,6,1,5,1,4,1,5,4,5) ) # from tuple
[10]:
{1, 4, 5, 6}

Again, we notice in the generated set there are no duplicates

REMEMBER: Sets are useful to remove duplicates from a sequence

Mutable elements and hashes

Let’s see again the definition from the beginning:

A set is a mutable unordered collection of immutable distinct elements

So far we only created the set using immutable elements like numbers and strings.

What happens if we place some mutable elements, like lists?

>>> s = { [1,2,3], [4,5] }

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-40-a6c538692ccb> in <module>
----> 1 s = { [1,2,3], [4,5]  }

TypeError: unhashable type: 'list'

We obtain TypeError: unhashable type: 'list', which literally means Python didn’t manage to calculate the hash of the list. What could this particular dish ever be?

What is the hash? The hash of an object is a number that Python can associate to it, for example you can see the hash of an object by using the function with the same name:

[11]:
hash( "This is a nice day" )  # string
[11]:
-1276273449930679549
[12]:
hash( 111112222223333333344444445555555555 )   # number
[12]:
651300278308214397

Imagine the hash is some kind of label with these properties:

  • it is too short to completely describe the object to which it is associated (that is: given a hash label, you cannot reconstruct the object it represents)

  • it is enough long to identify almost uniquely the object…

  • … even if in the world there might be different objects which have associated exactly the same label

What’s the relation with our sets? The hash has various applications, but typically Python uses it to quickly find an object in collections which are based on hashes, like sets and dictionaries. How much fast? Very: even with homongous sets, we always obtain an answer in a constant very short time! In other words, the answer speed does not depend on the set dimension (except for pathological cases we don’t review here).

This velocity is permitted by the fact that given some object to search, Python is able to rapidly calculate its hash label: then, with the label in the hand, so to speak, it can manage to quickly find in the memory store whether there are objects which have the same label. If they are found, they will almost surely be very few, so Python will only need to compare them with the searched one.

*Immutable* objects always have the same hash label from when they are created until the end of the program. Instead, the mutable ones behave differently: each time we change an object, the hash also changes. Imagine a market where employees place food by looking at labels and separating accordingly for example the coffee in the shelves for the breakfast and bleach in the shelves for detergents. If you are a customer and you want some coffee, you look at signs and directly go toward the shelves for breakfast stuff. Image what could happen if an evil sorcerer could transform the objects already placed into other objects, like for example the coffee into bleach (let’s assume that at the moment of the transmutation the hash label also changes). Much confusion would certainly follow, and, if we aren’t cautious, also a great stomachache or worse.

So to offer you the advantage of a fast search while avoiding disastrous situations, Python imposes to place inside sets only objects with a stable hash, that is immutable objects.

QUESTION: Can we insert a tuple inside a set? Try to verify your intuition with a code example.

Show answer

Empty set

WARNING: If you write {} you will obtain a dictionary, NOT a set !!!

To create an empty set we must call the function set():

[13]:
s = set()
[14]:
s
[14]:
set()

EXERCISE: try writing {} in the cell below and look at the object type obtained with type

Show solution
[15]:
# write here


QUESTION: Can we try inserting a set inside another set? Have a careful look at the set definition, then verify your suppositions by writing some code to create a set which has another set inside.

WARNING: To perform the check, DO NOT use the set function, only use creation with curly brackets

Show answer

QUESTION: If we write something like this, what do we get? (careful!)

set(set(['a','b']))
  1. a set with a and b inside

  2. a set containing another set which contains a and b as elements

  3. an error (which one?)

Show answer

QUESTION: Have a look at following expressions, and for each of them try to guess which result it produces (or if it gives an error):

  1. {'oh','la','la'}
    
  2. set([3,4,2,3,2,2,2,-1])
    
  3. {(1,2),(2,3)}
    
  4. set('aba')
    
  5. str({'a'})
    
  6. {1;2;3}
    
  7. set(  1,2,3  )
    
  8. set( {1,2,3} )
    
  9. set( [1,2,3] )
    
  10. set( (1,2,3) )
    
  11. set(  "abc"  )
    
  12. set(  "1232"  )
    
  13. set( [ {1,2,3,2} ] )
    
  14. set( [ [1,2,3,2] ] )
    
  15. set( [ (1,2,3,2) ] )
    
  16. set( [ "abcb"   ] )
    
  17. set( [ "1232"   ] )
    
  18. set((1,2,3,2))
    
  19. set([(),()])
    
  20. set([])
    
  21. set(list(set()))
    

Exercise - dedup

Write some brief code to create a list lb which contains all the elements of the list la without duplicates and alphabetically sorted.

  • DO NOT change original list la

  • DO NOT use cycles

  • your code should work for any la

la = ['c','a','b','c','d','b','e']

After your code, you should obtain:

>>> print(la)
['c', 'a', 'b', 'c', 'd', 'b', 'e']
>>> print(lb)
['a', 'b', 'c', 'd', 'e']
Show solution
[16]:
la = ['c','a','b','c','d','b','e']

# write here


la = ['c', 'a', 'b', 'c', 'd', 'b', 'e']
lb = ['a', 'b', 'c', 'd', 'e']

Frozenset

INFO: this topic is optional for the purposes of the book

In Python also exists immutable sets which are called frozenset. Here we just remind that since frozensets are immutable they do have associated a hash label and thus they can be inserted as elements of other sets. For other info we refer to the official documentation.

Operators

Operator

Result

Description

len(set)

int

the number of elements in the set

el in set

bool

verifies whether an element is contained in the set

set | set

set

union, creates a NEW set

set & set

set

intersetion, creates a NEW set

set - set

set

difference, creates a NEW set

set ^ set

set

symmetric difference, creates a NEW set

==,!=

bool

checks whether two sets are equal or different

len

[17]:
len( {'a','b','c'}  )
[17]:
3
[18]:
len( set() )
[18]:
0

Exercise - distincts

Given a string word, write some code that:

  • prints the distinct characters present in word as alphabetically ordered (without the square brackets!), together with their number

  • prints the number of duplicate characters found in total

Example 1 - given:

word = "ababbbbcdd"

after your code it must print:

word     : ababbbbcdd
4 distincts : a,b,c,d
6 duplicates

Example 2 - given:

word = "cccccaaabbbb"

after your code it must print:

word     : cccccaaabbbb
3 distinct : a,b,c
9 duplicates
Show solution
[19]:
# write here


word     : ababbbbcdd
4 distincts : a,b,c,d
6 duplicates

Union

The union operator | (called pipe) produces a NEW set containing all the elements from both the first and second set.

eiu3

[20]:
{'a','b','c'} | {'b','c','d','e'}
[20]:
{'a', 'b', 'c', 'd', 'e'}

Note there aren’t duplicated elements

EXERCISE: What if we use the +? Try writing in a cell {'a','b'} + {'c','d','e'}. What happens?

Show solution
[21]:
# write here


QUESTION: Look at the following expressions, and for each try guessing the result (or if they give an error):

  1. {'a','d','b'}|{'a','b','c'}
    
  2. {'a'}|{'a'}
    
  3. {'a'|'b'}
    
  4. {1|2|3}
    
  5. {'a'|'b'|'a'}
    
  6. {{'a'}|{'b'}|{'a'}}
    
  7. [1,2,3] | [3,4]
    
  8. (1,2,3) | (3,4)
    
  9. "abc" | "cd"
    
  10. {'a'} | set(['a','b'])
    
  11. set(".".join('pacca'))
    
  12. '{a}'|'{b}'|'{a}'
    
  13. set((1,2,3))|set([len([4,5])])
    
  14. {()}|{()}
    
  15. {'|'}|{'|'}
    

QUESTION: Given two sets x and y, the expression

len(x | y) <= len(x) + len(y)

produces:

  1. an error (which one?)

  2. always True

  3. always False

  4. sometimes True sometimes False according to values of x and y

Show answer

Exercise - everythingbut 1

Write some code which creates a set s4 which contains all the elements of s1 and s2 but does not contain the elements of s3.

  • Your code should work with any set s1, s2, s3

Example - given:

s1 = set(['a','b','c','d','e'])
s2 = set(['b','c','f','g'])
s3 = set(['b','f'])

After your code you should obtain:

>>> print(s4)
{'d', 'a', 'c', 'g', 'e'}
Show solution
[22]:
s1 = set(['a','b','c','d','e'])
s2 = set(['b','c','f','g'])
s3 = set(['b','f'])

# write here


Intersection

The intersection operator & produces a NEW set which contains all the common elements of the first and second set.

okoerioe

[23]:
{'a','b','c'} & {'b','c','d','e'}
[23]:
{'b', 'c'}

QUESTION: Look at the following expressions, and for each try guessing wthe result (or if it gives an error):

  1. {0}&{0,1}
    
  2. {0,1}&{0}
    
  3. set("capra") & set("campa")
    
  4. set("cba") & set("dcb")
    
  5. {len([1,2,3]),4} & {len([5,6,7])}
    
  6. {1,2}&{1,2}
    
  7. {0,1}&{}
    
  8. {0,1}&set()
    
  9. set([1,2,3,4,5][::2]) & set([1,2,3,4,5][2::2])
    
  10. {((),)}&{()}
    
  11. {(())}&{()}
    

Difference

The difference operator - produces a NEW set containing all the elements of the first set except the ones from the second:

3423dde

[24]:
{'a','b','c','d'} - {'b','c','e','f','g'}
[24]:
{'a', 'd'}

QUESTION: Look at the following expressions, and for each try guessing the result (or if it gives an error):

  1. {3,4,2}-2
    
  2. {1,2,3}-{3,4}
    
  3. '{"a"}-{"a"}'
    
  4. {1,2,3}--{3,4}
    
  5. {1,2,3}-(-{3,4})
    
  6. set("chiodo") - set("chiave")
    
  7. set("prova") - set("prova".capitalize())
    
  8. set("BarbA") - set("BARBA".lower())
    
  9. set([(1,2),(3,4),(5,6)]) - set([(2,3),(4,5)])
    
  10. set([(1,2),(3,4),(5,6)]) - set([(3,4),(5,6)])
    
  11. {1,2,3} - set()
    
  12. set() - {1,2,3}
    

QUESTION: Given two sets x and y, what does the following code produce? An error? Is it simplifiable?

(x & y) | (x-y)
Show answer

Symmetric difference

The symmetric difference of two sets is their union except their intersection, that is all elements except the common ones:

kjdfslkj

In Python you can directly express it with the ^ operator:

[25]:
{'a','b','c'} ^ {'b','c','d','e'}
[25]:
{'a', 'd', 'e'}

Let’s check the result corresponds to the definition:

[26]:
s1 = {'a','b','c'}
s2 = {'b','c','d','e'}

(s1 | s2) - (s1 & s2)
[26]:
{'a', 'd', 'e'}

QUESTION: Look at the following expressions, and for each try guessing the result (or if it gives an error):

  1. {'p','e','p','p','o'} ^ {'p','a','p','p','e'}
    
  2. {'ab','cd'} ^ {'ba','dc'}
    
  3. set('brodino') ^ set('bordo')
    
  4. set((1,2,5,3,2,3,1)) ^ set((1,4,3,2))
    

QUESTION: given 3 sets A, B, C, what’s the expression to obtain the azure part?

sewqe

Show answer

QUESTION: If we use the following values in the previous exercise, what would the set which denotes the azure part contain?

A = {'a','ab','ac','abc'}
B = {'b','ab','bc','abc'}
C = {'c','ac','bc','abc'}

Once you guessed the result, try executing the formula you obtained in the previous exercise with the provided values and compare the results with the solution.

Show answer

Membership

As for any sequence, when we want to check whether an element is contained in a set we can use the in operator which returns a boolean value:

[27]:
'a' in {'m','e','n','t','a'}
[27]:
True
[28]:
'z' in {'m','e','n','t','a'}
[28]:
False

in WHEN USED IN SETS IS VERY FAST

The speed of in operator DOES NOT depend on the set dimension

This is a substantial difference with respect to other sequences we’ve already seen: if you try searching for an element with in in strings, lists or tuples, and the element to find is toward the end (or there isn’t at all), Python will have to look through the whole sequence.

not in

To check whether something is not belonging to a sequence, we can use two forms:

not in - form 1:

[29]:
"carrot" not in {"watermelon","banana","apple"}
[29]:
True
[30]:
"watermelon" not in {"watermelon","banana","apple"}
[30]:
False

not in - forma 2

[31]:
not "carrot" in {"watermelon","banana","apple"}
[31]:
True
[32]:
not "watermelon" in {"watermelon","banana","apple"}
[32]:
False

QUESTION: Look at the following expressions, and for each try guessing the result (or if it gives an error):

  1. 2*10 in {10,20,30,40}
    
  2. 'four' in {'f','o','u','r'}
    
  3. 'aa' in set('aa')
    
  4. 'a' in set(['a','a'])
    
  5. 'c' in (set('parco') - set('cassa'))
    
  6. 'cc' in (set('pacca') & set('zucca'))
    
  7. [3 in {3,4}, 6 in {3,4} ]
    
  8. 4 in set([1,2,3]*4)
    
  9. 2 in {len('3.4'.split('.'))}
    
  10. 4 not in {1,2,3}
    
  11. '3' not in {1,2,3}
    
  12. not 'a' in {'b','c'}
    
  13. not {} in set([])
    
  14. {not 'a' in {'a'}}
    
  15. 4 not in set((4,))
    
  16. () not in set([()])
    

QUESTION: the following expressions are similar. What do they have in common? What is the difference with the last one (beyond the fact it is a set)?

  1. 'e' in 'abcde'
    
  2. 'abcde'.find('e') >= 0
    
  3. 'abcde'.count('e') > 0
    
  4. 'e' in ['a','b','c','d','e']
    
  5. ['a','b','c','d','e'].count('e') > 0
    
  6. 'e' in ('a','b','c','d','e')
    
  7. ('a','b','c','d','e').count('e') > 0
    
  8. 'e' in {'a','b','c','d','e'}
    
Show answer

Equality

We can check whether two sets are equal by using the equality operator ==, which given two sets return True if they contain the same elements or False otherwise:

[33]:
{4,3,6} == {4,3,6}
[33]:
True
[34]:
{4,3,6} == {4,3}
[34]:
False
[35]:
{4,3,6} == {4,3,6, 'hello'}
[35]:
False

Careful about removal of duplicates !

[36]:
{2,8} == {2,2,8}
[36]:
True

To verify the inequality, we can use the != operator:

[37]:
{2,5} != {2,5}
[37]:
False
[38]:
{4,6,0} != {2,8}
[38]:
True
[39]:
{4,6,0} != {4,6,0,2}
[39]:
True

Beware of duplicates and order!

[40]:
{0,1} != {1,0,0,0,0,0,0,0}
[40]:
False

QUESTION: Look at the following expressions, and for each try guessing the result (or if it gives an error):

  1. {2 == 2, 3 == 3}
    
  2. {1,2,3,2,1} == {1,1,2,2,3,3}
    
  3. {'aa'} == {'a'}
    
  4. set('aa') == {'a'}
    
  5. [{1,2,3}] == {[1,2,3]}
    
  6. set({1,2,3}) == {1,2,3}
    
  7. set((1,2,3)) == {(1,2,3)}
    
  8. {'aa'} != {'a', 'aa'}
    
  9. {set() != set()}
    
  10. set('scarpa') == set('capras')
    
  11. set('papa') != set('pappa')
    
  12. set('pappa') != set('reale')
    
  13. {(),()} == {(())}
    
  14. {(),()} != {(()), (())}
    
  15. [set()] == [set(),set()]
    
  16. (set('gosh') | set('posh')) == (set('shopping') - set('in'))
    

Methods like operators

There are methods which behave like the operators|, &, -, ^ by creating a NEW set.

NOTE: differently from operators, these methods accept as parameter any sequence, not just sets:

Method

Result

Description

Related operator

set.union(seq)

set

union, creas a NEW set

|

set.intersection(seq)

set

intersection, creates a NEW set

&

set.difference(seq)

set

difference, creates a NEW set

-

set.symmetric_difference(seq)

set

symmetric difference, creates a NEW set

^

Methods which MODIFY the first set on which they are called (and return None!):

Method

Result

Description

setA.update(setB)

None

union, MODIFIES setA

setA.intersection_update(setB)

None

intersection, MODIFIES setA

setA.difference_update(setB)

None

difference, MODIFIES setA

setA.symmetric_difference_update(setB)

None

symmetric difference, MODIFIES setA

union

We’ll only have a look at union/update, all other methods behave similarly

With union, given a set and a generic sequence (so not necessarily a set) we can create a NEW set:

[41]:
sa = {'g','a','r','a'}
[42]:
la = ['a','g','r','a','r','i','o']
[43]:
sb = sa.union(la)
[44]:
sb
[44]:
{'a', 'g', 'i', 'o', 'r'}

EXERCISE: with union we can use any sequence, but that’s not the case with operators. Try writing {1,2,3} | [2,3,4] and see what happens.

Show solution
[45]:
# write here


We can verify union creates a new set with Python Tutor:

[46]:
sa = {'g','a','r','a'}
la = ['a','g','r','a','r','i','o']
sb = sa.union(la)

jupman.pytut()
[46]:

update

If we want to MODIFY the first set instead, we can use the methods ending with update:

[47]:
sa = {'g','a','r','a'}
[48]:
la = ['a','g','r','a','r','i','o']
[49]:
sa.update(la)
[50]:
print(sa)
{'r', 'i', 'g', 'a', 'o'}

QUESTION: what did the call to update return?

Show answer

Let’s look what at happened with Python Tutor - we also added a x = to put in evidence what was returned by calling .update:

[51]:
sa = {'g','a','r','a'}
la = ['a','g','r','a','r','i','o']
x = sa.update(la)
print(sa)
print(x)

jupman.pytut()
{'r', 'i', 'g', 'a', 'o'}
None
[51]:

QUESTION: Look at the following expressions, and for each try guessing the result (or if it gives an error):

  1. set('case').intersection('sebo') == 'se'
    
  2. set('naso').difference('caso')
    
  3. s = {1,2,3}
    s.intersection_update([2,3,4])
    print(s)
    
  4. s = {1,2,3}
    s = s & [2,3,4]
    
  5. s = set('cartone')
    s = s.intersection('parto')
    print(s)
    
  6. sa = set("mastice")
    sb = sa.difference("mastro").difference("collo")
    print(sa)
    print(sb)
    
  7. sa = set("mastice")
    sb = sa.difference_update("mastro").difference_update("collo")
    print(sa)
    print(sb)
    
[ ]:

Exercise - everythingbut 2

Given sets s1, s2 e s3, write some code which MODIFIES s1 so that it also contains the elements of s2 but not the elements of s3:

  • Your code should work with any set s1, s2, s3

  • DO NOT create new sets

Example - given:

s1 = set(['a','b','c','d','e'])
s2 = set(['b','c','f','g'])
s3 = set(['b','f'])

After your code you should obtain:

>>> print(s1)
{'a', 'g', 'e', 'd', 'c'}
Show solution
[52]:
s1 = set(['a','b','c','d','e'])
s2 = set(['b','c','f','g'])
s3 = set(['b','f'])

# write here


{'e', 'c', 'a', 'd', 'g'}

Other methods

Method

Result

Description

set.add(el)

None

adds the specified element - if already present does nothing

set.remove(el)

None

removes the specified element - if not present raises an error

set.discard(el)

None

removes the specified element - if not present does nothing

set.pop()

obj

removes an arbitrary element from the set and returns it

set.clear()

None

removes all the elements

setA.issubset(setB)

bool

checks whether setA is a subset of setB

setA.issuperset(setB)

bool

checks whether setA contains all the elements of setB

setA.isdisjoint(setB)

bool

checks whether setA has no element in common with setB

add method

Given a set, we can add an element with the method .add:

[53]:
s = {3,7,4}
[54]:
s.add(5)
[55]:
s
[55]:
{3, 4, 5, 7}

If we add the same element twice, nothing happens:

[56]:
s.add(5)
[57]:
s
[57]:
{3, 4, 5, 7}

QUESTION: If we write this code, which result do we get?

s = {'a','b'}
s.add({'c','d','e'})
print(s)
  1. prints {'a','b','c','d','e'}

  2. prints {{'a','b','c','d','e'}}

  3. prints {'a','b',{'c','d','e'}}

  4. an error (which one?)

Show answer

QUESTION: Look at the following code, which result does it produce?

x = {'a','b'}
y = set(x)
x.add('c')
print('x=',x)
print('y=',y)
  1. an error (which one?)

  2. x and y will be the same (how?)

  3. x and y will be different (how?)

Show answer
[58]:
x = {'a','b'}
y = set(x)
x.add('c')

jupman.pytut()
[58]:

remove method

The remove method takes the specified element out of the set. If it doesn’t exist, it produces an error:

[59]:
s = {'a','b','c'}
[60]:
s.remove('b')
[61]:
s
[61]:
{'a', 'c'}
[62]:
s.remove('c')
[63]:
s
[63]:
{'a'}
s.remove('z')

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-266-a9e7a977e50c> in <module>
----> 1 s.remove('z')

KeyError: 'z'

Exercise - bababiba

Given a string word of exactly 4 syllabs of two characters each, create a set s which contains tuples with 2 characters each. Each tuple must represent a syllab taken from word.

  • to add elements to the set, only use add

  • your code must work for any word of 4 bisyllabs

Example 1 - given:

word = "bababiba"

after your code, it must result:

>>> print(s)
{('b', 'a'), ('b', 'i')}

Example 2 - given

word = "rubareru"

after your code, it must result:

>>> print(s)
{('r', 'u'), ('b', 'a'), ('r', 'e')}
Show solution
[64]:
word = "bababiba"
#word = "rubareru"

# write here


{('b', 'i'), ('b', 'a')}

discard method

The discard method removes the specifed element from the set. If it doesn’t exists, it does nothing (we may also say it silently discards the element):

[65]:
s = {'a','b','c'}
[66]:
s.discard('a')
[67]:
s
[67]:
{'b', 'c'}
[68]:
s.discard('c')
[69]:
s
[69]:
{'b'}
[70]:
s.discard('z')
[71]:
s
[71]:
{'b'}

Exercise - trash

✪✪ A waste processing plant receives a load of trash, which we represent as a set of strings:

trash = {'alkenes','vegetables','mercury','paper'}

To remove the contaminant elements which might be present (NOTE: they’re not always present), the plant has exactly 3 filters (as list of strings) which will apply in series to the trash:

filters = ['cadmium','mercury','alkenes']

In order to check whether filters have effectively removed the contaminant(s), for each applied filter we want to see the state of the processed trash.

At the end, we also want to print all and only the contaminants which were actually removed (put them together in the variable separated)

  • DO NOT use if commands

  • DO NOT use cycles (the number of filters is fixed to 3, so you can jsut copy and paste code)

  • Your code must work for any list filters of 3 elements and any set trash

Example - given:

filters = ['cadmium','mercury','alkenes']
trash = {'alkenes','vegetables','mercury','paper'}

After your code, it must show:

Initial trash: {'mercury', 'alkenes', 'vegetables', 'paper'}
Applying filter for cadmium : {'mercury', 'alkenes', 'vegetables', 'paper'}
Applying filter for mercury : {'alkenes', 'vegetables', 'paper'}
Applying filter for alkenes : {'vegetables', 'paper'}

Separated contaminants: {'mercury', 'alkenes'}
Show solution
[72]:
filters = ['cadmium','mercury','alkenes']
trash = {'alkenes','vegetables','mercury','paper'}

separated = trash.intersection(filters) # creates a NEW set

# write here


Initial trash: {'vegetables', 'mercury', 'alkenes', 'paper'}
Applying filter for cadmium : {'vegetables', 'mercury', 'alkenes', 'paper'}
Applying filter for mercury : {'vegetables', 'alkenes', 'paper'}
Applying filter for alkenes : {'vegetables', 'paper'}

Separated contaminants: {'alkenes', 'mercury'}

issubset method

To check whether all elements in a set sa are contained in another set sb we can write sa.issubset(sb). Examples:

[73]:
{2,4}.issubset({1,2,3,4})
[73]:
True
[74]:
{3,5}.issubset({1,2,3,4})
[74]:
False

WARNING: the empty set is always considered a subset of any other set

[75]:
set().issubset({3,4,2,5})
[75]:
True

issuperset method

To verify whether a set sa contains all the elements of another set sb we can write sa.issuperset(sb). Examples:

[76]:
{1,2,3,4,5}.issuperset({1,3,5})
[76]:
True
[77]:
{1,2,3,4,5}.issuperset({2,4})
[77]:
True
[78]:
{1,2,3,4,5}.issuperset({1,3,5,7,9})
[78]:
False

WARNING: the empty set is always considered a subset of any other set

[79]:
{1,2,3,4,5}.issuperset({})
[79]:
True

isdisjoint method

A set is disjoint from another one if it doesn’t have any element in common, we can check for disjointness by using the method isdisjoint:

[80]:
{1,3,5}.isdisjoint({2,4})
[80]:
True
[81]:
{1,3,5}.isdisjoint({2,3,4})
[81]:
False

QUESTION: Given a set x, what does the following expression produce?

x.isdisjoint(x)
  1. an error (which one?)

  2. always True

  3. always False

  4. True or False according to the value of x

Show answer

Exercise - matrioska

✪✪ Given a list sets of exactly 4 sets, we define it a matrioska if each set contains all the elements of the previous set (plus eventually others). Write some code which PRINTS True if the sequence is a matrioska, otherwise PRINTS False.

  • DO NOT use if

  • your code must work for any sequence of exactly 4 sets

  • HINT: you can create a list of 3 booleans which verify whether a set is contained in the next one …

Example 1 - given:

sets = [{'a','b'},
        {'a','b','c'},
        {'a','b','c','d','e'},
        {'a','b','c','d','e','f','g','h','i'}]

after your code, it must print:

Is the sequence a matrioska? True

Example 2 - given:

sets = [{'a','b'},
        {'a','b','c'},
        {'a','e','d'},
        {'a','b','d','e'}]

after your code, it must print:

Is the sequence a matrioska? False
Show solution
[82]:
sets = [{'a','b'},
        {'a','b','c'},
        {'a','b','c','d','e'},
        {'a','b','c','d','e','f','g','h','i'}]


#sets = [{'a','b'},
#        {'a','b','c'},
#        {'a','e','d'},
#        {'a','b','d','e'}]


# write here


Is the sequence a matrioska? True

Exercise with functions

WARNING: The following exercises require to know:

If you are a beginner, you might skip them and come back later

Exercise - syllabs

Write a function syllabs which given a string word made by only bisyllabs and a set found, finds all the distinct bisyllabs and puts them into the set found.

  • NOTE: the function syllabs return NOTHING !

Example 1:

>>> found = set()
>>> syllabs("banana", found)
>>> print(found)
{'an', 'ba'}

Example 2:

>>> found = set()
>>> syllabs("bonobo", found)
>>> print(found)
{'bo', 'on'}
Show solution
[83]:
# write here


Exercise - distinguish

✪✪ Write a function distinguish which given a list big_list containing sublists of two characters each, RETURN a NEW LIST containing all the distinct sublists (ignoring the duplicated sublists)

  • the returned list must have the elements in the same order in which they were found in big_list

  • to know fast whether a sublist was already found, use a set

  • DO NOT search in lists (so no count, index, in in lists - they’re slow!)

  • DO NOT remove from lists (so no remove from lists - it’s slow!)

  • HINT: lists are mutable, can we place them in a set? If it’s not possible, what can we do?

Example:

>>> big_list= [ ['d','d'],['a','b'],['d','d'],['c','a'],['c','a'],['d','d'],['a','b'] ]
>>> print(distinguish( big_list))
[['d', 'd'], ['a', 'b'], ['c', 'a']]
#NOTE: variable big_list MUST NOT be modified:
>>> print(big_list)
[ ['d','d'],['a','b'],['d','d'],['c','a'],['c','a'],['d','d'],['a','b'] ]
Show solution
[84]:
# write here


Verifify comprehension

WARNING

The following exercises contain tests with asserts. To understand how to execute them, read first Error handling and testing

Exercise - intersectron

intersectron

Given a list sets containing an arbitrary number of sets, RETURN a NEW set which contains the elements common to all sets.

To solve the exercise, you can intersecate a set at a time with a for cycle (slow) or with the technique described here (short and fast).

  • try to solve it in both ways

  • BEWARE of the empty list!

  • your code must work with any number of sets (the image is just an example)

Show solution
[85]:
def inter_for(sets):

    raise Exception('TODO IMPLEMENT ME !')

# TEST START - DO NOT TOUCH !
assert inter_for([]) == set()
assert inter_for([set(),set()]) == set()
assert inter_for([set(),set(),set()]) == set()
assert inter_for([{'a'},{'a'},{'a'}]) == {'a'}
assert inter_for([{'a','b'},{'b'},{'b'}]) == {'b'}
assert inter_for([{'a'},{'a','b'},{'a'}]) == {'a'}
assert inter_for([{'c'},{'c'},{'c','b'}]) == {'c'}
assert inter_for([{'a','b'},{'a','b'},{'a','b'}]) == {'a','b'}
assert inter_for([{'a','b','c'},{'a','b','c','d'},{'b','c','d'}, {'b','c'}]) == {'b','c'}
# check we didn't modify the input sets
s = {'a','b'}
assert inter_for([s,{'b','c'}]) == {'b'}
assert s == {'a','b'}
# TEST END
Show solution
[86]:
def inter_fast(sets):

    raise Exception('TODO IMPLEMENT ME !')

# TEST START - DO NOT TOUCH !
assert inter_fast([]) == set()
assert inter_fast([set(),set()]) == set()
assert inter_fast([set(),set(),set()]) == set()
assert inter_fast([{'a'},{'a'},{'a'}]) == {'a'}
assert inter_fast([{'a','b'},{'b'},{'b'}]) == {'b'}
assert inter_fast([{'a'},{'a','b'},{'a'}]) == {'a'}
assert inter_fast([{'c'},{'c'},{'c','b'}]) == {'c'}
assert inter_fast([{'a','b'},{'a','b'},{'a','b'}]) == {'a','b'}
assert inter_fast([{'a','b','c'},{'a','b','c','d'},{'b','c','d'}, {'b','c'}]) == {'b','c'}
# check we didn't modify the input sets
s = {'a','b'}
assert inter_fast([s,{'b','c'}]) == {'b'}
assert s == {'a','b'}
# TEST END