Lists union and intersecion

[arbiel]

Hi

Is it possible to take the union and the intersection of lists. If affirmative, which module contains these methods, and where to find it ?

Thank you for the advice

Arbiel

[buran]

what exactly union and intersect would do on lists? Usisally these are terms applied to sets. Provide sample input and desired output

[arbiel]

Yes, those terms apply to mathematical entities, with the following meaning :

list1=['a', 'b']
list2=['c']

union (list1, list2) : ['a', 'b, 'c']
intersection (list1, list2) : []

>>> dia
['Dasia', 'Dialytika', 'Macron', 'Oxia', 'Perispomeni', 'Prosgegrammeni', 'Psili', 'Tonos', 'Varia', 'Vrachy', 'Ypogegrammeni']
>>> mots
['Greek', 'Small', 'Letter', 'Upsilon', 'with', 'Dialytika', 'and', 'Oxia']
>>>

intersection (dia,mots) : ['Dialytika', 'Oxia']

Arbiel

[buran]

List and sets are different types in python. Sets hold unique/non-repeating elements, while lists can have repeating elements and there comes the tricky part.

As I said union and intersection are terms for sets. There is list.extend() method and for "intersection" you need to use list comprehension or loop

dia = ['Dasia', 'Dialytika', 'Macron', 'Oxia', 'Perispomeni', 'Prosgegrammeni', 'Psili', 'Tonos', 'Varia', 'Vrachy', 'Ypogegrammeni']
mots = ['Greek', 'Small', 'Letter', 'Upsilon', 'with', 'Dialytika', 'and', 'Oxia']

# convert to sets
result = set(dia).union(set(mots))
print(result)
print(list(result))

result2 = set(dia).intersection(set(mots))
print(result2)
print(list(result2))

# if order is imporatnt
result = [element for element in dia if element in set(mots)]
print(result)

dia.extend(mots)
print(dia)

[arbiel]

Hi buran

Thank you for your help. This exactly what I am looking for.

However I do not understand

you need to use list comprehension or loop

when I see that

result2 = set(dia).intersection(set(mots))

leads to the intersection of the sets.

Arbiel

[buran]

This is one way to do it

# convert to sets
result = set(dia).union(set(mots))
print(result)
print(list(result))
 
result2 = set(dia).intersection(set(mots))
print(result2)
print(list(result2))

However, this will not preserve order and set have unique elements, so if there are duplicates in the list (same element more than one time in list) they will be lost


And this is working with lists - order and repeating elements are preserved

# if order is imporatnt
result = [element for element in dia if element in set(mots)] # this is list comprehension, but it can be replaced by regular loop
print(result)
 
dia.extend(mots)
print(dia)