Python Programming for Biology: Bioinformatics and Beyond

Set manipulation

In this section we will illustrate some basic manipulations on sets using the following set,

containing three items:

s = {123, 54, 92}

The items in a set are not in any order and so it does not mean anything to access them

by index. In fact, there is no way to inquire about a specific item in a set. However, you

could  put  the  same  items  in  a  tuple  or  a  list  and  access  that,  but  even  then,  because  the

items in a set are unordered, the order in the resulting tuple or list is arbitrary:

x = tuple(s) # x now (123, 92, 54), s still a set

x = list(s) # x now [123, 92, 54], s still a set

You can check whether an item is present in a set, which is a computationally efficient

operation, like with dictionaries, but unlike lists and tuples:

54 in s # True

55 in s # False

You can also check whether an item is not in (absent from) a set:

54 not in s # False

55 not in s # True

The len() function returns the number of items in the set:

len(s) # 3

You can add items to a set:

---

s.add(77) # s now {123, 54, 92, 77}

If you try and add an item to a set that is already contained in the set then the operation

will effectively be ignored:

s.add(123) # s still {123, 54, 92, 77}

You can remove an item from a set:

s.remove(54) # s now {123, 54, 92, 77}

If you try to remove an item that is not in the set then you get an error.

You  can  fetch  and  remove  an  arbitrary  item  directly  from  a  set,  noting  that  here  the

pop() does not take any arguments:

y = s.pop() # s is one smaller, y is arbitrary

There are several functions that work with multiple sets, to do many of the operations

associated  with  set  theory  (as  are  often  illustrated  with  Venn  diagrams),  for  example,  to

find unions and intersections.

s = set{1, 2, 3, 4, 5}

t = set{4, 5, 6, 7}

a = s & t # Intersection: {4, 5}

b = s | t # Union: {1, 2, 3, 4, 5, 6, 7}

More set operations are listed in the reference section of the Appendices.