Clever use of set

I noticed that we can use set to simplify algorithm in many problems, including distribute candies and keyboard row. Thus, I conclude some clever use of set in python and do some comparison between list.

1. Introduction to set

We know that the core of data structure dict in Python is to map relationships between a set of key and value, while key can not be repeated. Set is similar to dict to some degree that elements in set is duplicate. Additionally, these elements are disorder, which means each print of set might be of different order inside. Same as the key in dict, elements in set must be constant instead of any variable that can be changed.

To create a set, we can call set() and pass elements in a list into it: s = set(['A', 'B', 'C'])

2. Methods & Properties

Properties

• The set representation set is very similar to a list with differences:

  • Sets can only store immutable objects
  • The objects stored in a set are unordered
  • Sets cannot have duplicate elements

• We can use {} to create sets

• Set() can be used to convert sequencesand dictionaries into sets.

• We can use len() to get the number of elements in a set

• We can use add() to add elements to a set

• update() adds elements from one set to another

• pop() removes an element from a set at random, usually the last element.

• remove() removes the specified element from the set

• clear() clears the set

Examples:

set_a={8,7,'pthon',True,8.8,7,7,7}  # elements in set cannot be changed
print('1.',set_a,type(set_a))    	# elements in set in disordered and duplicate
dict_a={'a1':'python','b2':'java','c':8}
set_b =set(dict_a)    				# we can use set() to transform sequence to set (transform key in dict)
print('2.',set_b,type(set_b))

Operations on sets

& 	# Intersection
| 	# Concurrency
- 	# Difference set operations
^ 	# Also or set

<= 		# Check if a set is a subset of another set
< 		# Check if a set is a true subset of another set
>= 		# Check if a set is a superset of another set
> 		# Check if a set is a true superset of another set

See keyboard row for example.

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