Python Programming/Dictionaries

Tuples Sets A dictionary in Python is a collection of unordered values accessed by key rather than by index. The keys have to be hashable: integers, floating point numbers, strings, tuples, and, frozensets are hashable. Lists, dictionaries, and sets other than frozensets are not hashable. Dictionaries were available as early as in Python 1.4. Overview Dictionaries in Python at a glance: dict1 = {} # Create an empty dictionary dict2 = dict() # Create an empty dictionary 2 dict2 = {"r": 34, "i": 56} # Initialize to non-empty value dict3 = dict([("r", 34), ("i", 56)]) # Init from a list of tuples dict4 = dict(r=34, i=56) # Initialize to non-empty value 3 dict1["temperature"] = 32 # Assign value to a key if "temperature" in dict1: # Membership test of a key AKA key exists del dict1["temperature"] # Delete AKA remove equalbyvalue = dict2 == dict3 itemcount2 = len(dict2) # Length AKA size AKA item count isempty2 = len(dict2) == 0 # Emptiness test for key in dict2: # Iterate via keys print (key, dict2[key]) # Print key and the associated value dict2[key] += 10 # Modify-access to the key-value pair for key in sorted(dict2): # Iterate via keys in sorted order of the keys print (key, dict2[key]) # Print key and the associated value for value in dict2.values(): # Iterate via values print (value) for key, value in dict2.items(): # Iterate via pairs print (key, value) dict5 = {} # {x: dict2[x] + 1 for x in dict2 } # Dictionary comprehension in Python 2.7 or later dict6 = dict2.copy() # A shallow copy dict6.update({"i": 60, "j": 30}) # Add or overwrite; a bit like list's extend dict7 = dict2.copy() dict7.clear() # Clear AKA empty AKA erase sixty = dict6.pop("i") # Remove key i, returning its value print (dict1, dict2, dict3, dict4, dict5, dict6, dict7, equalbyvalue, itemcount2, sixty) Dictionary notation Dictionaries may be created directly or converted from sequences. Dictionaries are enclosed in curly braces, {} >>> d = {'city':'Paris', 'age':38, (102,1650,1601):'A matrix coordinate'} >>> seq = [('city','Paris'), ('age', 38), ((102,1650,1601),'A matrix coordinate')] >>> d {'city': 'Paris', 'age': 38, (102, 1650, 1601): 'A matrix coordinate'} >>> dict(seq) {'city': 'Paris', 'age': 38, (102, 1650, 1601): 'A matrix coordinate'} >>> d == dict(seq) True Also, dictionaries can be easily created by zipping two sequences. >>> seq1 = ('a','b','c','d') >>> seq2 = [1,2,3,4] >>> d = dict(zip(seq1,seq2)) >>> d {'a': 1, 'c': 3, 'b': 2, 'd': 4} Operations on Dictionaries The operations on dictionaries are somewhat unique. Slicing is not supported, since the items have no intrinsic order. >>> d = {'a':1,'b':2, 'cat':'Fluffers'} >>> d.keys() ['a', 'b', 'cat'] >>> d.values() [1, 2, 'Fluffers'] >>> d['a'] 1 >>> d['cat'] = 'Mr. Whiskers' >>> d['cat'] 'Mr. Whiskers' >>> 'cat' in d True >>> 'dog' in d False Combining two Dictionaries You can combine two dictionaries by using the update method of the primary dictionary. Note that the update method will merge existing elements if they conflict. >>> d = {'apples': 1, 'oranges': 3, 'pears': 2} >>> ud = {'pears': 4, 'grapes': 5, 'lemons': 6} >>> d.update(ud) >>> d {'grapes': 5, 'pears': 4, 'lemons': 6, 'apples': 1, 'oranges': 3} >>> Deleting from dictionary del dictionaryName[membername] Exercises Write a program that: Asks the user for a string, then creates the following dictionary. The values are the letters in the string, with the corresponding key being the place in the string. https://docs.python.org/2/tutorial/datastructures.html#looping-techniques Replaces the entry whose key is the integer 3, with the value "Pie". Asks the user for a string of digits, then prints out the values corresponding to those digits. External links 5.5. Dictionaries in Tutorial, docs.python.org 5.8. Mapping Types in Library Doc, docs.python.org Tuples Sets