CMU 15-110: Principles of Computing
Maps (Dictionaries)

1. Motivating Example: getState(city)
2. Creating Dictionaries
3. Dictionary Properties
4. Dictionary Operations
5. Example: isAnagram with a Dictionary

1. Motivating Example: getState(city)
   stateMap = { 'Pittsburgh':'PA', 'Chicago':'IL', 'Seattle':'WA', 'Boston':'MA' } def getState(city): if (city in stateMap): return stateMap[city] else: return None print(getState('Chicago')) print(getState('Toronto'))


2. Creating Dictionaries
   • Create an empty dictionary
     d = dict() print(d)
   
   
   • Create a dictionary with some values in it
     d = { 'cow':5, 'dog':98, 'cat':1 } print(d['cow']) print(d)


3. Dictionary Properties
   1. Dictionaries (aka 'maps') map keys to values: d[key] = value
   2. Keys are restricted: must be unique and must be immutable
   3. Values are unrestricted: may repeat and may be mutable
   4. Dictionaries are very efficient


4. Dictionary Operations
   

   Operation	Result
   len(d)	the number of keys in d
   for key in d	loop over each key in d
   key in d	test if d has the given key
   key not in d	test if d does not have the given key
   d[key]	get the value that is associated with the given key. Raises a KeyError if key is not in d.
   d[key] = value	set d[key] to value (replace old value if there was one)



5. Example: isAnagram with a Dictionary
   Compare this solution to our earlier version here:
   def letterCounts(s): # improved version now returns a dictionary instead of a list, # and now uses the character as the key, instead of (ord(c) - ord('A') counts = dict() for c in s.upper(): if ((c >= 'A') and (c <= 'Z')): if (c in counts): counts[c] += 1 else: counts[c] = 1 return counts def isAnagram(s1, s2): return (letterCounts(s1) == letterCounts(s2)) def testIsAnagram(): print('Testing isAnagram()...', end='') assert(isAnagram('', '') == True) assert(isAnagram('abCdabCd', 'abcdabcd') == True) assert(isAnagram('abcdaBcD', 'AAbbcddc') == True) assert(isAnagram('abcdaabcd', 'aabbcddcb') == False) print('Passed!') testIsAnagram()