Computer Science 15-110, Fall 2010
Class Notes:  One-Dimensional Lists

1. What is a dictionary?
2. Quick Example
3. How to...
1.  Create a dictionary
1.  Create an empty dictionary (with { })
2. Create a dictionary from a list of tuples (with dict())
2. Add a key/value
3. Delete a key (with del, pop, and clear)
4. Get a value from a key
5. Get a value from a missing key
1. Check if the key exists (with key in dict)
2. Use dict.get(key) and dict.get(key, default)
6. Modify a key/value
7. Get all keys (with dict.keys())
4. Usage
1. Can use mixed types for keys and values
2. Keys are case-sensitive
3. Don't use a dictionary like a list
5. Recommended Reading
6. Examples Using Dictionaries
1. Word Counts (from a list of words)
2. Sparse Matrices
3. Text Adventure
7. Suggested Practice Problems

1. What is a dictionary?
1. It is a map from keys to values
2. Adding, removing, getting, and setting are all fast (eg, much faster than using a sorted list of tuples)
3.  Keys are a set:  they are unique and unsorted
   
   
2. Quick Example
   
   # Quick Example using a dictionary
   says = { }             # creates an empty dictionary
   says["dog"] = "woof"
   says["cat"] = "meow"
   print says["dog"]      # woof
   print says["cat"]      # meow
   
   print ("cow" in says)  # False
   says["cow"] = "moo"
   print ("cow" in says)  # True
   print says["cow"]      # moo
   
   print says             # {'dog': 'woof', 'cow': 'moo', 'cat': 'meow'}
   
   says["dog"] = "bark"
   print says["dog"]      # bark
   print says             # {'dog': 'bark', 'cow': 'moo', 'cat': 'meow'}
   
   
3. How to...
   
   
1.  Create a dictionary
   
   # Create an empty dictionary (with {})
   d = { }  # or d = dict()
   print d
   
   # Create a dictionary from a list of tuples (with dict())
   d = dict([("dog", 10), ("ant", 0), ("cat", 3)])
   print d
   
   
2. Add a key/value
   
   # Add a key/value
   d = { }
   d["dog"] = 5
   d["cat"] = 7
   print d
   
   
3. Delete a key (with del, pop, and clear)
   
   d = dict([("dog", 10), ("ant", 0), ("cat", 3), ("cow", 7)])
   print d              # {'ant': 0, 'dog': 10, 'cow': 7, 'cat': 3}
   
   # Delete a key with del dict[key]
   del d["dog"]
   print d              # {'ant': 0, 'cow': 7, 'cat': 3}
   
   # Delete a key with dict.pop(key)
   value = d.pop("cat")
   print value          # 3
   print d              # {'ant': 0, 'cow': 7}
   
   # Delete all keys with dict.clear()
   d.clear()
   print d              # { }
   
   
4. Get a value from a key
   
   # Get a value from a key with dict[key]
   d = dict([("dog", 10), ("ant", 0), ("cat", 3), ("cow", 7)])
   print d["dog"]   # 10
   
   
5. Get a value from a missing key
   
   Problem:
   
   # Try to get a value from a missing key with dict[key]
   d = dict([("dog", 10), ("ant", 0), ("cat", 3), ("cow", 7)])
   print d["pig"]  # KeyError: 'pig'  (key not in dictionary!)
   
   Solutions:
   
   # Detect a missing key with (key in dict):
   d = dict([("dog", 10), ("ant", 0), ("cat", 3), ("cow", 7)])
   if ("pig" in d):
       print d["pig"]
   else:
       print "pig is not in", d
   
   # Get a value from a missing key with dict.get(key)
   d = dict([("dog", 10), ("ant", 0), ("cat", 3), ("cow", 7)])
   print d.get("dog")  # 10
   print d.get("pig")  # None
   
   # Get a value from a missing key with dict.get(key, default)
   d = dict([("dog", 10), ("ant", 0), ("cat", 3), ("cow", 7)])
   print d.get("dog", 42)  # 10
   print d.get("pig", 42)  # 42
   
   
6. Modify a key/value
   
   # Modify a key/value
   d = { }
   d["dog"] = 5
   d["cat"] = 7
   print d
   d["dog"] = 42
   print d
   
   
7. Get all keys (with dict.keys())
   
   # Get all keys
   d = dict([("dog", 10), ("ant", 0), ("cat", 3), ("cow", 7)])
   print d.keys()         # ['ant', 'dog', 'cow', 'cat']
   print sorted(d.keys()) # ['ant', 'cat', 'cow', 'dog']
   
   
4. Usage
   
   
1. Can use mixed types for keys and values
   
   # Can use mixed types for keys and values
   d = { }
   d["ant"] = True
   d[False] = 42
   d[999] = "gnu"
   print d   # {'ant': True, False: 42, 999: 'gnu'}
   
   
2. Keys are case-sensitive
   
   # Keys are case-sensitive
   d = { }
   d["ant"] = 3
   d["Ant"] = 4
   print d["ant"]  # 3
   print d["Ant"]  # 4
   d["cow"] = 32
   print d["COW"]  # crashes with KeyError, "COW" is not in the dictionary
   
   
3. Don't use a dictionary like a list
   
   # Don't use a dictionary like a list
   # eg, the following code is discouraged:
   def exampleOfIncorrectUseOfDictAsList():
       d = { }
       n = 4
       print "Enter", n, "strings: "
       for i in range(n):
           value = raw_input("Enter string #" + str(i) + ": ")
           d[i] = value
       print "And here are the strings you entered:"
       for i in range(n):
           print i, d[i]
   
   exampleOfIncorrectUseOfDictAsList()
   
   
5. Recommended Reading
1. http://docs.python.org/library/stdtypes.html#typesmapping
2. http://docs.python.org/tutorial/datastructures.html
3. http://diveintopython.org/getting_to_know_python/dictionaries.html
   
   
6. Examples Using Dictionaries
1. Word Counts (from a list of words)  (wordCountsWithBug.py  and  wordCounts.py)
2. Sparse Matrices  (sparseMatrices.py)
3. Text Adventure  (textAdventure.py)
   
   
7. Suggested Practice Problems
1. More Sparse Matrix math (sparseMatrixAdd, sparseMatrixMultiply, sparseMatrixInverse, etc)
2. contentsSortedByKey(dictionary):  Returns a list of the dictionary's key/value pairs (as tuples) with the keys in alphabetical order
3. contentsSortedByValue(dictionary):  Returns a list of the dictionary's key/value pairs (as tuples) with the values in alphabetical order (this is a bit trickier)

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