A sparse list

sparse1.py

Please type a street number (e.g., 42): 42
Grand Central Terminal is on that street.

Please type a street number (e.g., 42): 14
The Con Ed Building is on that street.

Please type a street number (e.g., 42): 15
Nothing famous is on that street.

Please type a street number (e.g., 42):

Use a dictionary instead of a sparse list.

If the keys are consecutive integers, store the information in a list. If the keys are integers that are far apart or irregularly spaced, store the information in a dictionary.

sparse2.py

The output is the same.

Things to try

1. The keys of a dictionary will usually be strings, and we have just seen that the keys could also be ints. But the keys cannot be lists:

   #Try to make a dictionary whose keys are lists.
   
   buildings = {
       [5, 34]: "The Empire State Building",
       [5, 88]: "The Guggenheim",
       [7, 57]: "Carnegie Hall",
       [1, 42]: "The United Nations",
       [7, 32]: "Penn Station"
   }
   

   Traceback (most recent call last):
     File "/Users/myname/python/index.py", line 6, in <module>
       [7, 32]: "Penn Station"
   TypeError: unhashable type: 'list'
   

   The keys of a dictionary must be immutable, because we don’t want to have to rearrange the dictionary every time someone changes a key. For example, the keys could be tuples.

   #a dictionary whose keys are tuples
   
   buildings = {
       (5, 34): "The Empire State Building",
       (5, 88): "The Guggenheim",
       (7, 57): "Carnegie Hall",
       (1, 42): "The United Nations",
       (7, 32): "Penn Station"
   }
   
   while True:
       try:
           ave = int(input("Please type an avenue number (e.g., 5): "))
       except:  #if user did not type a valid int
           sys.exit(0)
   
       try:
           st = int(input("Please type a street number (e.g., 34): "))
       except:  #if user did not type a valid int
           sys.exit(0)
   
       try:
           b = buildings[(ave, st)]   #Parentheses are optional.
       except KeyError:
           b = "Nothing famous"
   
       print(f"{b} is at that corner.")
       print()
   

   Please type an avenue number (e.g., 5): 5
   Please type a street number (e.g., 34): 34
   The Empire State Building is at that corner.
   
   Please type an avenue number (e.g., 5): 5
   Please type a street number (e.g., 34): 35
   Nothing famous is at that corner.
   
   Please type an avenue number (e.g., 5):