#                           Python Hash Tables


# In addition to Array/Lists, Python has a built-in data structure called
# 'dictionaries' or 'associative arrays'.  Most computer scientists however,
# will call these structures "hash tables".  An array indexes its elements
# using integers.  But often we wish to store data by some other attribute.
# For example, suppose I want a data strucutres that stores the 700 id
# numbers of the students.  I want to be able to look up a 700 number given
# a student's name.  An array is not really appropriate for this kind of
# structure, but a hash table is:

ID = {}   # creates an empty hash table (or 'dictionary')
# can also say ID = dict(), which creats a dict() object, {} is shortcut.

ID["john smartypants"] = 700123456   # store data indexed by strings.
ID["jane fourpointo"] = 700654321
ID["henry deanslist"] = 700111111

# The string used to "index" the hash table ID is called the "key".  To
# access the data stored in the table I need to know the key.  

print ID["john smartypants"]   # prints 700123456

# It's also possible to define an entire table by giving each key:value pair:
H = {'A':1, 'B':2, 'C':3}
print H['B']  # will print 2


# Looping through a hash table:  I can't use the while loops like with arrays
# becaue the elements are not numbered 0,1,2,3... anymore.  Instead, I have
# to use the following kinds of for-loops:

for k in ID.keys():
    print ID[k]   # prints number associated with each key.

# The above loop loops through every key and associated value in the table.

# Here's a similar loop
for key,value in ID.items():
    print key,":", value      # prints each key-value pair.

# Usually hash tables are indexed by strings, but they can also be indexed
# by any kind of immutable data.