15.iterating over dictionaries
print out all the items in the dictionary -that is,each key and each value
这是第一节就学过，复习第一节
syntax:
print d.items()
2.keys() and values()
syntax:
print d.keys()
print d.values()
3.the"in" operater
syntax:
for key in d:
print key,d[key],
for letter in "Eric":
print letters,
