Python中的输出顺序错误

Question

There is something wrong with my code. It does not print out the way I wanted it to be.

print("Hello Herp, welcome to Derp v1.0 :)")

inFile = input("Herp, enter symbol table file: ")
symTbl = {}
for line in open(inFile):
    i = line.split()
    symTbl[i[0]] = int(i[1])
print("Derping the symbol table (variable name => integer value)...")
for var1 in symTbl:
    print(var1 + " => " + str(symTbl[var1]))

When I open the textfile, it prints out this:

z => 30
y => 20
x => 10

Which is not right, I am expecting to have output like this:

x => 10
y => 20
z => 30

The original textfile is this:

x 10
y 20
z 30

Answer 1

You need to use an Ordered Dictionary. There is no guarantee in what order you will get your keys when you read them(using the for loop in your case) from a dictionary. The OrderedDict will always return the keys in the order they were entered.

from collections import OrderedDict
symTbl = OrderedDict()

Answer 2

An OrderedDict preserves the order of insertion, it does not sort by key. Sometimes that's what people want, sometimes it's not.

If you're only needing sorted keys once, you could do something like:

for key, value in sorted(list(symTbl.items())):
    print('{} ==> {}'.format(key, value))

If you need sorted values many times (IOW, inside a loop), you're better off with a treap, red-black tree or (on disk, in case your values don't fit in memory) BTree. EG: http://en.wikipedia.org/wiki/Treap

Answer 3

Or, you can just sort the dictionary:

for var1 in sorted(symTbl):
    print(var1 + " => " + str(symTbl[var1]))