比较两个字典的值

Question

I have two dictionaries as follows:

a = {1:2, 3:4, 5:6}
b = {1:5, 3:6, 7:1}

For every key in a , I want to check if the key exists in b , if so, I want to print the value of both keys, if it does not, I print 0 as the value of the key in b :

for key in a.keys():
    if key in b.keys():
        print key, a[key], b[key]
    else:
        print key, a[key], '0'

The output would be:

1 2 5
3 4 6
5 6 0

But, I also want to print the value of key in b if it does not exist in a , that is the opposite of the last statement, if key is in b but not in a , print the value of the key in b and 0 as the value of the key in a . The output would be:

1 2 5
3 4 6
5 6 0
7 0 1

It should be simple but I can't figure out how I can do it. Thanks!

Answer 1

If I understand correctly, you want to iterate through all keys from either dictionary, and print their values from the two dictionaries, using '0' if the key is missing from that dictionary. Something like this:

for key in set(a)|set(b):
    print key, a.get(key, '0'), b.get(key, '0')

set(a)|set(b) is the union of the sets of keys from each dictionary (ie it is a collection of distinct keys from either dictionary).

dictionary.get(key, '0') returns '0' if the key is missing from the dictionary.

Answer 2

for key in set(a.keys()) | set(b.keys()):
    print key, a.get(key, 0), b.get(key,0)

| means union in a set context. You can also convert the resulting set into a list and sort it before iterating.

Answer 3

Use dict b 's get method, supplying a default value for use when the key isn't found.

for key in a:
    print key, a[key], b.get(key, '0')