比较两个字典列表

Question

Say i have two lists of dictionaries:

a=[{'name':'A','color':'1'},
   {'name':'B','color':'2'}]
b=[{'name':'A','color':'3'},
   {'name':'c','color':'1'}]

I need something like this:

for i in a:
    if i['name'] is not existent in b['name']:
         do this.
    else:
        if i['color'] is < than the corresponding item in b list:
            do that.

I don't know how to fetch the element form the second list which causes the iteration to go on "else:".

I need to say that the first list is smaller(several hundred items) but the second one has several thousand dictionaries, each with about a hundred items- efficiency is very important.

I did think about making a list of all values for the key['name'] in both lists and compare that but it would mean iterating first time to make these lists and then reiterating over the lists in order to do this or do that. Thanks in advance!

Answer 1

You absolutely want to do an iteration over b before you start. The only obvious alternative is to iterate over b for every item in a , which is obviously worse.

b_dict = {x['name']: x for x in b}
for item in a:
    if item['name'] in b_dict:
        f(b_dict['name']) 
    else:
        pass  # whatever

You may be interested in the get() method of Python dictionaries, if you wish to avoid using in followed by immediately getting the element.

Answer 2

You can use a hash table or something like this:

a=[{'name':'A','color':'1'},
   {'name':'B','color':'2'}]
b=[{'name':'A','color':'3'},
   {'name':'c','color':'1'}]

for item in a:
    mach = list(filter(lambda x: x['name'] == item['name'], b))
    if mach:
        if int(item['color']) > int(mach[0]['color']):
            do that
    else:
        do this

Dict in Python is a kind of hash table with amortized O(1) . Then you can change your code to this:

a=[{'name':'A','color':'1'},
   {'name':'B','color':'2'}]
b=[{'name':'A','color':'3'},
   {'name':'c','color':'1'}]

b_dict = {item['name']:item['color'] for item in b}
for item in a:
    if item['name'] in b_dict and int(item['color']) < int(b_dict[item['name']]):
        print 'do this'
    else:
        print 'do that'

Answer 3

First make an index:

a=[{'name':'A','color':'1'},
   {'name':'B','color':'2'}]

b=[{'name':'A','color':'3'},
   {'name':'C','color':'1'}]

dic = {}
for i,item in enumerate(b):
    dic[item['name']] = i
# dic will be {'A':0,'C':1}
for item in a:
    if not item['name'] in dic:
        #do this
    else:
        if b[dic[item['name']]]['color'] > item['color']:
            #do that

Answer 4

Instead of making a list of values for both the lists, do it only for list B. And then change your if condition to

if i['name'] is not existent in names_in_b:

You will need to check the performance gain of this method, but with this solution, you will need just one iteration over B and then iteration over A follows.