CODEWITHSUNDEEP
pythondictionary
ItsMeLeoYi
ItsMeLeoYi

Reputation: 35

Adding items into a dictionary

Hello I have a problem with my code for a project I am working on. I am currently trying to manipulate dictionaries and add things into them, but there is something weird going on with my code. The first step to my function is to take a text file that looks like this:

A;X;Y;Z
B;Y;Z;X
C;Y;Z;X
D;Z;X;Y
E;Z;X;Y

I then take the lines of text and put them into a dictionary. Example:

defaultdict(<class 'set'>, {'E': {'Z', 'X', 'Y'}, 'C': {'Y', 'Z', 'X'}, 'A': {'X', 'Y', 'Z'}, 'D': {'Z', 'X', 'Y'}, 'B': {'Y', 'Z', 'X'}})

I wrote the for loop to do this, but the problem I have is that when adding items to the dictionary the code mixes up the values in the dictionary for some reason. 

def read_voter_preferences(file : open):
    votes_dict = defaultdict(set)
    for line in file:
        line = line.strip().split(";")
        for i in range(1,4):
            print(votes_dict)
            votes_dict[line[0]].add(line[i])
    return votes_dict

The output is this: defaultdict(<class 'set'>, {'E': {'X', 'Y', 'Z'}, 'C': {'X', 'Y', 'Z'}, 'A': {'X', 'Y', 'Z'}, 'D': {'X', 'Y', 'Z'}, 'B': {'X', 'Y', 'Z'}})

When the code adds items into a dictionary, it mixes up the order of the items of the dictionary which is important for this particular project. How do I fix this? Does .add() automatically sort the items in the list? Thank you!

Upvotes: 0

Views: 278

Answers (2)

Alex Martelli
Alex Martelli

Reputation: 882721

A set, like a dict, does not preserve order. If the order, as you say, is important, you must therefore use a list, not a set, to maintain it. So, if you're not worries about duplicates:

votes_dict = dict()
for line in file:
    line = line.strip().split(";")
    votes_dict[line[0]] = line[1:]
return votes_dict

Note that you don't really need a default_dict here.

If you need to both maintain order and remove duplicates, your life is harder, but still not too terrible; e.g:

votes_dict = dict()
for line in file:
    line = line.strip().split(";")
    votes_dict[line[0]] = thelist = []
    seen = set()
    for item in line[1:]:
        if item in seen: continue
        thelist.append(item)
        seen.add(item)
return votes_dict

You could cut corners, avoid creating and maintaining seen, and use something like:

votes_dict = dict()
for line in file:
    line = line.strip().split(";")
    votes_dict[line[0]] = thelist = []
    for item in line[1:]:
        if item in thelist: continue
        thelist.append(item)
return votes_dict

Checking in with a list rather than a set is bad in general, but fine for a very short list as you have here, so this one may be best. (Giving a local name as an alias to the list you're building is a technique worth remembering -- no reason to repeatedly index votes_dict:-).

Upvotes: 1

Greg Hewgill
Greg Hewgill

Reputation: 994659

The syntax

{'Z', 'X', 'Y'}

declares a set of items, where only membership is important, not order. So Python feels free to reorder the items when it displays the set.

To preserve the order, use a list:

['Z', 'X', 'Y']

Upvotes: 0

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