CODEWITHSUNDEEP
pythonlistsortingdictionary
user710756
user710756

Reputation: 35

how can I sort a dict with the order of a list in python

for example a is a dict, a = {1:2,2:3,3:4}, and b= [3,1]

I want to sort a to get a tuple list that if keys in a is in b, sort them in the order of b, else put them in the end of the tuple list.

I do it like this : 

sorted(a.items(), key = lambda (k, v): b.index(k) if k in b else a.keys().index(k))

but, I think it is wrong.

so I can I do it using python.

Thanks

Upvotes: 2

Views: 137

Answers (4)

martineau
martineau

Reputation: 123453

I think this does what you want:

a = {1:2,2:3,3:4}
b = [3,1]

print ([(k,a[k]) for k in b if k in a] +
       [(k,a[k]) for k in a if k not in b])

Output:

[(3, 4), (1, 2), (2, 3)]

This would also work:

head, tail = [], []
any((head if k in b else tail).append((k,a[k]))
    for k in (b + [k2 for k2 in a if k2 not in b]))
print head+tail

I wouldn't call it sorting a -- maybe ordering it.

Upvotes: 0

John La Rooy
John La Rooy

Reputation: 304147

A good way to handle the special cases is to use tuples as the sort key

>>> a = {1: 2, 2: 3, 3: 4}
>>> b= [3, 1]
>>> sorted(a.items(), key=lambda (k,v):(0, b.index(k)) if k in b else (1,))
[(3, 4), (1, 2), (2, 3)]

if b is long, it's a good idea to create a dict to speed up the index lookups

>>> b_dict = {k:v for v, k in enumerate(b)}
>>> sorted(a.items(), key=lambda (k,v):(k not in b_dict, b_dict.get(k)))
[(3, 4), (1, 2), (2, 3)]

Upvotes: 2

mbatchkarov
mbatchkarov

Reputation: 16049

Try this:

import sys
sorted(a.items(), key = lambda (k, v): b.index(k) if k in b else sys.maxint)

For the keys that are not in b, we return a very large value, which places them at the end of the sorted result.

Upvotes: 4

Daniel Frey
Daniel Frey

Reputation: 56863

I'm not sure about the expected output that you want, but maybe this helps:

a = {1:2,2:3,3:4}
b = [3,1]

r = [(x,a[x]) for x in b if x in a]

which gives r as a list:

[(3, 4), (1, 2)]

If this is not the expected output, maybe it helps as an intermediate step.

Upvotes: 0

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