Adjust / norm python dictionary values to certain size / length-CodePudding

I have a question regarding how to limit the value length of a dictionary:

real_vertrices_copy = {
     'key': {1,3,8,4,2},
     'key1': {9,4,2,4},
     'key2': {6,4,2},
     'key3': {9,3,5,3,5}
    }

The dictionary consists of key value pairs, with the values varying in length. I want to limit the length of the values to 3, so if the length of the values is > 3, I want to pop random elements as many times necessary to reduce it to len=3.

I tried it like this:

for x in real_vertrices_copy.values():
if len(x) > 20:
    real_vertrices_copy.popitem()

but this just removes random dictionary entries (I did not expect this to work, I have to admit)

Does anyone have an idea how to solve this?

Thank you and BR

CodePudding user response：

You could just pop three times from each set with more than three elements to get three arbitrary elements:

d = {
    'key': {1,3,8,4,2},
    'key1': {9,4,2,4},
    'key2': {6,4,2},
    'key3': {9,3,5,3,5}
}
d = {
    k: {v.pop() for _ in range(3)} if len(v) > 3 else v
    for k, v in d.items()
}

This assumes that there are no other references to the sets in d because the mutation via pop will be seen across all references.

CodePudding user response：

You can use a dictionary comprehension with random sampling (random.sample):

d = {'key': {1,3,8,4,2},
     'key1': {9,4,2,4},
     'key2': {6,4,2},
     'key3': {9,3,5,3,5}
    }

import random
out = {k: set(random.sample(list(v), 3)) for k,v in d.items()}

or, if it is possible that the input has less than 3 elements:

N = 3
out = {k: set(random.sample(list(v), N)) if len(v)>=N else v
       for k,v in d.items()}

example output:

{'key': {1, 3, 8}, 'key1': {2, 4, 9}, 'key2': {2, 4, 6}, 'key3': {3, 5, 9}}

classical loop

If you need a classical loop and want to get both kept and discarded items, take advantage of set operations:

N = 3
keep = {}
discard = {}
for k,v in d.items():
    keep[k] = set(random.sample(list(v), min(len(v), N)))
    discard[k] = v.difference(keep[k])
    
print(keep)
# {'key': {1, 2, 4}, 'key1': {9, 2, 4}, 'key2': {2, 4, 6}, 'key3': {9, 3, 5}}
print(discard)
# {'key': {8, 3}, 'key1': set(), 'key2': set(), 'key3': set()}

CodePudding user response：

Inplace update of real_vertrices_copy:

import random

real_vertrices_copy = {'key': {1,3,8,4,2},
     'key1': {9,4,2,4},
     'key2': {6,4,2},
     'key3': {9,3,5,3,5}
    }
for k, v in real_vertrices_copy.items():
    while len(v) > 3:
        v.remove(random.choice(tuple(v)))