Dictionary with a list as value-CodePudding

I have a list of tuples:

res=[(0, 0, 255, 0, 0),(1, 0, 255, 0, 0),(0, 1, 255, 0, 0),(1, 1, 255, 0, 0),
(4, 4, 0, 255, 0),(5, 4, 0, 255, 0),(4, 5, 0, 255, 0),(5, 5, 0, 255, 0)]

This is my idea:

keys = [l[2:] for l in res]
values = [l[:2] for l in res]
d=dict(zip(keys, values))

and this is my output:

{(255, 0, 0): (1, 1), (0, 255, 0): (5, 5)}

My output is wrong, I need this one:

{(255, 0, 0): [(0, 0),(1,0),(0,1),(1,1)], 
(0, 255, 0): [(4,4),(5,4),(4,5),(5,5)]}

Any ideas?

CodePudding user response：

Using collections.defaultdict:

from collections import defaultdict 

out = defaultdict(list)

for t in res:
    out[t[2:]].append(t[:2])

dict(out)

Or with a classical dictionary:

out = {}

for t in res:
    k = t[2:]
    if k not in out:
        out[k] = []
    out[k].append(t[:2])

Output:

{(255, 0, 0): [(0, 0), (1, 0), (0, 1), (1, 1)],
 (0, 255, 0): [(4, 4), (5, 4), (4, 5), (5, 5)]}

CodePudding user response：

You can use a dictionary with .setdefault() to accumulate the results, using slicing to generate the appropriate keys and values.

ans = {}

for entry in res:
    ans.setdefault(entry[2:], []).append(entry[:2])
    
print(ans)

This outputs:

{
 (255, 0, 0): [(0, 0), (1, 0), (0, 1), (1, 1)],
 (0, 255, 0): [(4, 4), (5, 4), (4, 5), (5, 5)]
}

CodePudding user response：

There are much more elegant solutions to this problem, but the point of the exercise is (likely) for you to practice logic and control flow, so I'll show a more beginner-appropriate solution:

output = {}
for entry in res:
    key, value = entry[2:], entry[:2]
    if key not in output:
        output[key] = []
    output[key].append(value)

As has been mentioned though, collections.defaultdict(list) is the prime candidate for a problem like this.