CODEWITHSUNDEEP
pythondictionary
Marla
Marla

Reputation: 340

Python: How to append list item to dictionary if item equals key?

I'm attempting to iterate through a list and if the list item equals a dictionary key, append the list item to the dictionary.

mylist = [1, 2, 3]

mydict = dict.fromkeys(mylist, [])

for item in mylist:
    for key in mydict:
        if key == item:
            mydict[key].append(item)

print(mydict)

Output:

{1: [1, 2, 3], 2: [1, 2, 3], 3: [1, 2, 3]}

Required output:

{1: [1], 2: [2], 3: [3]}

Much thanks!

Upvotes: 2

Views: 1664

Answers (4)

kederrac
kederrac

Reputation: 17322

by using:

mylist = [1, 2, 3]

mydict = dict.fromkeys(mylist, [])

you are creating a dict that has all the elements from mylist as keys and all the values from your dict are references to the same list, to fix you may use:

mydict = dict.fromkeys(mylist)

for item in mylist:
    mydict[item] = [item]

print(mydict)

output:

{1: [1], 2: [2], 3: [3]}

same thing but in a more efficient and compact form by using a dictionary comprehension:

mydict = {item: [item] for item in mylist}

Upvotes: 1

Eduardo Donato
Eduardo Donato

Reputation: 327

mylist = [1, 2, 3]

mydict = dict.fromkeys(mylist, [])

for item in mylist:
    for key in mydict:
        if key == item:
            mydict[key] = [item]

print(mydict)

Upvotes: 0

Yosua
Yosua

Reputation: 421

Is this what you wanted?

mylist = [1,2,3,3]
mydict = {}
for item in mylist:
    if item not in mydict:
        mydict[item] = [item]
    else:
        mydict[item].append(item)
print(mydict)

It will output: {1: [1], 2: [2], 3: [3, 3]}

Upvotes: 0

ForceBru
ForceBru

Reputation: 44838

That's because here:

mydict = dict.fromkeys(mylist, [])

mydict's values will be the same object [], so when you append to mydict[something], you'll be appending to the same list, no matter what something is.

All values are the same object, you append three numbers to it => all values are shown as the same list.

To avoid this, assign new lists to each key:

mydict = {}
for item in mylist:
    mydict.setdefault(item, []).append(item)

Or, you know:

mydict = {key: [key] for key in mylist}

Upvotes: 4

Related Questions