Cant get my python dictionary to work properly-CodePudding

nums = [3,2,2,3]

hash = {}
        
for i in range(len(nums)):
    if nums[i] not in hash:
        hash[nums[i]] = [i]
    else:
        hash[nums[i]] = hash[nums[i]].append(i)
        
print(hash)

What im trying to do here is add the indexes of occurrences of a certain element in a list as a list as a value to the key, which is the element itself.

For Example:- nums = [3,2,2,3] should return {3: [0,3], 2: [1,2] instead, what my code returns is this:- {2: None, 3: None}

Please tell me where I'm going wrong in my code. Thanks.

CodePudding user response：

hash[nums[i]] = hash[nums[i]].append(i)

append always returns none as it modifies the original list you instead need to do just

hash[nums[i]].append(i)

there everything else is fine. Also you can use the built in enumerate function instead of your for loop

nums = [3,2,2,3]

hash = {}
        
for i, item in enumerate(nums):
    if item not in hash:
        hash[item] = [i]
    else:
        hash[item].append(i)
        
print(hash)

CodePudding user response：

as juanpa.arrivillaga already commented, you should not overwrite your hash dict when appending to its values.

nums = [3, 2, 2, 3]

hash = {}
for i in range(len(nums)):
    if nums[i] not in hash:
        hash[nums[i]] = [i]
    else:
        hash[nums[i]].append(i)

print(hash)
# {3: [0, 3], 2: [1, 2]}

a slightly more compact approach using enumerate und setdefault gives you the same results.

hash = {}
nums = [3, 2, 2, 3]
for i, num in enumerate(nums):
    hash.setdefault(num, []).append(i)

print(hash)
# {3: [0, 3], 2: [1, 2]}