How to reduce on a list of tuples in python

Question

I have an array and I want to count the occurrence of each item in the array.

I have managed to use a map function to produce a list of tuples.

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Answer 1

Instead of using any external module you can use some logic and do it without any module:

track={}
if intr not in track:
    track[intr]=1
else:
    track[intr]+=1

Example code :

For these types of list problems there is a pattern :

So suppose you have a list :

a=[(2006,1),(2007,4),(2008,9),(2006,5)]

And you want to convert this to a dict as the first element of the tuple as key and second element of the tuple. something like :

{2008: [9], 2006: [5], 2007: [4]}

But there is a catch you also want that those keys which have different values but keys are same like (2006,1) and (2006,5) keys are same but values are different. you want that those values append with only one key so expected output :

{2008: [9], 2006: [1, 5], 2007: [4]}

for this type of problem we do something like this:

first create a new dict then we follow this pattern:

if item[0] not in new_dict:
    new_dict[item[0]]=[item[1]]
else:
    new_dict[item[0]].append(item[1])

So we first check if key is in new dict and if it already then add the value of duplicate key to its value:

full code:

a=[(2006,1),(2007,4),(2008,9),(2006,5)]

new_dict={}

for item in a:
    if item[0] not in new_dict:
        new_dict[item[0]]=[item[1]]
    else:
        new_dict[item[0]].append(item[1])

print(new_dict)

output:

{2008: [9], 2006: [1, 5], 2007: [4]}