How can I count occurrences of values in a list of dicts?

Question

I have this list of dicts:

[{'Description': 'LARCENY'}, {'Description': 'LARCENY'}, {'Description': 'BURGLARY'}, {'Description': 'ROBBERY - STREET'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'AUTO THEFT'}, {'Description': 'AUTO THEFT'}, {'Description': 'ROBBERY - STREET'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'BURGLARY'}, {'Description': 'BURGLARY'}, {'Description': 'LARCENY'}, {'Description': 'ROBBERY - COMMERCIAL'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'COMMON ASSAULT'}]

I need to make a function that iterates over those dictionaries and counts how many times each type of crime has occurred. The output should be a list of dicts like this:

[{'LARCENY' : 3}, {'BURGLARY' : 2}, {'ROBBERY - STREET' : 3}...

For each type of crime, I need to know how many times that crime as occurred. This is what I have so far:

result = {}
for k in data:
   if 'Description' in k:
    result[k['Description']] = result.get(k['Description'], 0) + 1 

But the output given gives me everything in the same dict, but I want them to be in a list of dicts, each crime with each dict.

Feel free to ask any question if you don't understand mine.

Answer 1

Looks like you want to maintain the order of occurrence of the crimes as well.

So, use collections.OrderedDict instead of normal dict.

>>> from collections import OrderedDict
>>> counter = OrderedDict()
>>> for item in data:
...     if 'Description' in item:
...         counter[item['Description']] = counter.get(item['Description'], 0) + 1

Now, just construct a new dictionary with each and every key, value pair out of the counter , like this

>>> from pprint import pprint
>>> pprint([{k: v} for k, v in counter.items()])
[{'LARCENY': 3},
 {'BURGLARY': 3},
 {'ROBBERY - STREET': 2},
 {'COMMON ASSAULT': 7},
 {'AUTO THEFT': 2},
 {'ROBBERY - COMMERCIAL': 1}]

---

From the comments,

After I get the desired output I will need to separate the keys and the values into diferent lists,

Then, don't create list of dictionaries. Directly create two lists, with keys and values , from the counter itself, like this

>>> list(counter.keys())
['LARCENY', 'BURGLARY', 'ROBBERY - STREET', 'COMMON ASSAULT', 'AUTO THEFT', 'ROBBERY - COMMERCIAL']
>>> list(counter.values())
[3, 3, 2, 7, 2, 1]

Answer 2

Given the result you've got so far...

>>> result = {'LARCENY' : 3, 'BURGLARY' : 2, 'ROBBERY - STREET' : 3}
>>> result = [{k:v} for k,v in result.items()]
>>> result
[{'BURGLARY': 2}, {'LARCENY': 3}, {'ROBBERY - STREET': 3}]

Answer 3

While your data structure doesn't make sense as noted above, you can solve this easily using the Counter class from collections.

from collections import Counter

crimes = [{'Description': 'LARCENY'}, {'Description': 'LARCENY'}, {'Description': 'BURGLARY'}, {'Description': 'ROBBERY - STREET'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'AUTO THEFT'}, {'Description': 'AUTO THEFT'}, {'Description': 'ROBBERY - STREET'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'BURGLARY'}, {'Description': 'BURGLARY'}, {'Description': 'LARCENY'}, {'Description': 'ROBBERY - COMMERCIAL'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'COMMON ASSAULT'}, {'Description': 'COMMON ASSAULT'}]

c = Counter()
for item in crimes:
    c[item["Description"]] += 1

print(c)

This gives the output of:

Counter({'COMMON ASSAULT': 7, 'BURGLARY': 3, 'LARCENY': 3, 'AUTO THEFT': 2, 'ROBBERY - STREET': 2, 'ROBBERY - COMMERCIAL': 1})

I'd recommend looking at the Counter class whenever you want to count things.