Python - Turn nested list into a dictionary?

Question

I am using a nested list which contains various types of crimes. To clarify, crimeDescription is formatted as such: [['Murder'], ['Assault'], ... , ['Stalking'], ['Rape']] . I am looking to create a dictionary that will hold every value/crime that is in this list and record its frequency, or how often it appears in the data set. Since the keys inside dictionaries can't be lists, I am attempting to access the string inside of each nested list, by calling its index i[0] in the for loop. When I run the following program:

#TypesOfCrime --> a List containing all different types of crime that's happened in LA

TypesOfCrime = []
CrimeFreq = {}
for i in crimeDescription: 
    if i not in TypesOfCrime:
        i = i[0]
        CrimeFreq[i] = 1
    elif i in TypesOfCrime:
        i = i[0]
        CrimeFreq[i] += 1

I get the following error:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-75-a8698e79e96a> in <module>()
      5     if i not in TypesOfCrime:
      6         TypesOfCrime.append(i)
----> 7         CrimeFreq[i[0]] = 1
      8     elif i in TypesOfCrime:
      9         CrimeFreq[i[0]] += 1

TypeError: 'float' object is not subscriptable

What am I doing wrong? How can I create a dictionary containing each different value that is in my nested list? Any advice will be helpful.

Answer 1

To count something with a dict, check if the key already exists and then increment the value, otherwise add the key with value 1:

crime_freq = {}

for current_crime in crimeDescription:
    crime = current_crime[0]
    if crime in crime_freq:
        crime_freq[crime] += 1
    else:
        crime_freq[crime] = 1

The modern way to solve your problem is to use a collections.Counter and a generator expression:

crime_freq = collections.Counter(crime[0] for crime in crimeDescription)

Answer 2

Do you want to if key not existence, created value OR if key existence, add value.

I recommendation using setdefault() method.

crime_freq = {}

for current_crime in crimeDescription:
    crime = current_crime[0]
    crime_freq.setdefault(crime, 0)
    crime_freq[crime] += 1

If using setdefault() method, code is simple.

I usually using setdefault() method frequently.