Counting qualitative values based on the date range in Pandas

问题

I am learning to use Pandas library and need to perform analysis and plot the crime data set below. Each row represents one occurrence of crime. date_rep column contains daily dates for a year.

Data needs to be grouped by month and instances of specific crime need to be added up per month, like in the table below.

The problem I am running into is that data in crime column is qualitative and I just cant find resources online that can help me solve this!

I have been reading up on groupby and different methods of sorting but what is the most efficient way of accomplishing this? Thank you in advance!

回答1:

To replicate something of your data:

In [29]: df = pd.DataFrame({'date_rep':pd.date_range('2012-01-01', periods=100),
    ...:                    'crm_cd_desc':np.random.choice(['robbery', 'traffic', 'assault'], size=100)})


In [30]: df.head()
Out[30]: 
  crm_cd_desc   date_rep
0     traffic 2012-01-01
1     traffic 2012-01-02
2     assault 2012-01-03
3     robbery 2012-01-04

In essence, what you want to do is a value counts:

In [31]: df['crm_cd_desc'].value_counts()
Out[31]: 
assault    36
traffic    34
robbery    30
dtype: int64

However, you want to do this for each month seperately. To group by month, you can use pd.Grouper inside groupby to specify the month:

In [34]: df.groupby(pd.Grouper(key='date_rep', freq='M'))['crm_cd_desc'].value_counts()
Out[34]: 
date_rep           
2012-01-31  traffic    12
            robbery    10
            assault     9
2012-02-29  assault    13
            traffic    11
            robbery     5
2012-03-31  assault    12
            robbery    10
            traffic     9
2012-04-30  robbery     5
            assault     2
            traffic     2
dtype: int64

And then unstack to get the result:

In [35]: df.groupby(pd.Grouper(key='date_rep', freq='M'))['crm_cd_desc'].value_counts().unstack()
Out[35]: 
            assault  robbery  traffic
date_rep                             
2012-01-31        9       10       12
2012-02-29       13        5       11
2012-03-31       12       10        9
2012-04-30        2        5        2

Instead of using value_counts, you can also group by both the month and the crime type and then calculate the length of each group:

In [46]: df.groupby([pd.Grouper(key='date_rep', freq='M'), 'crm_cd_desc']).size().unstack()
Out[46]: 
crm_cd_desc  assault  robbery  traffic
date_rep                              
2012-01-31        9       10       12
2012-02-29       13        5       11
2012-03-31       12       10        9
2012-04-30        2        5        2

来源：https://stackoverflow.com/questions/25321357/counting-qualitative-values-based-on-the-date-range-in-pandas