Louis - 1 year ago 117

Python Question

I would like to make a nice function to aggregate data among an array (it's a numpy record array, but it does not change anything)

you have an array of data that you want to aggregate among one axis: for example an array of

`dtype=[(name, (np.str_,8), (job, (np.str_,8), (income, np.uint32)]`

I did this function, and in the example it should be called as

`aggregate(data,'job','income',mean)`

`def aggregate(data, key, value, func):`

data_per_key = {}

for k,v in zip(data[key], data[value]):

if k not in data_per_key.keys():

data_per_key[k]=[]

data_per_key[k].append(v)

return [(k,func(data_per_key[k])) for k in data_per_key.keys()]

the problem is that I find it not very nice I would like to have it in one line: do you have any ideas?

Thanks for your answer Louis

PS: I would like to keep the func in the call so that you can also ask for median, minimum...

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Answer Source

Perhaps the function you are seeking is matplotlib.mlab.rec_groupby:

```
import matplotlib.mlab
data=np.array(
[('Aaron','Digger',1),
('Bill','Planter',2),
('Carl','Waterer',3),
('Darlene','Planter',3),
('Earl','Digger',7)],
dtype=[('name', np.str_,8), ('job', np.str_,8), ('income', np.uint32)])
result=matplotlib.mlab.rec_groupby(data, ('job',), (('income',np.mean,'avg_income'),))
```

yields

```
('Digger', 4.0)
('Planter', 2.5)
('Waterer', 3.0)
```

`matplotlib.mlab.rec_groupby`

returns a recarray:

```
print(result.dtype)
# [('job', '|S7'), ('avg_income', '<f8')]
```

You may also be interested in checking out pandas, which has even more versatile facilities for handling group-by operations.

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