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pythonarraysnumpypandasscipy
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Reputation: 4206

Binning a numpy array

I have a numpy array which contains time series data. I want to bin that array into equal partitions of a given length (it is fine to drop the last partition if it is not the same size) and then calculate the mean of each of those bins.

I suspect there is numpy, scipy, or pandas functionality to do this.

example:

data = [4,2,5,6,7,5,4,3,5,7]

for a bin size of 2:

bin_data = [(4,2),(5,6),(7,5),(4,3),(5,7)]
bin_data_mean = [3,5.5,6,3.5,6]

for a bin size of 3:

bin_data = [(4,2,5),(6,7,5),(4,3,5)]
bin_data_mean = [7.67,6,4]

Upvotes: 15

Views: 23922

Answers (4)

Alexandre Kempf
Alexandre Kempf

Reputation: 989

I just wrote a function to apply it to all array size or dimension you want.

  • data is your array
  • axis is the axis you want to been
  • binstep is the number of points between each bin (allow overlapping bins)
  • binsize is the size of each bin

  • func is the function you want to apply to the bin (np.max for maxpooling, np.mean for an average ...)

    def binArray(data, axis, binstep, binsize, func=np.nanmean):
    data = np.array(data)
    dims = np.array(data.shape)
    argdims = np.arange(data.ndim)
    argdims[0], argdims[axis]= argdims[axis], argdims[0]
    data = data.transpose(argdims)
    data = [func(np.take(data,np.arange(int(i*binstep),int(i*binstep+binsize)),0),0) for i in np.arange(dims[axis]//binstep)]
    data = np.array(data).transpose(argdims)
    return data

In you case it will be :

data = [4,2,5,6,7,5,4,3,5,7]
bin_data_mean = binArray(data, 0, 2, 2, np.mean)

or for the bin size of 3:

bin_data_mean = binArray(data, 0, 3, 3, np.mean)

Upvotes: 5

Óscar López
Óscar López

Reputation: 236004

Try this, using standard Python (NumPy isn't necessary for this). Assuming Python 2.x is in use:

data = [ 4, 2, 5, 6, 7, 5, 4, 3, 5, 7 ]

# example: for n == 2
n=2
partitions = [data[i:i+n] for i in xrange(0, len(data), n)]
partitions = partitions if len(partitions[-1]) == n else partitions[:-1]

# the above produces a list of lists
partitions
=> [[4, 2], [5, 6], [7, 5], [4, 3], [5, 7]]

# now the mean
[sum(x)/float(n) for x in partitions]
=> [3.0, 5.5, 6.0, 3.5, 6.0]

Upvotes: 5

Joe Kington
Joe Kington

Reputation: 284612

Just use reshape and then mean(axis=1).

As the simplest possible example:

import numpy as np

data = np.array([4,2,5,6,7,5,4,3,5,7])

print data.reshape(-1, 2).mean(axis=1)

More generally, we'd need to do something like this to drop the last bin when it's not an even multiple:

import numpy as np

width=3
data = np.array([4,2,5,6,7,5,4,3,5,7])

result = data[:(data.size // width) * width].reshape(-1, width).mean(axis=1)

print result

Upvotes: 25

TomAugspurger
TomAugspurger

Reputation: 28946

Since you already have a numpy array, to avoid for loops, you can use reshape and consider the new dimension to be the bin:

In [33]: data.reshape(2, -1)
Out[33]: 
array([[4, 2, 5, 6, 7],
       [5, 4, 3, 5, 7]])

In [34]: data.reshape(2, -1).mean(0)
Out[34]: array([ 4.5,  3. ,  4. ,  5.5,  7. ])

Actually this will just work if the size of data is divisible by n. I'll edit a fix.

Looks like Joe Kington has an answer that handles that.

Upvotes: 6

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