Calculate numpy.std of each pandas.DataFrame's column?
numpy, pandas, python
Solution
They're both right: they just differ on what the default delta degrees of freedom is. `np.std` uses 0, and `DataFrame.std` uses 1:
>>> prices.std(axis=0, ddof=0)
0 0.323259
1 0.173375
2 0.147740
dtype: float64
>>> prices.std(axis=0, ddof=1)
0 0.395909
1 0.212340
2 0.180943
dtype: float64
>>> np.std(prices.values, axis=0, ddof=0)
array([ 0.32325862, 0.17337503, 0.1477395 ])
>>> np.std(prices.values, axis=0, ddof=1)
array([ 0.39590933, 0.21234018, 0.1809432 ])
Problem
I want to get the `numpy.std` of each column of my `pandas.DataFrame`. Here is my code: ``` import pandas as pd import numpy as np prices = pd.DataFrame([[-0.33333333, -0.25343423, -0.1666666667], [+0.23432323, +0.14285714, -0.0769230769], [+0.42857143, +0.07692308, +0.1818181818]]) print(pd.DataFrame(prices.std(axis=0))) ``` Here is my code's output: ``` pd.DataFrame([[ 0.39590933], [ 0.21234018], [ 0.1809432 ]]) ``` And here is the right output (if calculate with `np.std`) ``` pd.DataFrame([[ 0.32325862], [ 0.17337503], [ 0.1477395 ]]) ``` Why am I having such difference? How can I fix that? NOTE: I have tried to do this way: ``` print(np.std(prices, axis=0)) ``` But I had the following error: ``` Traceback (most recent call last): File "C:\Users\*****\Documents\******\******\****.py", line 10, in <module> print(np.std(prices, axis=0)) File "C:\Python33\lib\site-packages\numpy\core\fromnumeric.py", line 2812, in std return std(axis=axis, dtype=dtype, out=out, ddof=ddof) TypeError: std() got an unexpected keyword argument 'dtype' ``` Thank you!