# [Numpy-discussion] [SciPy-User] Why slicing Pandas column and then subtract gives NaN?

C W tmrsg11 at gmail.com
Thu Feb 14 15:20:20 EST 2019

```Hi Paul,

Thanks for your response! I did not find a Pandas list for users, only for
developers. I'd love to be on there.

result = a.subtract(b.shift()).dropna()

This seems verbose, several layers of parenthesis follow by a dot method.
I'm new to Python, I thought Python code would be pity and short. Is this
what everyone will write?

Thank you!

On Wed, Feb 13, 2019 at 6:50 PM Paul Hobson <pmhobson at gmail.com> wrote:

> This is more a question for the pandas list, but since i'm here i'll take
> a crack.
>
>
>    - numpy aligns arrays by position.
>    - pandas aligns by label.
>
> So what you did in pandas is roughly equivalent to the following:
>
> a = pandas.Series([85, 86, 87, 86], name='a').iloc[1:4].to_frame()
> b = pandas.Series([15, 72, 2, 3], name='b').iloc[0:3].to_frame()
> result = a.join(b,how='outer').assign(diff=lambda df: df['a'] - df['b'])
> print(result)
>
>       a     b  diff
> 0   NaN  15.0   NaN
> 1  86.0  72.0  14.0
> 2  87.0   2.0  85.0
> 3  86.0   NaN   NaN
>
> So what I think you want would be the following:
>
> a = pandas.Series([85, 86, 87, 86], name='a')
> b = pandas.Series([15, 72, 2, 3], name='b')
> result = a.subtract(b.shift()).dropna()
> print(result)
> 1    71.0
> 2    15.0
> 3    84.0
> dtype: float64
>
>
>
> On Wed, Feb 13, 2019 at 2:51 PM C W <tmrsg11 at gmail.com> wrote:
>
>> Dear list,
>>
>> I have the following to Pandas Series: a, b. I want to slice and then
>> subtract. Like this: a[1:4] - b[0:3]. Why does it give me NaN? But it works
>> in Numpy.
>>
>> Example 1: did not work
>> >>>a = pd.Series([85, 86, 87, 86])
>> >>>b = pd.Series([15, 72, 2, 3])
>> >>> a[1:4]-b[0:3] 0   NaN 1   14.0 2   85.0 3   NaN
>> >>> type(a[1:4])
>> <class 'pandas.core.series.Series'>
>>
>> Example 2: worked
>> If I use values() method, it's converted to a Numpy object. And it works!
>> >>> a.values[1:4]-b.values[0:3]
>> array([71, 15, 84])
>> >>> type(a.values[1:4])
>> <class 'numpy.ndarray'>
>>
>> What's the reason that Pandas in example 1 did not work? Isn't Numpy
>> built on top of Pandas? So, why is everything ok in Numpy, but not in
>> Pandas?
>>
>> Thanks in advance!
>> _______________________________________________
>> NumPy-Discussion mailing list
>> NumPy-Discussion at python.org
>> https://mail.python.org/mailman/listinfo/numpy-discussion
>>
> _______________________________________________
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>
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