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

Fair enough. Python has been called the #1 language for data science. If I'm slicing a[2:5] out of range, why not throw an error. This is disappointing! I mean, why would you design a language to slice outside of range? Also, no other language I know have this strange behavior. On Fri, Feb 15, 2019 at 12:51 PM Paul Hobson wrote:

On Fri, Feb 15, 2019 at 5:12 AM Mike C wrote:

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The original data was in CSV format. I read it in using pd.read_csv(). It does have column names, but no row names. I don’t think numpy reads csv files.

If you read a file into a pandas structure, it will have row labels. The default labels are integers that correspond to the ordinal positions of the values.

Numpy reads files.

https://docs.scipy.org/doc/numpy-1.15.0/reference/generated/numpy.loadtxt.ht...

https://docs.scipy.org/doc/numpy-1.15.0/reference/generated/numpy.genfromtxt...

I prefer file IO in pandas, so I don't know which function will better suite your needs.

And also, when I do a[2:5]-b[:3], it does not throw any “index out of

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range” error. I was able to catch that, but in both Matlab and R. You get an error. This is frustrating!!

That's a feature of python in general, not numpy in particular. Every language has its own quirks. The more you immerse yourself in them, the quick you'll learn to adapt to them. -paul

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------------------------------ *From:* NumPy-Discussion on behalf of Juan Nunez-Iglesias < jni.soma@gmail.com> *Sent:* Friday, February 15, 2019 4:15 AM *To:* Discussion of Numerical Python *Subject:* Re: [Numpy-discussion] [SciPy-User] Why slicing Pandas column and then subtract gives NaN?

I don’t have index when I read in the data. I just want to slice two series to the same length, and subtract. That’s it!

I also don’t what numpy methods wrapped within methods. They work, but hard do understand.

How would you do it? In Matlab or R, it’s very simple, one line.

Why are you using pandas at all? If you want the Matlab equivalent, use NumPy from the beginning (or as soon as possible). I personally agree with you that pandas is too verbose, which is why I mostly use NumPy for this kind of arithmetic, and reserve pandas for advanced data table type functionality (like groupbys and joining on indices).

As you saw yourself, a.values[1:4] - b.values[0:3] works great. If you read in your data into NumPy from the beginning, it’ll be a[1:4] - b[0:3] just like in Matlab. (Or even better: a[1:] - b[:-1]). _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@python.org https://mail.python.org/mailman/listinfo/numpy-discussion

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