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Questions tagged [numpy]

NumPy is an extension of the Python language that adds support to large multidimensional arrays and matrixes, along with a large library of high-level mathematical functions for operations with these arrays.

About NumPy

From the NumPy homepage:

NumPy is the fundamental package for scientific computing with . It contains among other things:

  • a powerful N-dimensional array object
  • built-in universal functions (ufuncs)
  • sophisticated (broadcasting) operation
  • tools for integrating C/C++ and Fortran code
  • useful linear algebra, Fourier transform, and random number capabilities

Besides its obvious scientific uses, NumPy can also be used as an efficient multi-dimensional container of generic data. Arbitrary data-types can be defined. This allows NumPy to seamlessly and speedily integrate with a wide variety of databases.

NumPy provides reliable and efficient methods of data storage, manipulation, and analysis as it also integrates easily with other methods of data manipulation, notably Pandas and scikit-learn.

NumPy is released under the BSD license, enabling reuse with few restrictions.

Resources

Official Resources

Books

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Creating a Pandas DataFrame from a Numpy array: How do I specify the index column and column headers?

I have a Numpy array consisting of a list of lists, representing a two-dimensional array with row labels and column names as shown below: data = array([['','Col1','Col2'],['Row1',1,2],['Row2',3,4]]) I'd like the resulting DataFrame to have Row1 and…
user3132783
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What is the difference between flatten and ravel functions in numpy?

import numpy as np y = np.array(((1,2,3),(4,5,6),(7,8,9))) OUTPUT: print(y.flatten()) [1 2 3 4 5 6 7 8 9] print(y.ravel()) [1 2 3 4 5 6 7 8 9] Both function return the same list. Then what is the need of two…
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NumPy array is not JSON serializable

After creating a NumPy array, and saving it as a Django context variable, I receive the following error when loading the webpage: array([ 0, 239, 479, 717, 952, 1192, 1432, 1667], dtype=int64) is not JSON serializable What does this mean?
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How to convert a PIL Image into a numpy array?

Alright, I'm toying around with converting a PIL image object back and forth to a numpy array so I can do some faster pixel by pixel transformations than PIL's PixelAccess object would allow. I've figured out how to place the pixel information in a…
akdom
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What does axis in pandas mean?

Here is my code to generate a dataframe: import pandas as pd import numpy as np dff = pd.DataFrame(np.random.randn(1,2),columns=list('AB')) then I got the dataframe: +------------+---------+--------+ | | A | B …
jerry_sjtu
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Concatenating two one-dimensional NumPy arrays

I have two simple one-dimensional arrays in NumPy. I should be able to concatenate them using numpy.concatenate. But I get this error for the code below: TypeError: only length-1 arrays can be converted to Python scalars Code import numpy a =…
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Comparing two NumPy arrays for equality, element-wise

What is the simplest way to compare two NumPy arrays for equality (where equality is defined as: A = B iff for all indices i: A[i] == B[i])? Simply using == gives me a boolean array: >>> numpy.array([1,1,1]) == numpy.array([1,1,1]) array([ True, …
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How do I check which version of NumPy I'm using?

How can I check which version of NumPy I'm using? (FYI this question has been edited because both the question and answer are not platform specific.)
larus
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Saving a Numpy array as an image

I have a matrix in the type of a Numpy array. How would I write it to disk it as an image? Any format works (png, jpeg, bmp...). One important constraint is that PIL is not present.
M456
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Most efficient way to reverse a numpy array

Believe it or not, after profiling my current code, the repetitive operation of numpy array reversion ate a giant chunk of the running time. What I have right now is the common view-based method: reversed_arr = arr[::-1] Is there any other way to…
nye17
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What is the difference between ndarray and array in numpy?

What is the difference between ndarray and array in Numpy? And where can I find the implementations in the numpy source code?
flxb
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How to take column-slices of dataframe in pandas

I load some machine learning data from a CSV file. The first 2 columns are observations and the remaining columns are features. Currently, I do the following: data = pandas.read_csv('mydata.csv') which gives something like: data =…
cpa
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Converting numpy dtypes to native python types

If I have a numpy dtype, how do I automatically convert it to its closest python data type? For example, numpy.float32 -> "python float" numpy.float64 -> "python float" numpy.uint32 -> "python int" numpy.int16 -> "python int" I could try to…
conradlee
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How do I convert a numpy array to (and display) an image?

I have created an array thusly: import numpy as np data = np.zeros( (512,512,3), dtype=np.uint8) data[256,256] = [255,0,0] What I want this to do is display a single red dot in the center of a 512x512 image. (At least to begin with... I think I can…
jlswint
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numpy: most efficient frequency counts for unique values in an array

In numpy / scipy, is there an efficient way to get frequency counts for unique values in an array? Something along these lines: x = array( [1,1,1,2,2,2,5,25,1,1] ) y = freq_count( x ) print y >> [[1, 5], [2,3], [5,1], [25,1]] ( For you, R users…
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