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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

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How to print the full NumPy array, without truncation?

When I print a numpy array, I get a truncated representation, but I want the full array. Is there any way to do this? Examples: >>> numpy.arange(10000) array([ 0, 1, 2, ..., 9997, 9998, 9999]) >>>…
kame
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Dump a NumPy array into a csv file

Is there a way to dump a NumPy array into a CSV file? I have a 2D NumPy array and need to dump it in human-readable format.
Dexter
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How can the Euclidean distance be calculated with NumPy?

I have two points in 3D: (xa, ya, za) (xb, yb, zb) And I want to calculate the distance: dist = sqrt((xa-xb)^2 + (ya-yb)^2 + (za-zb)^2) What's the best way to do this with NumPy, or with Python in general? I have: import numpy a = numpy.array((xa…
Nathan Fellman
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How do I get indices of N maximum values in a NumPy array?

NumPy proposes a way to get the index of the maximum value of an array via np.argmax. I would like a similar thing, but returning the indexes of the N maximum values. For instance, if I have an array, [1, 3, 2, 4, 5], function(array, n=3) would…
Alexis Métaireau
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Convert pandas dataframe to NumPy array

I am interested in knowing how to convert a pandas dataframe into a NumPy array. dataframe: import numpy as np import pandas as pd index = [1, 2, 3, 4, 5, 6, 7] a = [np.nan, np.nan, np.nan, 0.1, 0.1, 0.1, 0.1] b = [0.2, np.nan, 0.2, 0.2, 0.2,…
mister.nobody.nz
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How to access the ith column of a NumPy multidimensional array?

Suppose I have: test = numpy.array([[1, 2], [3, 4], [5, 6]]) test[i] gets me ith line of the array (eg [1, 2]). How can I access the ith column? (eg [1, 3, 5]). Also, would this be an expensive operation?
lpl
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What does -1 mean in numpy reshape?

A numpy matrix can be reshaped into a vector using reshape function with parameter -1. But I don't know what -1 means here. For example: a = numpy.matrix([[1, 2, 3, 4], [5, 6, 7, 8]]) b = numpy.reshape(a, -1) The result of b is: matrix([[1, 2, 3,…
user2262504
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Is there a NumPy function to return the first index of something in an array?

I know there is a method for a Python list to return the first index of something: >>> l = [1, 2, 3] >>> l.index(2) 1 Is there something like that for NumPy arrays?
Nope
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What are the advantages of NumPy over regular Python lists?

What are the advantages of NumPy over regular Python lists? I have approximately 100 financial markets series, and I am going to create a cube array of 100x100x100 = 1 million cells. I will be regressing (3-variable) each x with each y and z, to…
Thomas Browne
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How to count the occurrence of certain item in an ndarray?

In Python, I have an ndarray y that is printed as array([0, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0, 1]) I'm trying to count how many 0s and how many 1s are there in this array. But when I type y.count(0) or y.count(1), it says numpy.ndarray object has no…
mflowww
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How do I read CSV data into a record array in NumPy?

I wonder if there is a direct way to import the contents of a CSV file into a record array, much in the way that R's read.table(), read.delim(), and read.csv() family imports data to R's data frame? Or is the best way to use csv.reader() and then…
hatmatrix
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Most efficient way to map function over numpy array

What is the most efficient way to map a function over a numpy array? The way I've been doing it in my current project is as follows: import numpy as np x = np.array([1, 2, 3, 4, 5]) # Obtain array of square of each element in x squarer = lambda…
Ryan
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pandas create new column based on values from other columns / apply a function of multiple columns, row-wise

I want to apply my custom function (it uses an if-else ladder) to these six columns (ERI_Hispanic, ERI_AmerInd_AKNatv, ERI_Asian, ERI_Black_Afr.Amer, ERI_HI_PacIsl, ERI_White) in each row of my dataframe. I've tried different methods from other…
Dave
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Pandas read_csv low_memory and dtype options

When calling df = pd.read_csv('somefile.csv') I get: /Users/josh/anaconda/envs/py27/lib/python2.7/site-packages/pandas/io/parsers.py:1130: DtypeWarning: Columns (4,5,7,16) have mixed types. Specify dtype option on import or set…
Josh
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Simple Digit Recognition OCR in OpenCV-Python

I am trying to implement a "Digit Recognition OCR" in OpenCV-Python (cv2). It is just for learning purposes. I would like to learn both KNearest and SVM features in OpenCV. I have 100 samples (i.e. images) of each digit. I would like to train with…
Abid Rahman K
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