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.

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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, ...
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700votes
23answers
1.2mviews

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 ...
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696votes
11answers
1.0mviews

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.
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689votes
20answers
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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 ...
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630votes
15answers
1.3mviews

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....
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588votes
8answers
1.0mviews

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 ...
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588votes
11answers
494kviews

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....
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575votes
17answers
866kviews

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?
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546votes
31answers
854kviews

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 ...
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545votes
11answers
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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 ...
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529votes
7answers
217kviews

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 ...
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525votes
17answers
639kviews

How to find all occurrences of an element in a list

index() will give the first occurrence of an item in a list. Is there a neat trick which returns all indices in a list for an element?
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518votes
7answers
947kviews

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 ...
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508votes
13answers
1.0mviews

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? ...
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461votes
11answers
501kviews

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. ...
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