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Questions tagged [matrix-indexing]

Indexing into a matrix is a means of selecting a subset of elements from the matrix/array

Matrix indexing is a way to reference a particular element in a matrix, by specifying its row and column number.

The notion extends to multi-dimensional arrays as well.

This is a fundamental concept in many languages such as MATLAB, Octave, Python+NumPy, R, Julia, etc...

There are several matrix-indexing methods:

  • row/column indexing: M(m,n) accesses the element on the m-th row on the n-th column.
  • linear indexing: this methods treats the matrix as a 1D list of elements and access the n-element. Note that there may be several ways of "flattening" the matrix row-first or column-first.
  • logical indexing: by specifying a mask

The use of indexing can often improve performance in the context of code , as it can replace for-loops, if conditions, etc.

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Using numpy, what is a good way to index into a matrix using another matrix whose entry values are column numbers?

Suppose I want to indepedently re-order each row of a matrix. Here is an example of that using np.argsort(): >>> A array([[88, 44, 77, 33, 77], [33, 55, 66, 88, 0], [88, 0, 0, 55, 88], [ 0, 22, 44, 88, 33], [33, 33,…
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Python implementation of Francis double step QR iteration algorithm does not converge

I am implementing the Francis double step QR Iteration algorithm using the notes and psuedocode from lecture https://people.inf.ethz.ch/arbenz/ewp/Lnotes/chapter4.pdf - Algorithm 4.5 The psuedocode is provided in Matlab I believe. Below is the…
Shourya Verma
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Fastest way to write in an numpy array at specific indexes?

I would like to get the fastest solution to write data in a 2D numpy array using an array of indexes. I have a large 2D boolean numpy array buffer import numpy as np n_rows = 100000 n_cols = 250 shape_buf = (n_rows, n_cols) row_indexes =…
Wang Lee
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How to access multiple elements in c++ Eigen array?

I want to retrieve certain elements in an Eigen array and return them as a vector. I use the following code: Eigen::ArrayXXi test; test.resize(5,5); test.setRandom(); Eigen::Matrix inds; inds<<0, 2, 3, 2, 3, 1; auto res =…
raincold23
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Getting a subset of numpy array indicies an easy way

Can this for loop be written in a simpler way? import itertools import numpy as np def f(a, b, c): # placeholder for a complex function print(a+b+c) a = np.arange(12).reshape(3, 4) for y, x in itertools.product(range(a.shape[0]-1),…
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Indexing a 4D NumPy Array with two 2D arrays

I have a 4D target NumPy array which I want to fill with values from a 2D source array, using two additional 2D arrays which specify the position in the second and third axis of the target array where the value from the source array should be…
jmsrtch
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Equivalent of NumPy index arrays with standard Python lists or arrays

I can use an array or a list to index into numpy.array, e.g.: a = np.array([1, 2, 3, 4]) print(a[[1, 3]]) will produce [2 4] Is there an equivalent construct to index into a standard Python list or array? Just to be more specific: indexing has to…
Paul Jurczak
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Accessing an individual element in a Numpy array of arbitrary dimensions

So. I am trying to access and modify the penultimate individual element of a series of Numpy arrays. array_1D = np.array([10,11,12,13, 14]) array_2D = np.array([[20,30,40,50,60], [43,54,65,76,87], [11,22,33,44,55]]) array_3D =…
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Python Matrix indexes

I have a matrix with 3 dimension: import numpy as np a = np.array([[['Mon',18,20,22,17],['Tue',11,18,21,18]], [['Wed',15,21,20,19],['Thu',11,20,22,21]], [['Fri',18,17,23,22],['Sat',12,22,20,18]]])** and if I do print(a[0,1,3]) and…
user17184578
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How to Convert Matrix Indexing Code from MATLAB to R

I have a one-dimensional matrix called h1 and a 3-dimensional matrix called nv in my MATLAB code. I then run the code h1(nv) to create a new 3 column matrix hc. I need to convert this code into R. I am not sure how to do this sort of matrix indexing…
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numpy - why Z[(0,2)] can be view for some cases and be copy in the others?

Continuation to the question numpy - why Z[(0,2)] is view but Z[(0, 2), (0)] is copy?. I got the answer and understood that comma triggering advanced index makes a totally different indexing. The answer also provided a way using __array_interface__…
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numpy - is Z[[1, 2],[1]] completely same with Z[(1,2),(1)]?

As per the answer of numpy - why Z[(0,2)] is view but Z[(0, 2), (0)] is copy?, now understood comma can trigger advanced indexing which is also warned in the document. Warning The definition of advanced indexing means that x[(1,2,3),]…
mon
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Multidimensional Indexing of an multidemensional matrix in Python

I want to use a multidimensional index matrix to access another multidimensional matrix. My problem is, that methods such as np.newaxis are not working because of broadcasting issues (mismatch of shapes). My data matrix has a shape of (5001, 3, 240,…
Bobbyyyyy
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NumPy - convert indices to bools in one line

I have NumPy N-dimensional bool array b. It was converted to indices of True values by i = np.nonzero(b). What is the shortest one-liner to convert i back to b. Of cause it can be done multi-line as: b = np.zeros(b_shape, dtype = np.bool_) b[i] =…
Arty
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Transfer python/numpy indexing to Tensorflow and improve performance

In an earlier question here, I asked for advice on faster item assignment to an array. Since then, I have done some progress, eg I expanded the recommended version to take care of 3-D arrays, which is intended to resemble the batch size of the later…
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