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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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Subvectors from vector using indexes, matlab

I have a vector, F0, with values I need to study. The vector f0 can be separated into sections A&B, A_starts has the indexes where sections of A start and A_ends has the indexes where A ends, the resulting elements of F0 are B. How can I extract the…
Josu Salinas Colina
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find value in array according to the row and column number

I have two matrix A and B,the first row of matrix A(1,:)=[1 2] refer to the number of row and column matrix B(1,2)=21,now I want to do this work for another rows of matrix A without loops? A=[1 2;2 3;1 3;3 3]; B=[1 21 34;45 65 87;4 55 66]; for…
abbas
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How to use a cell array directly returned from a function?

I have a path (without a file name at the end) as a string in Matlab and i want to receive the first parent directory (the directory after the last file seperator character) in it. At the moment i'm doing it like this: >>filePath =…
Semih SÜZEN
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Assigning values to a matrix through vector addition from an adjacency matrix

Very new to Matlab, I usually use STATA. I want to use the nchoosek fuction to get the sum of vectors in one matrix. I have a 21x21 adjacency matrix, with either 0 or 1 as the inputs. I want to create a new matrix, that will give me a sum of…
user6366660
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2D array indexing in python

tau = 0.5 dt = 0.01 Nt = int(tau/dt) grid_size = 100 dx = 1/grid_size x = np.linspace(0, grid_size, grid_size) t = np.linspace(dt, tau, Nt+1) Sw_g = [[0 for x1 in range(grid_size)] for y in range(Nt)] Sw_g[:Nt][grid_size-1] = 0.2 #…
Nitya Mohan
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Tensorflow Error when indexing a 4D array: ValueError: Shapes must be equal rank, but are 1 and 0

I'm modifying a simple CNN in Tensorflow and when I'm indexing a 4d array I get this error. My reproducable example is: from __future__ import print_function import pdb import numpy as np import tensorflow as tf from…
Ash
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How to find the index of the max/min element on a list?

I have a multidimentional list called dist. I want to get the position of the max element on this list/ matrix without using NumPy. How can I do it with built-in functions? I am using Python 3.6 dist = [[ 0.0, 1804.04, …
feys
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I have a A = 3×3 matrix in which position of elements are to be shuffled based on polynomial function result

A = [1 2 3; 4 5 6; 7 8 9]; Now reshaping the matrix A to form a row vector gives B. B = [1 4 7 2 5 8 3 6 9]; Evaluating polynomial function f(x) = (7x+6x^2+3x^3)mod 9 by putting values for 'x' ranging from (1,...,9) since there are 9…
mayfly
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Remove rows in matrix where column equals any value in vector

I have a matrix A that is size 967874 x 3 and a vector v that is length 7211. I want to create a new matrix B that equals matrix A minus all the rows in A where the 2nd column value equals any of the elements in v. What's a fast way to accomplish…
Austin
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accessing indices in two dimensions

I have a vector a and a second vector b to access a range of indices in a, all working as I would expect: a=1:5400; % a==[1 2 3 4 ...] value: (1x5400 double) b=[1 2 3]; % b value: (1x3 double) a(b); % returns [1 2 3] however if a b is a 2D matrix…
user2305193
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selecting rows from 2 matrices using an indicator vector in numpy

Suppose I have a vector: f = np.array([1,1,0,0]) #(4,) and 2 matrices: m1 = np.array([[1,2],[3,4],[5,6],[7,8]]) #(4,2) m2 = np.array([[10,20],[30,40],[50,60],[70,80]]) #(4,2) How can I create a new matrix m3 that selects rows from m1 where f == 1…
A.D
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Get bin edges from a histogram by index of the bin

I have a matrix m and plot a histogram of the third column. I search for the peak in the first 100 bins and get the frequency as a and the index of the bin as b. Now I need the edges of the bin with index b. How can I get them? nbins = 1000; histo…
Chaostante
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How to select a submatrix in Matlab?

Hello and happy new year! I have a problem, and I can't find a solution. I need to create these 2 matrices, C from B with submatrix. B = [ 1 2 3 4 5 6 7 9 7 5 3 1 -1 -3 4 8 16 32 64 128 256]; And I…
BillΤzik
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Values from elements that creates concentric square on distance "k" in NumPy ndarray

What is the fastest way to get values from elements that creates concentric square on some distance K from the center. I wrote code where I access every element value with numpy.ndarray.item, but it has shown poor results. To help you visualize…
felentovic
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solving of zero indexing issue in matlab

suppose that we have some vector y in matlab and let us suppose that there is such linear relationship between values of y of course first vector can be easily implemented by y(L:N-1), but related to matrix, we dont have index y[0] in matlab,…
user466534
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