Questions tagged [matrix]
In mathematics, a matrix (plural matrices) is a rectangular array of numbers, symbols, or expressions, arranged in rows and columns. The individual items in a matrix are called its elements or entries.
41,189
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Transpose/Unzip Function (inverse of zip)?
I have a list of 2-item tuples and I'd like to convert them to 2 lists where the first contains the first item in each tuple and the second list holds the second item.
For example:
original = [('a', ...
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What are the differences between numpy arrays and matrices? Which one should I use?
What are the advantages and disadvantages of each?
From what I've seen, either one can work as a replacement for the other if need be, so should I bother using both or should I stick to just one of ...
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Difference between numpy.array shape (R, 1) and (R,)
In numpy, some of the operations return in shape (R, 1) but some return (R,). This will make matrix multiplication more tedious since explicit reshape is required. For example, given a matrix M, if we ...
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How can I index a MATLAB array returned by a function without first assigning it to a local variable?
For example, if I want to read the middle value from magic(5), I can do so like this:
M = magic(5);
value = M(3,3);
to get value == 13. I'd like to be able to do something like one of these:
value =...
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votes
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answers
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How do you rotate a two dimensional array?
Inspired by Raymond Chen's post, say you have a 4x4 two dimensional array, write a function that rotates it 90 degrees. Raymond links to a solution in pseudo code, but I'd like to see some real world ...
293
votes
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answers
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What are the most widely used C++ vector/matrix math/linear algebra libraries, and their cost and benefit tradeoffs? [closed]
It seems that many projects slowly come upon a need to do matrix math, and fall into the trap of first building some vector classes and slowly adding in functionality until they get caught building a ...
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numpy matrix vector multiplication [duplicate]
When I multiply two numpy arrays of sizes (n x n)*(n x 1), I get a matrix of size (n x n). Following normal matrix multiplication rules, an (n x 1) vector is expected, but I simply cannot find any ...
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votes
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answers
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Transposing a 2D-array in JavaScript
I've got an array of arrays, something like:
[
[1,2,3],
[1,2,3],
[1,2,3],
]
I would like to transpose it to get the following array:
[
[1,1,1],
[2,2,2],
[3,3,3],
]
It's not ...
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votes
32
answers
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Bomb dropping algorithm
I have an n x m matrix consisting of non-negative integers. For example:
2 3 4 7 1
1 5 2 6 2
4 3 4 2 1
2 1 2 4 1
3 1 3 4 1
2 1 4 3 2
6 9 1 6 4
"Dropping a bomb" decreases by one the number of the ...
207
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12
answers
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views
Why is MATLAB so fast in matrix multiplication?
I am making some benchmarks with CUDA, C++, C#, Java, and using MATLAB for verification and matrix generation. When I perform matrix multiplication with MATLAB, 2048x2048 and even bigger matrices are ...
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votes
10
answers
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Numpy matrix to array
I am using numpy. I have a matrix with 1 column and N rows and I want to get an array from with N elements.
For example, if i have M = matrix([[1], [2], [3], [4]]), I want to get A = array([1,2,3,4])....
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votes
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answers
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R memory management / cannot allocate vector of size n Mb
I am running into issues trying to use large objects in R. For example:
> memory.limit(4000)
> a = matrix(NA, 1500000, 60)
> a = matrix(NA, 2500000, 60)
> a = matrix(NA, 3500000, 60)
...
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votes
10
answers
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Convert a 1D array to a 2D array in numpy
I want to convert a 1-dimensional array into a 2-dimensional array by specifying the number of columns in the 2D array. Something that would work like this:
> import numpy as np
> A = np.array([...
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votes
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answers
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numpy get index where value is true
>>> ex=np.arange(30)
>>> e=np.reshape(ex,[3,10])
>>> e
array([[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19],
[20, 21, 22, ...
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How to get element-wise matrix multiplication (Hadamard product) in numpy?
I have two matrices
a = np.matrix([[1,2], [3,4]])
b = np.matrix([[5,6], [7,8]])
and I want to get the element-wise product, [[1*5,2*6], [3*7,4*8]], which equals
matrix([[5, 12], [21, 32]])
I have ...
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votes
35
answers
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Looping in a spiral
A friend was in need of an algorithm that would let him loop through the elements of an NxM matrix (N and M are odd). I came up with a solution, but I wanted to see if my fellow SO'ers could come up ...
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votes
6
answers
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Select rows of a matrix that meet a condition
In R with a matrix:
one two three four
[1,] 1 6 11 16
[2,] 2 7 12 17
[3,] 3 8 11 18
[4,] 4 9 11 19
[5,] 5 10 15 20
I want to extract the ...
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votes
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answers
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Should I use a data.frame or a matrix?
When should one use a data.frame, and when is it better to use a matrix?
Both keep data in a rectangular format, so sometimes it's unclear.
Are there any general rules of thumb for when to use which ...
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votes
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answers
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How do I find the length (or dimensions, size) of a numpy matrix in python? [duplicate]
For a numpy matrix in python
from numpy import matrix
A = matrix([[1,2],[3,4]])
How can I find the length of a row (or column) of this matrix? Equivalently, how can I know the number of rows or ...
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votes
7
answers
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Apply a function to every row of a matrix or a data frame
Suppose I have a n by 2 matrix and a function that takes a 2-vector as one of its arguments. I would like to apply the function to each row of the matrix and get a n-vector. How to do this in R?
For ...
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votes
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answers
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How can I plot a confusion matrix? [duplicate]
I am using scikit-learn for classification of text documents(22000) to 100 classes. I use scikit-learn's confusion matrix method for computing the confusion matrix.
model1 = LogisticRegression()
...
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votes
19
answers
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Performance of Java matrix math libraries? [closed]
We are computing something whose runtime is bound by matrix operations. (Some details below if interested.) This experience prompted the following question:
Do folk have experience with the ...
150
votes
6
answers
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Reshape three column data frame to matrix ("long" to "wide" format) [duplicate]
I have a data.frame that looks like this.
x a 1
x b 2
x c 3
y a 3
y b 3
y c 2
I want this in matrix form so I can feed it to heatmap to make a plot. The result should look something like:
...
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149
votes
11
answers
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Convert a matrix to a 1 dimensional array
I have a matrix (32X48).
How can I convert the matrix into a single dimensional array?
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votes
8
answers
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how does multiplication differ for NumPy Matrix vs Array classes?
The numpy docs recommend using array instead of matrix for working with matrices. However, unlike octave (which I was using till recently), * doesn't perform matrix multiplication, you need to use the ...
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votes
9
answers
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Why are quaternions used for rotations?
I'm a physicist, and have been learning some programming, and have come across a lot of people using quaternions for rotations instead of writing things in matrix/vector form.
In physics, there are ...
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votes
6
answers
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Counting the number of non-NaN elements in a numpy ndarray in Python
I need to calculate the number of non-NaN elements in a numpy ndarray matrix. How would one efficiently do this in Python? Here is my simple code for achieving this:
import numpy as np
def ...
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votes
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answers
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Finding row index containing maximum value using R
Given the following matrix lets assume I want to find the maximum value in column two:
mat <- matrix(c(1:3,7:9,4:6), byrow = T, nc = 3)
mat
[,1] [,2] [,3]
[1,] 1 2 3
[2,] 7 8 ...
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votes
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answers
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How do I make a matrix from a list of vectors in R?
Goal: from a list of vectors of equal length, create a matrix where each vector becomes a row.
Example:
> a <- list()
> for (i in 1:10) a[[i]] <- c(i,1:5)
> a
[[1]]
[1] 1 1 2 3 4 5
[[...
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votes
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answers
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How can I apply a function to every row/column of a matrix in MATLAB?
You can apply a function to every item in a vector by saying, for example, v + 1, or you can use the function arrayfun. How can I do it for every row/column of a matrix without using a for loop?
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votes
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java Arrays.sort 2d array
I am looking to sort the following array based on the values of [][0]
double[][] myArr = new double[mySize][2];
so for example, myArr contents is:
1 5
13 1.55
12 100.6
12.1 .85
I want ...
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votes
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answers
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Sum rows in data.frame or matrix
I have a very large dataframe with rows as observations and columns as genetic markers. I would like to create a new column that contains the sum of a select number of columns for each observation ...
- 1,071
107
votes
4
answers
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Split a large dataframe into a list of data frames based on common value in column
I have a data frame with 10 columns, collecting actions of "users", where one of the columns contains an ID (not unique, identifying user)(column 10). the length of the data frame is about 750000 rows....
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votes
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answer
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How to subset matrix to one column, maintain matrix data type, maintain row/column names?
When I subset a matrix to a single column, the result is of class numeric, not matrix (i.e. myMatrix[ , 5 ] to subset to the fifth column). Is there a compact way to subset to a single column, ...
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votes
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answers
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undefined reference to `std::ios_base::Init::Init()'
I write this code to read 3 files, TM is the size of square matrix, LER the No. of rows of an array and from last value define a non-square matrix of (ler/2)*2
Then... the code read a file with some ...
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votes
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answer
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data type not understood
I'm trying to use a matrix to compute stuff. The code is this
import numpy as np
# some code
mmatrix = np.zeros(nrows, ncols)
print mmatrix[0, 0]
but I get 'data type not understood', and it works ...
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votes
5
answers
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Scale and mirror SVG object
How do I most easily first scale an object, say 2 * times it's current size and then flip it vertically and horizontally, or both?
As of now, I can either set "scale(2,2)" for it to become 2 times as ...
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votes
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In WebGL what are the differences between an attribute, a uniform, and a varying variable?
Is there an analogy that I can think of when comparing these different types, or how these things work?
Also, what does uniforming a matrix mean?
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votes
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Confusion between C++ and OpenGL matrix order (row-major vs column-major)
I'm getting thoroughly confused over matrix definitions. I have a matrix class, which holds a float[16] which I assumed is row-major, based on the following observations:
float matrixA[16] = { 0, 1, 2,...
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votes
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Pretty print 2D list?
Is there a simple, built-in way to print a 2D Python list as a 2D matrix?
So this:
[["A", "B"], ["C", "D"]]
would become something like
A B
C D
I found ...
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votes
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Very large matrices using Python and NumPy
NumPy is an extremely useful library, and from using it I've found that it's capable of handling matrices which are quite large (10000 x 10000) easily, but begins to struggle with anything much larger ...
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votes
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What is the fastest way to transpose a matrix in C++?
I have a matrix (relatively big) that I need to transpose. For example assume that my matrix is
a b c d e f
g h i j k l
m n o p q r
I want the result be as follows:
a g m
b h n
c I o
d j p
e k q
f ...
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votes
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Convert a matrix to a list of column-vectors
Say you want to convert a matrix to a list, where each element of the list contains one column. list() or as.list() obviously won't work, and until now I use a hack using the behaviour of tapply :
x &...
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Inverting a 4x4 matrix
I am looking for a sample code implementation on how to invert a 4x4 matrix. I know there is Gaussian eleminiation, LU decomposition, etc., but instead of looking at them in detail I am really just ...
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votes
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Find out if a matrix is positive definite with NumPy
How can I find out if a matrix is positive definite?
My matrix is a NumPy matrix. I was expecting to find any related method in the NumPy library, but I didn't have any success.
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votes
4
answers
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Elegant indexing up to end of vector/matrix
Is it possible in R to say - I want all indices from position i to the end of vector/matrix?
Say I want a submatrix from 3rd column onwards. I currently only know this way:
A = matrix(rep(1:8, each = ...
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votes
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How do I multiply matrices in PyTorch?
With numpy, I can do a simple matrix multiplication like this:
a = numpy.ones((3, 2))
b = numpy.ones((2, 1))
result = a.dot(b)
However, this does not work with PyTorch:
a = torch.ones((3, 2))
b = ...
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votes
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How can I create a correlation matrix in R?
I have 92 set of data of same type.
I want to make a correlation matrix for any two combinations possible.
i.e., I want a matrix of 92x92.
such that element (ci,cj) should be correlation between ci ...
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votes
8
answers
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How do I iterate through each element in an n-dimensional matrix in MATLAB?
I have a problem. I need to iterate through every element in an n-dimensional matrix in MATLAB. The problem is, I don't know how to do this for an arbitrary number of dimensions. I know I can say
for ...
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votes
8
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Android: How to rotate a bitmap on a center point
I've been looking for over a day for a solution to this problem but nothing helps, even the answers here. Documentation doesn't explain anything too.
I am simply trying to get a rotation in the ...
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