# 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.

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### Matrix rotation algorithm - follow-up

In a response to this post I wrote a solution in Python. It passed all the test cases given, so I would like help making it more pythonic and reducing the almost repeated lines. Link to the source of ...
1k views

### Spiral Scanning for matrix using recursion

I have written code for scanning an N x M matrix in spiral order using a recursive function. It uses 5 parameters - including 2 extra variables: call to keep ...
1k views

### Determine if an element exists in a sorted NxN matrix

The point of this algorithm is to see if an element exists in a NxN matrix that has its rows and columns sorted. What would you change? What did I do well? Both perspectives help so I am not left ...
142 views

### Matrix class with space-optimized triangular (lower and upper) variants

I've written a class for square numerical matrices (of doubles, in this case), in C++11. Since I intend to use this class later ...
157 views

### Basic Templated Matrix Library

I am a beginner C++ programmer and I have been working on a library to use for working with matrices. The library main features should be Working with matrices (addition, subtraction, arbitrary ...
97 views

### Probabilistic Matrix Inspection - Suggested by a paper, implemented by me

I have read this paper and I actually found it interesting. This is an attempt to implement the proposed algorithm. I would like to know: Is my algorithm correct by what is discribed in the paper? ...
114 views

### Picking seats in a matrix

I'm trying to write a Python class that creates a matrix of zeros, then uses a random number generator to pick 'seats' on the matrix. It changes the zero in that seat to a one, until the matrix is all ...
545 views

### Matrix Diagonal Difference

Problem Given a square matrix of size N×N, calculate the absolute difference between the sums of its diagonals. Input Format The first line contains a single ...
6k views

### Divide all elements in the column of a sparse matrix

Using Python Scipy, I am trying to divide all numbers in all columns of a sparse matrix (400K × 500K, density 0.0005), by the sum of the squares of all numbers in a column. If a column is ...
2k views

### Normalize Sparse Matrix along rows in C++ Eigen

I wrote a function to normalize sparse matrix along rows using C++ Eigen; however, I feel that it can be improved, and that I am not using some built in functions of Eigen. ...
536 views

### Linear algebra, reduced row echelon form - function 1

This is a derivative post from here. This is just a general review, so the question is the same: Is there something... That you would consider as a bad practice and why? That is just bad in some ...
64 views

### Fast execution of function which resembles matrix multiplication

I am trying to make the following function as fast as possible on large matrices. The matrices can have dimensions in the range of 10K-100K. The matrix values are always between 0 and 1. The function ...
634 views

### Householder transformation

I'm trying to make this Householder method run as quickly as possible. It runs fine now, but I'm still a bit new to java, is there anything you guys see to make this run faster? ...
280 views

### Quaternion rotations and preparing matrices for a shader

I am implementing an OpenGL ES 2.0 renderer in c. I want to use quaternions for rotations. Please take a look at the way I am implementing the rotation math. Everything looks as expected when the ...
2k views

### Calculating Euclidean norm for each vector in a sparse matrix

Below is a naive algorithm to find nearest neighbours for a point in some n-dimensional space. ...
791 views

### Leetcode 54: Spiral Matrix

Problem statement Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order. For example, Given the following matrix: [   [ 1, 2, 3 ], ...
558 views

### 3D matrix rotation in homogeneous coordinate space

Assuming that the framework is in place to handle the difference between row and column major matrices, I am curious to know if in a header based library such implementation is semantically and ...
2k views

### Implementing the CutHill-McKee Algorithm

I am very much interested in the Reverse Cuthil McKee Algorithm. I have seen Fortran and C or C++ implementations of it, and I decided that it would be a nice exercise to implement it in Python. I ...
187 views

### Multivariate normal density using Woodbury matrix inversion lemma

I'm trying to speed up by multivariate normal density function by taking advantage of the fact that the covariance matrix is of the form A + U C U'. The inverse of such a matrix can be calculated ...
350 views

### Encryption/decryption by matrix multiplication in C

I need to encrypt and decrypt text, image and video with a program in C with a matrix. My program is working but it's really slow, how can I improve its speed ? specially for the decrypt part. Each ...
1k views

### Sparse matrix compressed sparse row (CSR) in Python 2.7

Brief introduction for CSR: The compressed sparse row (CSR) or compressed row storage (CRS) format represents a matrix M by three (one-dimensional) arrays, that respectively contain nonzero ...
108 views

### Reading sparse matrix from binary file

I have binary files containing sparse matrices. Their format is: number of rows int length of a row int column index int value float Reading ...
75 views

I'm working on 2-D top-down graphics engine, and need a way to simulate clouds. My idea was that I could create a layer comprised of a matrix of rectangles whose alpha value would be picked from a ...
460 views

### Template Matrix Class: implemented some basic functionalities

Basically I have written a matrix class for addition, multiplication and scalar multiplication. I need your review of the class implementation below in terms of efficiency, memory consumption and new ...
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### Transposing a 2D array

Would you have any suggestions for improvements for the code below? ...
134 views

### Image-grouping using bit matrices

It might be easier to read the explanation of what I was trying to do, and implement it a completely different way, rather than go through the code. But the code does what I want it to do, just ...
329 views

### Create matrices of a certain format

I need to create matrices that follow a specific format: the matrix is a square NxN matrix and the off-diagonals all have the same value (param) and the diagonals ...
1k views

### Are there any memory issues with this Eigen QR wrapper?

I am writing a wrapper to Eigen QR for my personal use and I am wondering if there are any memory leaks or undocumented behavior in my implementation, especially in the function ...
12k views

### Rotate an image by 90 degrees

Looking for review, good practices, optimizations, clean code tips etc. ...
228 views

### A Matrix Library in C++;

I have written a Matrix library that contains all the main properties of matrices. It's a relatively long project, I am hoping it is ok to post here since I really want to have it reviewed. The ...
2k views

### Fixed size matrix implementation

I implemented a fixed-size matrix class which supports basic matrix operations (cannot use C++11). What do you think, and how could it be improved? ...
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### Finding the maximum row of a 4x4 matrix

I have written a recursive function that I believe will return the 'maximum' row of the 4x4 matrix it is fed. By 'maximum' I mean that the maximum row of the matrix. ...
148 views

### Simple matrix library in C++

Today I started learning C++ and at the end of the day have made a simple generic matrix class. I'm looking for feedback on techniques and features. It's still not complete, though. But everything ...
358 views

### Printing an N*N array in the rectangular/circular/spiral fashion

The problem is to print an N*N array and fill it with numbers(starting from 1 at (0,0)) in the rectangular/circular/spiral fashion, going inwards, ending at the N*N value. Also find and count the ...
5k views

### 4x4 matrix implementation in C++

I've been doing some 3D graphics in OpenGL lately and I needed a way to work with 4x4 matrices. My implementation supports the following operations: Matrix-matrix addition. Matrix-matrix subtraction. ...
503 views

### Dynamic matrix in web browser

Today, my project was to make a matrix in my web browser. It is really slow, so help with optimizing it would be nice. It runs slow on my PC, yet alone my iPod 4 which this is going to be for. ...
1k views

### Path sum - Dijkstra's algorithm in F#

I'm learning F# and I've decided to solve Project Euler's Problem #81 with Dijkstra's algorithm. In the 5 by 5 matrix below, the minimal path sum from the top left to the bottom right, by only ...
134 views

### Getting a transpose of a matrix of size of n*n where n=2^m using multithreading

The following is a c++ source code to get a matrix (std::vector<std::vector<T>>) transpose in parallel. The span is $\Theta(\lg^2(n))$ while the work ...
133 views

### General matrix class

I have written a matrix class using python, allowing me to perform a variety of operations on matrices. I'm pretty happy with how it performs, and everything works fine. However, I feel like the way I'...
3k views

### Calculate co-occurrences of some words in documents dataset

It's a function to calculate the co-occurrences of some words in a news dataset, such as Techcrunch, Wired. The words list could be locations, products or people names. Words list example: ["New ...
92 views

### Extract the largest value for each day from a matrix

I have a matrix in which the right-most elements are repeated YYYYMMDD dates in descending order, for example: ...
134 views

### Calculating diagonal difference

I was trying to solve a problem concerning the difference of the diagonal sums of a matrix (using C99): ...
4k views

### Optimize vector rotation

I have a trivial function that rotates 2d vectors, and a method in a class representing a polygon that rotates every point in the polygon around an origin. The code is fairly optimized as it is, but I ...
117 views

### Rotate a matrix, using transposition and reflection

I want to compare two solutions to the following question from Cracking The Coding Interview by Gayle Laakman McDowell : 1.7 Rotate Matrix - Given an image represented by an NxN matrix, where each ...
189 views

### Multiplying a Matrix with its Transpose

Currently, I am in the process of optimizing a MIPS assembly program that takes a n x n matrix and multiplies it with its transpose. I am trying to optimize my matrix calculation algorithm so that it ...
81 views

### Manually writing a function matrixpower() in R

Problem 1 - Matrix powers in R R does not have a built-in command for taking matrix powers. Write a function matrixpower() with two arguments mat and k ...
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### Determinant using Gauss Elimination

We had a question in examination to compute determinant of general square matrix. Now I had made the normal program using Laplace Formula of determinant, ie, recursive solution. Now I was thinking of ...
419 views

### Determinant of a matrix of doubles

I have been working with some matrix functions and need the determinant. I have looked at several examples, but they all seem to use a hard coded size for the matrix, or calculate for a matrix of int, ...
I have tried to obtain speed gains by transferring Matlab calculations to C and call it using .mex. My goal is to operate on a matrix $B$ which has roughly the dimension 10000x1000  \frac{1}{\...