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Derek O'Connor


University College Dublin, Retired

138 total contributions since 2006

Professional Interests: Algorithms and Data Structures, Numerical Analysis

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Answered
Is there a better way to randomly generate a Doubly Stochastic Matrix?
@Matt, You are a true blue Matlabber. With just matrix indexing you have written a function which is many times faster than mine...

5 years ago | 0

Question


Is there a better way to randomly generate a Doubly Stochastic Matrix?
Here is a profile of the call |n = 2*10^3; M = DStochMat02(n,ones(n)./n);| More specifically, can the hot-spot, statement 1...

5 years ago | 2 answers | 0

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answers

Answered
Memory Utilization of a program in matlab.
A few years ago I suggested to Mathworks that they include memory usage in their Profiler. They said they were thinking about it...

5 years ago | 0

Answered
How to count consuctive 0's inbetween values of 1
This discussion might be useful: <http://www.mathworks.co.uk/matlabcentral/answers/16356-finding-sequences-of-1-s-values>

5 years ago | 0

Submitted


The Bellman-Ford-Moore Shortest Path Algorithm
A simple, efficient sparse implementation of the original Bellman-Ford-Moore Shortest Path Algorithm

5 years ago | 12 downloads |

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Answered
Poisson random number generator
This is a cleaner fix of |PoissonSamp| % ------------------------------------------------------------- function S = Po...

5 years ago | 0

Answered
Poisson random number generator
I would like to thank Kang Wook Lee of Berkeley for pointing out an error in the code above. The last line should be |S(i) = k-1...

5 years ago | 0

Answered
Matlab gives wrong determinant value of 2x2 Matrix without warning.
@Nitin, The "issue" here is not round-off. It is the ill-condition of your matrix. Floating point arithmetic and its rounding...

5 years ago | 1

Answered
Matlab gives wrong determinant value of 2x2 Matrix without warning.
The products above are too big to fit in 32-bit integers. You can get the correct result by switching to 64-bit integers: ...

5 years ago | 0

Answered
Variation on the travelling salesman problem
@Walter, you're right, I mis-interpreted the problem. So back to the Zero-One LP solution. Given 2 3 1 A = ...

5 years ago | 0

Answered
Variation on the travelling salesman problem
It can be solved as a _Zero-One Linear Program_: Let |xij = 1| if the number |aij| is chosen for row |i|, zero otherwise. ...

5 years ago | 0

Answered
Inversion of a boolean matrix
*WRONG AGAIN* In testing Walter's suggestion about row and column sums I realized that |isInvBool2| is wrong. Try |B = [true...

5 years ago | 1

| accepted

Answered
Inversion of a boolean matrix
I am adding this crude |O(n^2)| invertibility test as a separate answer because my previous answer has become too long. %...

5 years ago | 0

Question


Why are the BLAS functions in R2011b much faster than R2008a?
Here are the results of a simple Matrix Benchmark test with |n = 10^3; A = rand(n,n)| Dell Precision 690, 2x4-Core Xeon 354...

5 years ago | 1 answer | 0

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answer

Answered
Do you think that MATLAB is expensive?
The academic version of Matlab at about €500 is too expensive for me, a retired lecturer. Cleve Moler in his SIAM oral history i...

5 years ago | 3

Answered
Poisson random number generator
I prefer this: % ------------------------------------------------------------- function S = PoissonSamp(lambda,ns); ...

5 years ago | 0

Answered
Poisson random number generator
@Ahmed, you're correct, it is not correct. The function below is a Matlab translation of Kroese's algorithm. It seems to wor...

5 years ago | 0

Answered
Poisson random number generator
Dirk Kroese has excellent notes here: <http://www.maths.uq.edu.au/~kroese/mccourse.pdf>, which are based on his book: D.P. Kr...

5 years ago | 0

Answered
When to use the Secant Method of finding roots?
I would like to qualify what John says: "Methods like the secant method are rarely very good choices anyway." This is arguabl...

5 years ago | 1

Answered
randsample issues and generating random numbers from large populations
Here are two functions for taking a sample |S| of size |Ns| from a large, simply-defined population |P| of size |Np|. Both use ...

5 years ago | 1

Answered
Random sample without replacement
If you don't have access to *|R2011b|* and |randsample|, then the function below is reasonably fast on my |Dell Precision 690, ...

5 years ago | 1

Answered
Generate a Sequence in MATLAB
L1 = L(:)'

5 years ago | 0

Responded


Re: Finding all possible paths using Yen's Algorithm
NetScience <acute4ever@gmail.com> wrote in message <d260a92e-39c1-4553- --- snip --- Would the Floyd-Warshall All Pairs Shorte...

6 years ago | 7548 views

Answered
Solve for x in (A^k)*x=b (sequentially, LU factorization)
Contrary to what Walter says, LU Decomposition is a great help in this problem. See my solution notes to Lab Exercise 6 --- LU...

6 years ago | 0

Answered
speed up finding common values in multiple matrices
Here are some tests on loopy alternatives to vectorized functions. %----------------------------------- function [z,A,...

6 years ago | 0

Answered
Finding Sequences of 1's values
Jan, I'm using the Add-an-Answer window because the comment window is hard to use for all but short, text-only replies. I post...

6 years ago | 0

Answered
Finding Sequences of 1's values
This is a more general and simpler solution than my previous answer. %-------------------------------------------------------...

6 years ago | 0

Answered
Finding Sequences of 1's values
If you don't have any toolboxes then this plain Matlab function may help. It is based loosely on the WordCount procedure in Kern...

6 years ago | 1

Answered
Inversion of a boolean matrix
*THEOREM*. _If a Boolean matrix B possesses a one-sided inverse, that inverse is also a two-sided inverse. Furthermore suc...

6 years ago | 3

Responded


Re: (a+b) -a is not b ??
a = 10^5; b = 10^5 + 10^(-11); c= b-a gives 1.455191522836685e-011 The problem occurs in the expression b = 10^5 + 10^(-11). ...

6 years ago | 2895 views

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