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Derek O'Connor


University College Dublin, Retired

Con actividad desde 2011

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Professional Interests: Algorithms and Data Structures, Numerical Analysis

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Respondida
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...

casi 12 años hace | 0

Pregunta


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...

alrededor de 12 años hace | 2 respuestas | 0

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Respondida
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...

alrededor de 12 años hace | 0

Respondida
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>

alrededor de 12 años hace | 0

Enviada


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

alrededor de 12 años hace | 3 descargas |

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

más de 12 años hace | 1

Respondida
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...

más de 12 años hace | 1

Respondida
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...

más de 12 años hace | 1

Respondida
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: ...

más de 12 años hace | 0

Respondida
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 = ...

más de 12 años hace | 0

Respondida
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. ...

más de 12 años hace | 0

Respondida
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...

más de 12 años hace | 1

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Respondida
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. %...

casi 13 años hace | 0

Pregunta


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...

casi 13 años hace | 1 respuesta | 0

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Respondida
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...

casi 13 años hace | 4

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

casi 13 años hace | 1

Respondida
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...

casi 13 años hace | 0

Respondida
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...

casi 13 años hace | 1

Respondida
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...

casi 13 años hace | 1

Respondida
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 ...

casi 13 años hace | 1

Respondida
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, ...

casi 13 años hace | 1

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

casi 13 años hace | 0

Respondida
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...

casi 13 años hace | 1

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

alrededor de 13 años hace | 0

Respondida
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...

alrededor de 13 años hace | 0

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

alrededor de 13 años hace | 0

Respondida
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...

alrededor de 13 años hace | 1

Respondida
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...

alrededor de 13 años hace | 4

Respondida
[DEPRECATED] What frustrates you about MATLAB?
At Walter Roberson's request I'm reposting this answer here. *Boredom Forecast*: High(78/100). _Discussion of Syntax_. First,...

alrededor de 13 años hace | 1

Respondida
Problem with execution of "nested if" commands.
*Boredom Forecast*: _Discussion of Syntax_. High: (78/100) First, get the preferred indentation by using "smart indent" in the...

alrededor de 13 años hace | 0

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