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John D'Errico


Retired

4.491 total contributions since 2006

Mainly retired from Eastman Kodak. (Of course, Kodak itself is now semi-retired. I don't think I had any influence in that.) I still write MATLAB code as I find something interesting, and I do attempt to write new tools to put on the File Exchange when I think I can make a contribution.

However, I DON'T answer direct e-mail questions, and I do NOT do homework. In general, your homework is YOUR problem, not mine. Please do NOT e-mail me with your homework problems or student projects. If I was willing to answer your questions, I would very rapidly become overwhelmed, because one question is never just one simple question.

When I'm not doing something with MATLAB, you might find me playing bridge, either in the club or online on BBO.

Professional Interests: MATLAB, numerical analysis, mathematical modeling

Avocational interests: Bridge, woodworking, woodturning

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Answered
isprime function seems to have poor performance
First, using isprime to test for primality of a Mersenne number is a bad idea. The Lucas-Lehmer test, is as I recall, much more ...

ongeveer 6 uur ago | 1

Answered
How can I solve a complicated symbolic integral
Your need is not relevant. Mathematics is not something that just works because you want it to. (Unless you have a magic wand, a...

ongeveer 9 uur ago | 1

Answered
MATLAB for invoice comparisons
Of course it can. It would require learning MATLAB. It would require a clear understanding of what you are comparing, and what ...

ongeveer 9 uur ago | 0

Answered
Integers can only be combined with integers of the same class, or scalar doubles.
READ THE ERROR MESSAGE! R = uint8(1:5); >> R./double(R) Error using ./ Integers can only be combined with integers of the ...

ongeveer 13 uur ago | 0

Answered
How to change non zero value into 0?
You want to reset the last 10 non-zeros to zero, in each column. So, let me build an example matrix. A = rand(20,5) > 0.25 A =...

ongeveer 19 uur ago | 3

Answered
How to generate matrices that satisfies constraints on sum of row elements and sum of column elements?
You have 9 unknowns, with 6 linear equality constraints. The unknowns are bounded between 0 and 100. You don't say an upper boun...

5 dagen ago | 1

Answered
Matrix Equality Check Failed Using == and isequal
NEVER test for exact equality of floating point matrices. It won't happen, at least not unless you are very, very lucky. a_inv ...

6 dagen ago | 0

| accepted

Answered
randperm non uniformly distributed
Sigh. This is NOT a question of non-uniformity. Just a question of not understanding how to recognize non-uniformity, and partia...

6 dagen ago | 1

| accepted

Answered
How to plot below multivalued function
But it is not a function, in some standard mathematical sense of the word. https://en.wikipedia.org/wiki/Function_(mathematics)...

6 dagen ago | 0

Answered
How to do discontinuous piecewise linear model fitting?
SLM is not designed to solve the discontinuous linear case, nor would I ever decide to modify the code to allow that. (Sorry, bu...

7 dagen ago | 1

| accepted

Answered
lagrangian polynomial function giving the incorrect answer
A good try at some code, with three errors in it. Well, essentially two errors, but you made one of those errors twice so I'm no...

8 dagen ago | 2

| accepted

Answered
Vector integration; is this the most correct and/or accurate calculation?
This is a slightly complex question. Is it the correct use of trapz? Of course. Is it the most accurate possible way to integr...

8 dagen ago | 0

| accepted

Answered
how to pick up all combination of numbers from multiple vectors
Don't store your vectors separately. Instead, learn to use tools like cell arrays, which make things hugely more efficient. V =...

8 dagen ago | 2

| accepted

Answered
How can I solve the non linear algebraic equation system with a parameter
You CANNOT use fsolve with a symbolic parameter in the problem. CANNOT. Fsolve is a numerical tool. It finds a root (if it can t...

9 dagen ago | 0

Answered
How can I solve a sparse linear system efficiently in MATLAB?
B changes in each iteration. It is different. We are given no clue as to how it changes, or how much. We don't even know anythin...

9 dagen ago | 0

Answered
Repeat numbers of X based on Y to produce Z
help repelem

10 dagen ago | 0

Answered
Finding cube with most Values
Why would you expect there exists built-in code to do something totally unusual like this? Built-in code is written to solve com...

10 dagen ago | 0

Answered
Fitting Gamma to a curve with two peaks
This is a mistake that many people make. They know how to fit a model to a curve. But the model they know how to use does not ...

11 dagen ago | 0

Answered
Determining values in array (of 1s and 0s) that neighbour similar values
As an alternative to a tool like imerode, you can just use conv2. The minor virtue is that conv2 lives in MATLAB proper, with no...

11 dagen ago | 0

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Answered
How to calculate of average numbers every n rows?
Pretty easy. What you need to learn is how MATLAB stores numbers in an array, how to reshape things so that you get what you wan...

11 dagen ago | 1

| accepted

Answered
Deleted surf script on system
Sorry, but we cannot send you a copy of code. However, you can just download a new copy of MATLAB. Go to the support page. htt...

11 dagen ago | 0

Answered
hook for an optimization problem.
That is a bad idea. You want to adjust your objective function in an optimization, based on iteration count, or some other info...

12 dagen ago | 0

| accepted

Answered
Fibonacci sequence up to first 256 elements
While this is surely homework, you got it almost correct, with a few major errors. Preallocate the vector!!!!!!!!!!!!!! Never ...

12 dagen ago | 0

| accepted

Answered
How to work rdivide function
Please don't post answers to questions as you did to another question when you just want to ask a question. Regardless, it look...

14 dagen ago | 2

| accepted

Answered
How to perform piece-wise linear regression to determine break point?
The simple way? Use a tool that can solve the problem. Don't write code to do work when you don't understand the fundamentals be...

14 dagen ago | 0

Answered
Finding standard errors in curve fitting tool
In real research, maybe YOU always show it that way. That is not true for everybody. Standard for you is not necessarily standar...

14 dagen ago | 0

Answered
How to do exponential curve fitting in the form of y=a*exp(-b*x)+ a constant
Sigh. You give us x as: x = 1:119; Then a vector y, of length 132. Since x was so simply created, I'll assume it is just 1:32...

15 dagen ago | 0

Answered
problem with fsolve optimization error message
You seem to misunderstand how things work. You have provided ONE unknown to solve for. However, there are TWO equations that ...

15 dagen ago | 0

Answered
Find combinations of two variables in functions which must equal certain value
Algebra? If we have q = 9 = sqrt(z1*z2) then it is trivial. z1*z2 = 9^2 So the curve is given as z2 = 81/z1 Plott...

16 dagen ago | 0

| accepted

Answered
Solving for a matrix
You have what? 2 unknowns? I've seen various versions of these equations that you have written. But you do not have 16 unknowns....

17 dagen ago | 0

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