John D'Errico

Retired

Last seen: Today | Active since 2006

Followers: 29 Following: 0

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A retired applied mathematician, consultant, sometime mechanical engineer, numerical analyst, woodworker, bridge player. Please DON'T contact me, as too many students then decide I am their personal consultant if I allow it. I won't do consulting by mail.

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Answered
Logic Code required to be run in finite time
I think you misunderstand an important idea of computing. You NEVER want to use brute force on big problems. Those problems alwa...

2 days ago | 1

Answered
Find blocks iof non-zero values in Matrix
Sure. It should even be pretty easy. A = randi(200,[120,240]); % make up a matrix First, I'll convert it to binary, where a 1 ...

6 days ago | 0

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assigning values with []
@feynman feynman The funny thing is, I have at times wanted a syntax like you wrote to work in MATLAB, but it won't. We could p...

7 days ago | 1

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Answered
How would I use a 2D array based on a topographical map to determine the steepest direction at each point?
You should be talking to your teacher, as these are questions they should be answering. But what does the gradient function do? ...

9 days ago | 1

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Answered
Solving odes with ode45 and then checking fixed points for constant behavior,
How many times will you ask this question? Can you do better than 13-14 digits? No. Not in double precision. And a different O...

10 days ago | 0

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Answered
I have a dataset of x and y coordinates in UTM units, and cumulative erosion or sedimentation in depth (m) units, I need the volume of erosion or accretion in cubic metres
Difficult, since you don't show the data. A picture of numbers is useless. A bigger problem is your data is surely not on some n...

14 days ago | 0

Answered
In a (possibly directed) graph, is there a simple way to find all nodes reachable for a given node?
First, create a graph. M = eye(100) + triu(rand(100) < 0.035); spy(M) You will agree this matrix satisfies the requirements, ...

14 days ago | 0

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Answered
Evaluating stability of fixed points in a system of differential equations,
First, there is nothing wrong with a loop! You know how to evaluate stability for each case. Just write the loop. Could you do s...

17 days ago | 0

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Answered
linprog
Think about it. Do you need that constant at all? (NO.) LINPROG finds a location (in your case, in terms of x and y) that minim...

17 days ago | 0

Answered
how to design digital filter
Are there free toolboxes to do filter design? You might find something on the file exchange, and what you find there comes at ...

23 days ago | 0

Answered
why does this not work?
It failed because datetime did not recognize that string as a list of distinct times. Just because you know what you want code t...

24 days ago | 3

Answered
Solving for a variable in an equation
First, you need to recognize there is NO analytical solution to your problem. So using solve means it will just toss and turn fo...

25 days ago | 0

Answered
Does inverse modelling is possible with ANFIS?
In general, absolutely no. This is impossible. Why do I say that? Consider even an absolutely trivial linear model. I don't need...

28 days ago | 0

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Answered
Why is vpa not converting double accurately?
Quite interesting. When I first saw the title, I assumed it would be an obvious mistake. Then I saw the poster, and knew it woul...

29 days ago | 1

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Answered
finding partial solutions from ill-conditioned equations
The largest subset? It may be impossible to compute ANY subset accurately. You may be able to only compute a linear combination ...

1 month ago | 0

Answered
What is evaluated to standard output is not being assigned to my variable
This is a not uncommon misunderstanding about cell arrays. d = {'.'; '..' '2022' '2024' '2025'} b = d(3:end) % the "righ...

1 month ago | 1

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Answered
Create similar curves through specific data points
The question is becoming more clear now, based on your response to my first answer. I'll post this as a separate answer, since t...

2 months ago | 0

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Answered
Create similar curves through specific data points
You have asked a confusing question, which usually means you are yourself confused, perhaps about what you really want to do. It...

2 months ago | 0

Answered
Solving linear system modulo prime^n when matrix is univertable
You can't use LU anyway, since it is not designed to work on that problem. Nor can you use slash or backslash, which again, do n...

2 months ago | 0

Answered
Differentiating inside a matlab function
Oh, you were so close. Actually, you did not even need to tell diff what variable to differentiate with respect to. syms x(t) ...

2 months ago | 0

Answered
How to generate an prime random number??
A random prime? Impossible to do. That is, something that many seem to misunderstand is you cannot generate a random number on s...

2 months ago | 0

Answered
How to make multiple lines of best fit into one scatter graph
Is it possible to do in MATLAB? Of course it is. It of course depends on the skill of the person writing the code. It depends on...

2 months ago | 0

Answered
Find best fit curve from multiple curves of different size
On the right end, you have an infinite slope. On the left end, not so much of a problem. But it will be better to do a couple of...

2 months ago | 0

Answered
How does interp1 work?
Why not post the actual data you used that caused the problem, instead of only a tiny fragment of it? You certainly CAN post t...

2 months ago | 2

Answered
Matlab Dimpulse function documentaion not found
It seems dimpulse (or at least one version thereof) can be found in the control TB. which dimpulse -all However, given the loc...

2 months ago | 0

Answered
part 3 example was wrong
Answers is not official MATLAB support. So telling us a problem exists does nothing. Contact support directly. https://www.ma...

2 months ago | 1

Answered
I'm trying to find solutions to variables in a system of non linear eqations, that represent a geometrical puzzle.
A problem is, we don't actually know what is the geometric system you are trying to solve, nor have you told us the solution you...

2 months ago | 0

Answered
Integrating a very long expression containing symbolic values
Is there some law of mathematics that everything you can or will write down has a solution? (No.) That int did not find a soluti...

2 months ago | 0

Answered
Can someone help me fix my bisection function?
Good code. Well, at least in terms of the documentation. Excellent in that respect. Readable code is important when you need to ...

2 months ago | 0

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Answered
What is the interpolation algorithm behind MATLAB’s pcolor with FaceColor = 'interp' that creates sharp, defined boundaries？
There are three simple ways to interpolate inside a 2-d rectangular region. But two of them are analogues of each other. First,...

2 months ago | 0