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# John D'Errico

### Retired

4 875 total contributions since 2006

(Please don't contact me. I won't answer questions.) A retired mathematician, numerical animalist, woodworker, bridge player.

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Volume of overlapping ellipsoids
The simple solution is just to fill a 3-d array with voxels that are inside each ellipsoid. So, if a voxel is already filled, th...

ungefär 9 timmar ago | 0

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generate 0 and 1
rand() < 0.6 If you want a random binary sequence of length n, then this is sufficient: rand(1,n) < 0.6

en dag ago | 0

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interrupt a function when it takes too long
This is not what parfor is designed to solve, nor even what you could reasonably solve with parfor. Since you are calling the f...

en dag ago | 0

To create a code that differentiate any function
Well, you write a complete version of a set of symbolic tools, that can parse any function provided, and know the rules for diff...

2 dagar ago | 0

How can i extrapolate data?
Extrapolation is a dangerous thing to do. Best left to someone with sufficient expertise that they know when they are doing some...

2 dagar ago | 1

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How to Get Answers in Non-PolyGamma Form?
(I'm sorry that my answer is coming 9 years too late to be of any significant value to you personally. But someone else might fi...

3 dagar ago | 0

Making contours in polar plot
You could at least spell my name right, since you had it right in front of you. Regardless, it turns out that you changed your ...

4 dagar ago | 0

How to fit custom polynomial equation?
You could download polyfitn from the file exchange. But if you already know how to use the curve fitting toolbox, then just spe...

4 dagar ago | 0

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How to efficiently plot functions?
Wyhy not try it? I don't have t or y, so I'll make them up. t = (1:10)'; y = rand(10,12); plot(t,y) Actually, I don't need...

4 dagar ago | 1

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How can I creat a lower triangular with one vektor avoiding for loops?
Will it always be a matrix of that form? Or will you next ask how to fill the lower triangle with a fully general set of numbers...

4 dagar ago | 1

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How to use for loop and if else condition for multiple conditions
First, you don'r need a for loop that runs over system, sine you re just assigning it one value! That is, this: for system = 1 ...

5 dagar ago | 0

Distance between a point and the intersection of two spheres
This is confusing, possibly because of how you are trying to solve it. I think you are working too hard. The intersection betwe...

5 dagar ago | 0

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How to plot polynomial function correctly?
Hmm. Why does this feel like deja vu? (Answer: I just answered almost the same question for someone else, not 5 minutes ago.) d...

5 dagar ago | 1

Plotting a graph with a confusing equation
How to lie with statistics. ;-) (Yes, that is the name of a little book I read long ago.) My point is, not that you or anyone i...

5 dagar ago | 1

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Matrices with different numbers of equations
You know the size of the matrix. You already know you need to use zeros to initialize it. A = zeros(2*N,2*N); b = zeros(2*N,1)...

5 dagar ago | 1

plotting a natural cubic spline
Are you allowed to use existing code? So if you have the curve fitting toolbox, you can just use csape. x = 0:4; y = [1 3 3 4 ...

5 dagar ago | 1

Find the vector sum combination that gives the minimum vector length
First, NEVER store data in multiple variables like this. Instead, LEARN TO USE ARRAYS. There is no reason why you cannot store a...

6 dagar ago | 2

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Unrecognized function or variable 'carsmall'
This is not a question of you updating your version. I can see what you did, but not what you were hoping to do. Suppose I try t...

6 dagar ago | 1

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Is the Trapz feature always accurate?
trapz is not a "feature". It is a piece of software that embodies the trapezoidal rule, something that students may be exposed t...

6 dagar ago | 1

Why do I get this error in my Taylor Expansion?
It might seem a subtle point. But what is the shape of sqrt(sin(x)), where x is near zero? (I'll just disregard the +2, since th...

6 dagar ago | 1

What is the reason that a Hessian should be positive definite? And what is the relation between an ill-conditioned Hessian and convergence?
(This is not really a MATLAB question, but you are asking a question about the behavior of MTLAB solvers. Therefore close enoug...

7 dagar ago | 1

Why can't I write this row vector?
Very often, people who are just starting to use MATLAB, think that if you don't know the value of something, then it must be sym...

7 dagar ago | 2

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Hyerbolic Functions causing issues with Contour Plots.
Ugh. Learn how to use MATLAB, and scrap the crapola inpuptdlg interface. Next, learn NOT to use variable names like a lower cas...

8 dagar ago | 0

Linears equations summation+combination optimization
Let me see. The use of globals here is unecessary, better would have been to use anonymous functions. Globals just complicate ho...

9 dagar ago | 1

Making a Matrix Strictly Diagonally-Dominant
Can you solve this? Yes, sometimes, and there is no need for random permutations of the matrix. In fact, that is a poor solution...

10 dagar ago | 0

How would i adapt this code to make it work for a whole phrase, not just one word?
Simplest would be to extract each word, one at a time. For example, consider this sentence (taken from your question): phrase =...

10 dagar ago | 0

How can i replace all positive values of a matrix whit their square root ?
If you need to do it in one line, this will work: A(A>0) = sqrt(A(A>0)); Or, you could do the test in advance, saving some CPU...

10 dagar ago | 1

How to quickly find the column index of the last non-zero element in all rows in a sparse matrix?
It is important that if your matrix is sparse, that we use sparse matrix examples to test anything out. Regardless, I would alw...

10 dagar ago | 3

Find ones blocks in upper triangular matrix
Let me plead for you to change how you are doing things. What you are asking to do is perhaps the most silly form of information...

11 dagar ago | 0

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