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John D'Errico


Retired

3,815 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
How likely is it that I can use simple linear regression to predict?
No real world data truly follows a perfectly linear model. Even where there is an essentially linear domain, there are always li...

3 days ago | 0

Answered
Order of logical operators
You need to recognize how MATLAB evaluates a logical expression. Many people use that shorthand, to indicate two coupled compari...

4 days ago | 1

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Answered
is it possible that answers can be accepted?
Being an MVP does not give you the privilege to doubly accept an answer. Read the help for Answers. It tells you what privileges...

4 days ago | 1

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Answered
Plotting the area defin
You know how to use meshgrid! In fact, I know that, because you used it in your other question. So it you want to plot someth...

4 days ago | 1

Answered
Start point power law fit
I'm not positive what tools you are using for the fit. I will assume the curve fitting toolbox. Almost all optimizers (nonlin...

5 days ago | 0

Answered
Looking for some tips on a Simulation Project!
Is there a better way? Not really. In fact, your plan is actually a good one, and arguably the best approach. The true art will ...

5 days ago | 1

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Answered
assemble global stiffness matrix
K = zeros(10,10); k = [1 -1; -1 1]; for n = 1:9; rind = n + [0 1]; cind = rind; K(rind, cind) = K(rind, c...

5 days ago | 0

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Answered
Create a polar function from a polar regression
Your question is slightly ambiguous. Are you looking to find a model of the form z=f(r,theta)? Or do you want some model that li...

5 days ago | 0

Answered
What are Contour Levels
A contour of the function z(x,y) is a set of points in the (x,y) plane, such that z(x,y) is fixed at some constant value. That c...

5 days ago | 0

Answered
Operands to the || and && operators must be convertible to logical scalar values
LEARN TO USE THE DEBUGGER!!!!!!!!! I did this: dbstop if error Next, I ran your code. Well, I tried to run it. f...

6 days ago | 3

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Answered
Exponential fit using Linear Regression
Write down the model you want to use. DO IT! You can't solve this unless you understand & think about what you are doing. That r...

6 days ago | 0

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Answered
How to calculate the ratio of two primes in a given set of primes?
As long as the set is sorted, you never need to worry about the ratio being greater than than 1. How do you compute a base 3 ...

8 days ago | 1

Answered
gamma function for N>100
No. You will not make it work for large values, when they exceed the limits of gamma itself. Your code is not better than what y...

8 days ago | 1

Answered
find rows of Zeros in matrix
Time to learn MATLAB. How do you find that something is zero? A == 0 That returns 1 where an element of A is zero. EXACT...

9 days ago | 1

Answered
If statement with many logical or.
Your problem is EXACTLY in that you are trying to compare floating point numbers, for exact equality. This is something you shou...

9 days ago | 2

Answered
Function not defined but it is in fact defined
Did you actually save it as an m-file? Is it saved to your search path? NOT saved in one of the MATLAB supplied directories? ...

9 days ago | 0

Answered
Volume of irregular shape
Not sure I see the problem. You apparently have cross sections in say x and y, presumably, at various values for z. That means y...

10 days ago | 0

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Answered
How to make an exponential function taking as input a vector?
inp = 3; syms n n.^(0:inp-1) ans = [ 1, n, n^2]

11 days ago | 0

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Answered
How can I do least square problem to get certain area?
Compute the area of the polygon. I'll call it A. polyarea should suffice. Recognize that area scales with the square of the u...

11 days ago | 0

Answered
How to underline an undefined function or variable
I believe that capability would not exist, because it is easy enough to create a variable or function name on the fly. It would ...

12 days ago | 2

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Answered
How to find point(s) of intersection between two scatter surfaces and the zero plane?
Hint: contour 1. That is, use contour to identify the level surface at 0 for each surface. The result will be a set of piecew...

13 days ago | 0

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Answered
How can I plot a line of one data point (x,y) using linear interpolation
Linear interpolation is meaningless with one point. Said another way, there are infinitely many lines that pass through one poin...

14 days ago | 0

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Answered
How can I fit the data if I know the relationship of x and y, but don't have expression form of y=.....
Use polyfit. Seriously. Your model is a linear one in those parameters. Here is your model: ((x+5.637)^(y-2))/1000 = a + b*...

14 days ago | 0

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Answered
What is the simplest way to extract lengths of NaN sequences from a vector?
Given sequences in a vector of NaNs, you find the location of the start, then find the end. the length is given by the differenc...

15 days ago | 1

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Answered
Conversion of 100 jpg images to PGM
Use a loop? Surely you can get the directory contents, so the names of all images in a directory, using a tool like dir. Now jus...

16 days ago | 0

Answered
How to divide anonymous functions
But what does this do? @(x) f(x)./df(x)

16 days ago | 0

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Answered
Taylor Series for sine function
(Note: you might decide to read how I coded trig functions in my HPF toolbox. But that would be wild overkill for basic homework...

16 days ago | 0

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Answered
how to solve this equation (vdmoins/2)*(1-cos((x + alpha)*(2 * pi)/360)) + (vpiston/2)*(1+cos((x)*(2 * pi)/360))==291
First, check that a root exists. vdmoins = 537; alpha = 45; vpiston = 426; fun = @(x) (vdmoins/2)*(1-cos((x + alph...

16 days ago | 0

Answered
How to create symbolic polynomials from a coefficient vector and symbolic vector?
As Walter shows, you can do it using provided tools easily enough. Just to understand how things work, you can also do it direct...

17 days ago | 0

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