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Adam Danz
Last activity 2023-10-13

Approximating π with durations

Over the weekend I came across a pi approximation using durations of years and weeks (image below, Wolfram, eq. 89), accurate to 6 digits using the average Gregorian year (365.2425 days).

Here it is in MATLAB. I divided by 1 week at the end rather than multiplying by its reciprocal because you can’t divide a numeric by a duration in MATLAB (1/week).

weeks = @(n)n*days(7);
piApprox = ((years(13)-weeks(6))/years(13) + weeks(3)) / weeks(1)
% piApprox = 3.141593493469302

Here’s a breakdown

The first argument becomes 12.885 yrs / 13 yrs or 0.99115
Add three weeks: 0.99115 + 3 weeks = 21.991 days
The reduced fraction becomes 21.991 days / 7 days

Now it looks a lot closer to the more familiar approximation for pi 22/7 but with greater precision!

https://www.youtube.com/watch?v=h376W93gQq4

Walter Roberson
Last activity 2023-10-6

full body keyboard using flag semaphore

This person used computer version to build a keyboard input, and used standard flag semaphore for the positions.

Flag semaphore is used mostly by sailors to be able to communicate optically over a distance; it does not need anything more than make-shift flags (but binoculars or telescopes can help.) Trained users can go faster than you might guess.

Hans Scharler
Last activity 2024-7-27

What's everyone doing today?

Chen, Rena, and I are at a community management event. It's great to be with others talking about relationships, trust, and co-creation.

community

https://www.youtube.com/watch?v=UTUnVEiO-TU

Walter Roberson
Last activity 2023-12-27

Tricking AI vision systems

A research team found a way to trick a number of AI systems by injecting carefully placed nonsense -- for example being able able to beat DeepMind's Go game.

https://www.youtube.com/watch?v=SsGEcLwjgEg

Walter Roberson
Last activity 2023-10-4

"Cody" bridge rotates up to move out of the way

This video discusses the "Cody" bridge, which is a pedestrian bridge over a canal that has been designed to move up and out of the way when ships need to travel through. The mathematics of the bridge movement are discussed and diagrammed. It is unique and educational.

https://fb.watch/njjZFdCnC1/

Walter Roberson
Last activity 2023-10-4

touch microscope developed

Recently developed: a "microscope" based on touch and stereo vision.

Using touch removes the possibility of optical confusion -- for example, black on touch is only due to shape, not due to the possibility that the object has a black patch.

Sorry, you might need a Facebook account to watch the video.

Adam Danz
Last activity 2024-3-6

To hold or not to hold

I'm curious how the community uses the hold command when creating charts and graphics in MATLAB. In short, hold on sets up the axes to add new objects to the axes while hold off sets up the axes to reset when new objects are added.

When you use hold on do you always follow up with hold off? What's your reasoning on this decision?

Can't wait to discuss this here! I'd love to hear from newbies and experts alike!

graphics

Mike Croucher
Last activity 2023-10-3

Totally new solution framework for Ordinary Differential Equations (ODEs) in MATLAB

The way we've solved ODEs in MATLAB has been relatively unchanged at the user-level for decades. Indeed, I consider ode45 to be as iconic as backslash! There have been a few new solvers in recent years -- ode78 and ode89 for example -- and various things have gotten much faster but if you learned how to solve ODEs in MATLAB in 1997 then your knowledge is still applicable today.

In R2023b, there's a completely new framework for solving ODEs and I love it! You might argue that I'm contractually obliged to love it since I'm a MathWorker but I can assure you this is the real thing!

I wrote it up in a tutorial style on The MATLAB Blog https://blogs.mathworks.com/matlab/2023/10/03/the-new-solution-framework-for-ordinary-differential-equations-odes-in-matlab-r2023b/

The new interface makes a lot of things a much easier to do. Its also setting us up for a future where we'll be able to do some very cool algorithmic stuff behind the scenes.

Let me know what you think of the new functionality and what you think MathWorks should be doing next in the area of ODEs.

Mayla
Last activity 2023-9-19

Problem 1899. Convert a Cell Array into an Array

Thats the task:

Given a square cell array:

x = {'01', '56'; '234', '789'};

return a single character array:

y = '0123456789'

I wrote a code that passes Test 1 and 2 and one that passes Test 3 but I'm searching a condition so that the code for Test 3 runs when the cell array only contains letters and the one for Test 1 and 2 in every other case. Can somebody help me?

This is my code:

 y = []
[a,b]=size(x)
%%TEST 3
delimiter=zeros(1,a)
delimiter(end)=1
delimiter=repmat(delimiter,1,b)
delimiter(end)=''
delimiter=string(delimiter)
y=[]
for i=1:a*b
      y = string([y x(i)])
end
y=join(y,delimiter)
y=erase(y,'0')
y=regexprep(y,'1',' ')
%%TEST 1+2
for i=1:a*b
      y = string([y x(i)])
      y=join(y)
  end
  y=erase(y,' ' )

Mayla
Last activity 2023-9-13

Problem 2022. Find a Pythagorean triple

That's the question: Given four different positive numbers, a, b, c and d, provided in increasing order: a < b < c < d, find if any three of them comprise sides of a right-angled triangle. Return true if they do, otherwise return false .

I wrote this code but it doesn't pass test 7. I don't really understand why it isn't working. Can somebody help me?

function flag = isTherePythagoreanTriple(a, b, c, d)
a2=a^2
b2=b^2
c2=c^2
d2=d^2
format shortG
  if a2+b2==c2
      flag=true
  else if a2+b2==d2
          flag=true      
  else if a2+c2==d2
           flag=true
  else if c2+b2==d2
           flag=true
  else flag=false
  end
  end
  end
  end
end

Mayla
Last activity 2023-10-24

Problem 44952. Find MPG of Lightest Cars

That's the question:

The file cars.mat contains a table named cars with variables Model, MPG, Horsepower, Weight, and Acceleration for several classic cars.

Load the MAT-file. Given an integer N, calculate the output variable mpg.

Output mpg should contain the MPG of the top N lightest cars (by Weight) in a column vector.

I wrote this code and the resulting column vector has the right values but it doesn't pass the tests. What's wrong?

function mpg = sort_cars(N)
load cars.mat
sorted=sortrows(cars,4)
mpg = sorted(1:N,2)
end

Joel Hottinger
Last activity 2023-6-16

No longer able to rate difficulty of qustions on Cody

I recently have found that I am no longer able to give my difficulty rating for questions on Cody after sucessfully completing a question. This is obviously not a big deal, I was just wondering if this was an issue on my end or if there was some change that I was not aware of.

The option to rate does not pop up after solving a problem, and the rating in general does not even show up anymore when answering questions (though it is visible from problem groups).

cody

KALYAN ACHARJYA
Last activity 2023-7-3

Will the MATLAB Answers community diminish/obsolete with the rise of AI-based chatbots?

The MATLAB Answers community is an invaluable resource for all MATLAB users, providing selfless assistance and support. However, with the emergence of AI-based chatbots, like chatGPT, there may be concerns about the future relevance and utility of the MATLAB Answer community. What are your thoughts?

Christian Schröder
Last activity 2023-1-30

Cody: what are the criteria for being able to view all solutions?

When solving problems over on Cody, I can almost always view all solutions to a problem after submitting a correct solution of my own. Very rarely, however, this is not the case, and I instead get the following message:

This solution is locked. To view this solution, you need to provide a solution of the same size or smaller.

You may solve another problem from Community group to unlock all the solutions to this problem.

If this happens, then again, I can almost always rectify this by submitting a (correct) solution to a different problem (I take it that the Community group is the implicit group of all problems on Cody --- is it?). But sometimes that, too, fails.

So my question is, why? What are the criteria that determine when all solutions are, in fact, unlocked?

(There is a related question here, but I feel the posted answer does not answer the question.)

cody

Christian Schröder
Last activity 2022-12-31

Cody: where's the Modeling and Simulation Challenge group?

Simple question: I noticed there's a Modeling & Simulation Challenge Master badge over on Cody, but I can't find the corresponding group. So: where is it? Does it still exist at all?

cody