Help on a Function

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Dhmhgr
Dhmhgr 2019-11-30
评论： Image Analyst 2019-12-2
Greetings!
I would like your help on a function I'd like to make on Matlab.
I want to make a function called R,
[Roots] = R(X,Y,eps) and this function will be returning a table with 2 columns showing the X and Y of the points where abs(Y)<eps
Thank you in advance.
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dpb
dpb 2019-11-30
I suggest reading the Getting Started tutorial sections in the online documentation...it outlines basic syntax and operations that will get you started. There are lots of examples of writing simple functions with the doc for function
Logical addressing is the trick for such questions as this...see what
eps=0.5;
x=rand(10,1);
x(x<eps)
does.
Dhmhgr
Dhmhgr 2019-12-1
I see, but it didn't help me.

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回答（3 个）

Image Analyst
Image Analyst 2019-12-1
Try this (untested)
% This function will be returning a table with 2 columns showing the X and Y of the points where abs(Y)<eps
function [Roots] = R(X,Y,eps)
Roots = []; % Initialize to no values.
indexes = abs(Y) < eps;
if isempty(indexes)
% No Y satisfies the criteria.
return;
end
xSmall = reshape(X(indexes), [], 1); % Extract indexes and make sure they're in a column vector.
ySmall = reshape(Y(indexes), [], 1); % Extract indexes and make sure they're in a column vector.
Roots = table(xSmall, ySmall);
end
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Dhmhgr
Dhmhgr 2019-12-1
So, what should the correct answer be? The first one by the person "Image Analyst"?
Image Analyst
Image Analyst 2019-12-1
Of course. dpb was merely showing what would happen if I didn't use reshape to turn the data into a column vector before passing in to the table() function. It was not a separate answer in itself, and I did use reshape() so I did it correctly.

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Dhmhgr
Dhmhgr 2019-12-1
??
  1 个评论
Image Analyst
Image Analyst 2019-12-1
I didn't notice at first that we were using eps. That is probably a bad thing to use (unless it's a trick question) because no value of Y can be less than eps unless it's pure zero.
Try this (with a variable named minValue):
X = rand(1, 100);
Y = rand(1, 100);
minValue = 0.1;
Roots = R(X,Y,minValue)
% This function will be returning a table with 2 columns showing the X and Y of the points where abs(Y)<eps
function [Roots] = R(X,Y,minValue)
Roots = []; % Initialize to no values.
indexes = abs(Y) < minValue;
if isempty(indexes)
% No Y satisfies the criteria.
return;
end
xSmall = reshape(X(indexes), [], 1); % Extract indexes and make sure they're in a column vector.
ySmall = reshape(Y(indexes), [], 1); % Extract indexes and make sure they're in a column vector.
Roots = table(xSmall, ySmall);
end
You'll see
Roots =
11×2 table
xSmall ySmall
_________________ ___________________
0.970592781760616 0.0838213779969326
0.792207329559554 0.0781755287531837
0.849129305868777 0.00463422413406744
0.743132468124916 0.0844358455109103
0.489764395788231 0.0759666916908419
0.679702676853675 0.0496544303257421
0.890903252535798 0.0964545251683886
0.257508254123736 0.0597795429471558
0.243524968724989 0.0154034376515551
0.929263623187228 0.0430238016578078
0.567821640725221 0.0811257688657853

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Dhmhgr
Dhmhgr 2019-12-2
编辑：Walter Roberson 2019-12-2
Okay, function made correctly.
I have an other problem regarding the same exercise.
I have:
x=linspace(-2,2,100) y=x.^4-3*x.^3+x.^2+8*x
I want to give eps a number so that by calling the R function I will be able to find the points where abs(y)<eps.
What should the eps number be?
  2 个评论
Walter Roberson
Walter Roberson 2019-12-2
What should the eps number be?
42.
Image Analyst
Image Analyst 2019-12-2
eps, which remember I said must be renamed, can be anything. Whatever you want. Whatever you use, R will find the points, if any exists, or null if they don't.

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