Help on a Function
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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.
4 个评论
dpb
2019-11-30
OK, simple enough. What have you got so far and where, specifically did you get stuck?
Dhmhgr
2019-11-30
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
2019-12-1
回答（3 个）
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
5 个评论
Dhmhgr
2019-12-1
Walter Roberson
2019-12-1
Yes, that code will work with row vectors. Image Analyst took care of the detail that the values passed into table() should normally be column vectors.
dpb
2019-12-1
Continuing w/ Walter's explanation, because if row vectors,
>> X=1:10;Y=randn(size(X));
>> table(X,Y)
ans =
1×2 table
X Y
_____________ _____________
[1x10 double] [1x10 double]
>>
you see you get a row vector for each of the two variables as the one record in the table...this probably is not what is wanted.
Dhmhgr
2019-12-1
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.
Dhmhgr
2019-12-1
0 个投票
??
1 个评论
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
Dhmhgr
2019-12-2
编辑：Walter Roberson
2019-12-2
2 个评论
Walter Roberson
2019-12-2
What should the eps number be?
42.
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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2019-12-2
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