# Help on a Function

1 view (last 30 days)
Dhmhgr on 30 Nov 2019
Commented: Image Analyst on 2 Dec 2019
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

Show 1 older comment
Dhmhgr on 30 Nov 2019
I'm new to Matlab and I saw this interesting exercise online and there is no solution given. I don't know what to do, but I'd like to know how this type of exercises are getting solved.
dpb on 30 Nov 2019
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 on 1 Dec 2019
I see, but it didn't help me.

Image Analyst on 1 Dec 2019
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

dpb on 1 Dec 2019
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 on 1 Dec 2019
So, what should the correct answer be? The first one by the person "Image Analyst"?
Image Analyst on 1 Dec 2019
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 on 1 Dec 2019
??

#### 1 Comment

Image Analyst on 1 Dec 2019
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 on 2 Dec 2019
Edited: Walter Roberson on 2 Dec 2019
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?