# Why can't I write this row vector?

1 view (last 30 days)
Armando MAROZZI on 22 Mar 2020
Commented: Armando MAROZZI on 22 Mar 2020
I simply want a 1x3 row vector like this: . I tried the following:
[x(1,3); x(2,3); x(3,3)]
but I got the following error:
Unrecognized function or variable 'x'.
Why? How to sort it out?

John D'Errico on 22 Mar 2020
Edited: John D'Errico on 22 Mar 2020
Very often, people who are just starting to use MATLAB, think that if you don't know the value of something, then it must be symbolic. That is not always true. I'll give some examples later. But to do what you are asking right now, just do this:
X = sym('X',[1,3])
X =
[X1, X2, X3]
As I said, you can have an unknown that will then be estimated. For example, if we stay in the purely numerical domain, we can solve for an unknown vector. This eample is pretty simple, but...
fun = @(X) sum((X - [1 2 3]).^2);
Xinit = [4 4 4];
Xfinal = fminsearch(fun,Xinit)
Xfinal =
0.99999144502901 1.99997109582809 2.99995541073201
Of course, the minimum arises at the vector [1 2 3].

#### 1 Comment

Armando MAROZZI on 22 Mar 2020
Thanks a lot for the explanation John. As you mention that example, I grab the opportunity to ask you another question. I am solving a symbolic non-linear system of equations. I did all the steps correctly I think but when I run:
x = fsolve(fun, x0, option)
I get:
fsolve stopped because the sum of squared function values, r, has gradient with
relative norm 1.490116e-08; this is less than options.OptimalityTolerance = 1.000000e-06.
However, r = 3.000000e+00, exceeds sqrt(options.FunctionTolerance) = 1.000000e-03.
As far as I understand it, I shall change "option", right? At the moment is:
Default properties:
Algorithm: 'trust-region-dogleg'
FiniteDifferenceStepSize: 'sqrt(eps)'
FiniteDifferenceType: 'forward'
FunctionTolerance: 1.0000e-06
MaxFunctionEvaluations: '100*numberOfVariables'
MaxIterations: 400
OptimalityTolerance: 1.0000e-06
OutputFcn: []
StepTolerance: 1.0000e-06
TypicalX: 'ones(numberOfVariables,1)'
UseParallel: 0
I'll edit it and re-run the code?

Image Analyst on 22 Mar 2020
Define x somehow before that line, like these examples
x = magic(3)
x = rand(3)
x = randi(100, 3, 3)
or whatever.

Armando MAROZZI on 22 Mar 2020
But I want it to stay symbolic. I want it to appear exactly as it is written, no numbers. Any hint?
Image Analyst on 22 Mar 2020
I don't have that toolbox so I don't know what kind of assumptions it makes. I've added it to the product list, which you forgot to do when you created this posting.
What are you going to do after that? How are you going to use x? Or that 3 element, 1-D column vector you're trying to make from the 2-D x matrix?
Armando MAROZZI on 22 Mar 2020
sorry I am moving from R to Matlab and I make silly mistakes. I'm writing a function to solve a non-linear system of equations. I think it's easier for me to write the row vector directly in the script and not trying to save it in the original command section