How can I plot a series of points in a vector with two coordinates x&y considering that I want to name these points respectively S1, S2....S10, and I want to use them in other functions just by using their "name"?

I want to plot a series of points in a vector with two coordinates x&y and I want to name these points respectively S1, S2....S10, and then I wantto use them in other functions just by using their "name". How can I write the code to be able to do this? The vector will be: S=[S1,S2,S3,S4,S5,S6,S7,S8,S9,S10] While the coordinates will be: x=[0 0.5 1 0.7 -1 -0.5 0.5 0.5 -0.5]; y=[1 -1 1 0.5 0 0.8 0.8 0 -0.8 -0.8]; I want to plot them with the respective letters, and then use them in functions just by typing the letter. Thank you in advance.

[Stephen23]

You should avoid magically accessing variable names. Read this to know why:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Answer 1

[Jan]

This sounds like a rather complicated and indirect way of programming. Of course your could simply create a set of variables:

S1 = [0,    1]
S2 = [0.5, -1]
...

But as you find in many (hundreds!) discussions in this forum, this way is known to cause more troubles than it solves. Hiding indices in names of variables is not efficient, unclear and deprecated:

• https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval
• https://www.mathworks.com/matlabcentral/answers/57445-faq-how-can-i-create-variables-a1-a2-a10-in-a-loop

Using vectors and indices is the standard method and there are good reasons for this.

But to be honest, I'm not sure, what this exactly means:

 I want to plot them with the respective letters, and then use them
 in functions just by typing the letter

[Jenny]

I want the letter to show next to the plotted point in the graph. Then depending on the distance of each point from the point P(0;0), I want to take only those points that fullfil a specific condition (for example dist≤1) and put them in the final vector. This is why I wanted to know if it's possible to copy only the letter in the new vector after the initialization of each point with the respective letter. This is the idea, but I don't know how to do it.

[Jan]

I still do not understand which letter you actually mean. What is "the graph"? Do you mean a legend? Selecting a subset of points matching a certain criterion is the perfect job for indices.

Let's start with the inputs you have:

x = [0 0.5 1 0.7 -1 -0.5 0.5 0.5 -0.5];
y = [1 -1 1 0.5 0 0.8 0.8 0 -0.8 -0.8];

Now determine the distance to [0,0]:

dist2 = x .^ 2 + y .^ 2;  % Cheaper to compare the squared distance
match = dist2 < 1.0 ^ 2;  % Yes, I know that 1^2 equal 1  :-)
idx   = find(match)
xm    = x(match)
ym    = y(match)

This works well without using "letters".

Where do you want to type which letter? Why do you think that these letters are useful? In which kind of graph should they appear where (a hand drawn sketch perhaps?)? Or could the problem be solves easier with arrays and indices?

[Jenny]

S1 would be for the coordinates of a sensor. I have to put 10 sensors to cover a surface. I need the letters to identify the position for each sensor. Do I need any other coordinate for the position of the sensor, like angle or I don't know? Maybe it can be solved with arrays and indices. I would really appreciate it if you can help because I have been trying for hours and I couldn't do it. Thank you very much.

[Jenny]

I am trying to create the second algorithm of this paper, LNSA. It is a school exercise. https://www.google.al/url?sa=t&source=web&rct=j&url=http://www.mdpi.com/1424-8220/17/6/1303/pdf&ved=2ahUKEwiftPH7g9bYAhVBOywKHTmXAZEQFjABegQIEhAB&usg=AOvVaw1XURnfpO9ZTeUNr3jAgyzS

[Jan]

@Jenny: I really want to help you, but I still do not know, what you are talking of. Which letter? Is "S1" a letter? Your x and y look really fine for the identification of a sensor in 2D. Why do you need an angle?

I'm sure that using names as S1, S2, ... is a bad idea in general, because you would hide indices in the names, which causes complicated and inefficient methods to access the data.

I recommend to make a short break. Coffee, a piece of cake. (This is meant seriously.) Then come back to the forum and explain the problem you want to solve. What are the available inputs, which operation do you want to apply, what do you want as result?

[Steven Lord]

Jenny, you can label the points on your plot using text. This will allow you to label each one S1, S2, etc. without storing the underlying data in variables named S1, S2, etc.

[Jenny]

It is supposed that in the center I have a sensor P with coordinates (0;0). P has a radius R=1. Then we have the distance of the other sensors from P, that will be r=sqrt(√(x^2+y^2)); Then we have a condition: If r-R<rho, rho=0.08, then the coordinates of the sensors (points) that fulfill the condition should be put in the final vector, and this vector should be printed as the result in the screen after the code execution. Can you help me please? I wrote a "code", but it doesn't give me any result.

[Jan]

@Jenny: If you post your code, the readers can suggest improvements. Sometimes it is easier to fix an existing code than to write it completely from scratch. Then we could e.g. know, how your inputs look like. Do you get the position of the other sensors as an array? Or do you use random points? Or an even grid of coordinates?

What about the code I had suggested already - with a tiny modification:

R     = 1.0;
dist2 = sqrt(x .^ 2 + y .^ 2);
match = dist2 - R < 0.08;  % Or abs(dist2 - R) < 0.08 ??
xm    = x(match)
ym    = y(match)