Vector and matrix index operations

Hi, My question is about vectors and matrix: Assuming I have vector (array) of strings: myvect={'101AA21' '101AA22' '101AA23' '102AA21' '102AA22'}; I have a matrix mymatrix=magic(5); I want to define a variable from myvect for each column of mymatrix such the results should be: 101AA21=mymatrix(:,1); 101AA22=mymatrix(:,2), etc

in order to do this:

myvect={'101AA21' '101AA22' '101AA23' '102AA21' '102AA22'}; mymatrix=magic(5); mynew_vect=char(myvect);

for i=1:length(mynew_vect) mynew_vect(i)== mymatrix(:,i); end

plot(101AA21, 102AA22, '-r');

Statment mynew_vect(i)== mymatrix(:,i); it is wrong but how can be defined . The real vector is 300, matrix is 70000X300

 Thank you

[Stephen23]

https://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html

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

Answer 1

[Keith Hooks]

You won't be able to vectors/matrices long with a variable name. I suggest using a table or structure for mynew_vect instead. However, the variable names you have are invalid for a structure element. They need to start with a character.

A table might have performance advantages over a structure that large. I couldn't say either way.

Using a structure:

names={'D101AA21' 'D101AA22' 'D101AA23' 'D102AA21' 'D102AA22'}; 
mymatrix=magic(5); 

for i=1:length(names) 
    mynew_vect.(names{i})= mymatrix(:,i);
end

And with a table:

names={'101AA21' '101AA22' '101AA23' '102AA21' '102AA22'}; 
mymatrix=magic(5); 

for i=1:length(names) 
    mynew_table.names{i}= mymatrix(:,i);
end

[Ionut Anghel]

Thank you for you answer. The problem is that those names(variables) '101AA22', '101AA23' exits exact in this format. And in the final report, the customers want exact this format.

[Stephen23]

MATLAB variable names must start with a letter character: "A valid variable name starts with a letter, followed by letters, digits, or underscores"

Also you should not define variable names dynamically, but use a structure, table or cell array instead, as Keith Hooks described.

You could easily keep a cell array of those names for using in "the final report", and keep the corresponding data in a cell / table / structure. This would be much more robust, because it makes your code completely independent of how the customer names their variables. It would make your life easier.

[John D'Errico]

That the customers want an illegal variable name is not relevant. They cannot have it. Variables cannot have names that start with a number.

And even if they decide to choose better names, I would STRONGLY suggest following Stephen's excellent recommendations here. It is just a bad idea to do what you wish to do.

[Ionut Anghel]

Hi, Those variables are signals in a power plant defined in the early 1978. In order to get their licence some calculations are with specials codes. So, I cannot change the name of the variables. I write this part of the program to compare experimental data with their system code calculations. So, after 37 years of using those names for their parameters/variables they are very reluctant to any changes. Any way, thank you all. I proved all your solutions.

Answer 2

[Thorsten]

Use eval:

myvect={'101AA21' '101AA22' '101AA23' '102AA21' '102AA22'}; 
mymatrix=magic(5);  
for i = 1:numel(myvect)
  eval(['var_' myvect{i} '= mymatrix(i,:);']);
end

plot(var_101AA21, var_101AA22)

And yes, I agree that this is awful code, but the closed to be able to write something as simple as (which seems to be what wanted here, for whatever reasons):

plot(101AA21, 101AA22)

[Steven Lord]

DON'T use EVAL. Here's a slight modification of your code that avoids the EVAL while still getting close to the desired (illegal) code.

 myvect={'101AA21' '101AA22' '101AA23' '102AA21' '102AA22'}; 
 mymatrix = magic(5);
 V = struct;
 for k = 1:numel(myvect)
   N = ['x' myvect{k}];
   V.(N) = mymatrix(k, :);
 end
 plot(V.x101AA21, V.x101AA22)

Some prefix on the field name is necessary because MATLAB identifiers must start with a letter; I chose x.

Answer 3

[F.]

What do you think about something like this :

myvect={'101AA21' '101AA22' '101AA23' '102AA21' '102AA22'}; 
mymatrix = magic(5);
Tmp1 = find( strcmp( myvect, '101AA21' ) );
Tmp2 = find( strcmp( myvect, '102AA21' ) );
plot( mymatrix(:,Tmp1), mymatrix(:,Tmp2) );

But perhaps, I didn't well understand what would you want to do with this variables...