Select variable automatically in matlab

[mathia]

Hi,
I want to select my variable automatically in MATLAB so I used the following commands: (may variable for example are: Mpoint1,Mpoint5,Mpoint14,...)

for i=1:50
if exist (eval(['Mpoint' num2str(i)]));
statements
else
continue
end
end
but it gave me the following error:

Error using exist
The first input to exist must be a string.

Anybody know how can I do that?

Mathia

 

[gsal]

baby steps...baby steps...


I don't know matlab, but I would suggest to first focus on getting the eval call to work, in the first place...then, you can worry about the loop, etc.

So, just try eval by itself until it is working...does it need a comma between the two arguments? I know you mean to concatenate, but it still may need to be separated by a comma in the list and EVAL will glue them togehter...that what EVAL does, I think.

Also, I read that EVAL takes any valid MATLAB statement...is what you provide to EVAL a valid statement? It is not really much of a command or assignment...will it work? maybe you have to assigne it to something and it will still return the value...

...play around, don't be shy...you are not going to break anything.

 

[mathia]

I tested eval. It works correctly, but i have that error wen I use exist eval...!

 

[gsal]

You may be overseeing the context within which the result is being used. 'exist' expects a string, not a command output, which is what eval returns.

You may need something like "strcat" instead...this is purely a string concatenation function...this may be all you need, not really eval.

 

[alphy]

s,

There are a few things that could be causing this error. First, make sure that all of your variables are actually strings. You can use the class() function to check the data type of your variables. If any of them are not strings, you can use the num2str() function to convert them.

Another possible issue is that the eval() function is not necessary in this case. You can use the exist() function directly on the variable name without using eval().

Finally, it's always a good idea to use error-checking in your code. You can use the isvarname() function to check if a given string is a valid variable name before using it in your code.

I hope this helps and good luck with your project!