How to identify variables in workspace from strings in cell arrays and then make changes to the variables themselves?

Hi all,

I have a cell array containing strings say for eg {'var1','var2','var3',....'varn'} . Now i have values of all these variables in my workspace.So what i want to do is , first to get values of these variables from workspace.Then i want to compare the lengths of these variables.If their lengths are not equal,i want to make them equal using interpolation.I am so very confused that i cant decide how to go ahead on this.Maybe i need to use regexp,whos,..but i dont know how to use them here.

[Stephen23]

@sangeet pillai: accessing variable names dynamically will make your code slow, buggy, obfuscated, and hard to debug. There are much better alternatives. Read these to know why this is a bad idea, and what the better alternatives are:

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

or any of the hundreds of other times that dynamically named variables has been discussed on this forum.

Answer 1

[Adam]

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

The reason you are confused is because naming variables in this way is really not the way to solve anything. I don't know how you got the variables in the first place, maybe you individually named them all, but this is the type of problem you come to when you do that. Naming them this way in the first place is usually where people come unstuck, but even if they succeed in doing so accessing them afterwards is a real pain.

Use a struct with dynamic field names if you really want numbered items or even better just use an array - it is what they are for!

[sangeet pillai]

hi Adam,these are not the actual variable names.these are the names i just created for example.the actual variables names are quite different and somewhat lengthy

[sangeet pillai]

the actual variable names dont have any relation to each other like these names.ok.so instead of var1,var2,...let me say i have a cell array with variable names {'sky','food','raincoat','blue'...}

[Adam]

But if they are all unrelated variables why would you need a piece of code that just does a common thing to a group of seemingly unconnected variables?

As a general rule (though far from exclusively) if you have two distinct variables they are distinct because they are not explicitly related to each other so that doing the same thing to both of them wouldn't really make any sense. If they are related in that way then they ought to be grouped in an array or a struct or a class or some other container that represents their interdependency.

doc whos

will give you a listing of workspace variables that you can potentially manipulate, but anything that requires you to be fishing out variables in this way sounds like code that is going in very much the wrong direction.

Variable names are just handles in the code, really just for code readability, they should never turn into something that has to be explicitly accessed by their name in the program itself. Variables in a program really could all be called var1, var2, var3, etc and the program would work identically, we just don't wish to do that as programmers because the meaning of the code would be totally obscure and debugging it would be total nightmare.

[sangeet pillai]

i have to solve an equation like sky+food-raincoat*blue .But the dimensions of these variables are different. so what i did is that i wrote a code that could identify different variables in an equation by removing special characters and splitting it. So now i have a cell array with names of individual variables (in string) like cell={'sky',food',raincoat','blue'} .

Now i already have values for these variables in workspace . What i want to do is that i want to use the strings in my cell array to check for the values of those variables in workspace . Then i want to first check if their dimensions are same. If not same,then i want to do something like interpolation or something to make their dimensions same in order to be able to perform operations on them.

I hope i am clear.

[Guillaume]

You've made yourself perfectly clear the first time. What we're telling you is that you shouldn't be in a situation where you need to do what you ask and a redesign of your original program would make the whole problem go away.

Your 'sky', 'food', etc. variables are obviously related since you want to use them together in an equation. In that case, they should be a single variable (in your case a cell array). When you created the content of each, instead of creating it in individual variables, you should have put added as elements of the cell array.

Instead of

sky = somethingthatcreatessky %created some way or another
food = somethingthatcreatesfood %created some way or another
raincoat = somethingthatcreatesraincoat
%etc.

You would have

eqargs = {somethingthatcreatessky, ...
          somethingthatcreatesfood, ...
          somethingthatcreatesraincoat, ...
          etc.

Your resizing problem is then trivial:

maxlength = max(cellfun(@numel, eqargs));
eqargs = cellfun(@(arg) interp1(arg, linspace(1, numel(arg), maxlength)), eqargs, 'UniformOutput', false);  %or something similar
result = eqargs{1}+eqargs{2}-eqargs{3}*eqargs{4};

[sangeet pillai]

Hi,so the variables sky,food,raincoat...are present in a .dat file. I am using mdfimport which loads all those variables in the workspace. Ok so i think u are saying that i should find out a way so that instead of storing them as variables in workspace,i read those variables and store them in a cell. Am in getting that correct?

[Adam]

mdfimport doesn't exist in my Matlab so I have no idea what it does.

[sangeet pillai]

mdfimport imports data from .dat files and saves all variables in base workspace

https://in.mathworks.com/matlabcentral/fileexchange/9622-mdf-import-tool-and-function

[Stephen23]

Someone wrote code that imports that data directly into the workspace, which is not a good design decision because it makes processing those variables difficult:

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

Also note that that MDF file reader is very old and is (according to the comments) no longer maintained by the author. So instead of using an old, buggy MDF file reader, you might like to consider using a newer, better written one, such as this:

https://www.mathworks.com/matlabcentral/fileexchange/47518-mdf-reader-zip

[sangeet pillai]

ok.So what i did is that instead of reading all the required variables from .dat file and importing them into the workspace,i imported them into a structure(say data for example). So now i have a structure containing the field names like sky,food,raincoat.... .Also i have the cell array containing strings like {'sky','food','raincoat'...} . I now intend to check if the structure contains sky then i want to replace the values of data.sky .Then i want to check the next string food which would replace the values of data.food . But i dont know how to tell matlab to change data.sky / data.food using data structure and cell array of strings containing the field names of structure.

[sangeet pillai]

How do i tell matlab to change structure.cellarray{1,1} .Here structure contains fields sky,food,..with their old values. while cellarray contains names of only those fields that i am interested in replacing values. like if cellarray{1,1}='sky' then i want to replace data.sky .cellarray{1,2}='food' so i want to replace data.food and so on.

[Adam]

myCell = { 'sky','food','raincoat' };

myStruct.( myCell{ 1 } );

[sangeet pillai]

ok.i think i got it.i just needed to place cellarray in brackets.so i did structure.(cellarray{1,1}) and it worked.I think i am proceeding in the right direction

[Adam]

Accessing structure fields using dynamic strings like this can be very useful. I don't do it much myself, even with classes which I use instead of structs, but I have certainly found it very neat at those times I have done.

[sangeet pillai]

Hello guys , I have successfully completed my intended task which wouldnt have been possible without your help. So i would like to thank Stephen Cobeldick , Adam & Guillaume .You saved me a lot of time and headache . If it were not for your timely help , i would have made this task even more complex trying to complete it through alternate means and thereby confusing myself more n more

[Stephen23]

@sangeet pillai: we are glad to help you. Please consider accepting Adam's answer, as a small indication of your appreciation.

[sangeet pillai]

yes.sure.I was thinking the same but didnt know which answer to accept as all your answers helped me in a way